Localization of L-lactate dehydrogenase in mitochondria

Empowering mitochondrial metabolism: Exploring L-lactate supplementation as a promising therapeutic approach for metabolic syndrome

Mitochondrial dysfunction plays a critical role in the pathogenesis of metabolic syndrome (MetS), affecting various cell types and organs. In MetS animal models, mitochondria exhibit decreased quality control, characterized by abnormal morphological structure, impaired metabolic activity, reduced energy production, disrupted signaling cascades, and oxidative stress. The aberrant changes in mitochondrial function exacerbate the progression of metabolic syndrome, setting in motion a pernicious cycle. From this perspective, reversing mitochondrial dysfunction is likely to become a novel and powerful approach for treating MetS. Unfortunately, there are currently no effective drugs available in clinical practice to improve mitochondrial function. Recently, L-lactate has garnered significant attention as a valuable metabolite due to its ability to regulate mitochondrial metabolic processes and function. It is highly likely that treating MetS and its related complications can be achieved by correcting mitochondrial homeostasis disorders. In this review, we comprehensively discuss the complex relationship between mitochondrial function and MetS and the involvement of L-lactate in regulating mitochondrial metabolism and associated signaling pathways. Furthermore, it highlights recent findings on the involvement of L-lactate in common pathologies of MetS and explores its potential clinical application and further prospects, thus providing new insights into treatment possibilities for MetS.

Distinct lactate metabolism between hepatocytes and myotubes revealed by live cell imaging with genetically encoded indicators

The process of glycolysis breaks down glycogen stored in muscles, producing lactate through pyruvate to generate energy. Excess lactate is then released into the bloodstream. When lactate reaches the liver, it is converted to glucose, which muscles utilize as a substrate to generate ATP. Although the biochemical study of lactate metabolism in hepatocytes and skeletal muscle cells has been extensive, the spatial and temporal dynamics of this metabolism in live cells are still unknown. We observed the dynamics of metabolism-related molecules in primary cultured hepatocytes and a skeletal muscle cell line upon lactate overload. Our observations revealed an increase in cytoplasmic pyruvate concentration in hepatocytes, which led to glucose release. Skeletal muscle cells exhibited elevated levels of lactate and pyruvate levels in both the cytoplasm and mitochondrial matrix. However, mitochondrial ATP levels remained unaffected, indicating that the increased lactate can be converted to pyruvate but is unlikely to be utilized for ATP production. The findings suggest that excess lactate in skeletal muscle cells is taken up into mitochondria with little contribution to ATP production. Meanwhile, lactate released into the bloodstream can be converted to glucose in hepatocytes for subsequent utilization in skeletal muscle cells.

How the 'Aerobic/Anaerobic Glycolysis' Meme Formed a 'Habit of Mind' Which Impedes Progress in the Field of Brain Energy Metabolism

The division of glycolysis into two separate pathways, aerobic and anaerobic, depending on the presence or absence of oxygen, respectively, was formulated over eight decades ago. The former ends with pyruvate, while the latter ends with lactate. Today, this division is confusing and misleading as research over the past 35 years clearly has demonstrated that glycolysis ends with lactate not only in cancerous cells but also in healthy tissues and cells. The present essay offers a review of the history of said division and the more recent knowledge that has been gained about glycolysis and its end-product, lactate. Then, it presents arguments in an attempt to explain why separating glycolysis into aerobic and anaerobic pathways persists among scientists, clinicians and teachers alike, despite convincing evidence that such division is not only wrong scientifically but also hinders progress in the field of energy metabolism.

Tracing the lactate shuttle to the mitochondrial reticulum

Isotope tracer infusion studies employing lactate, glucose, glycerol, and fatty acid isotope tracers were central to the deduction and demonstration of the Lactate Shuttle at the whole-body level. In concert with the ability to perform tissue metabolite concentration measurements, as well as determinations of unidirectional and net metabolite exchanges by means of arterial–venous difference (a-v) and blood flow measurements across tissue beds including skeletal muscle, the heart and the brain, lactate shuttling within organs and tissues was made evident. From an extensive body of work on men and women, resting or exercising, before or after endurance training, at sea level or high altitude, we now know that Organ–Organ, Cell–Cell, and Intracellular Lactate Shuttles operate continuously. By means of lactate shuttling, fuel-energy substrates can be exchanged between producer (driver) cells, such as those in skeletal muscle, and consumer (recipient) cells, such as those in the brain, heart, muscle, liver and kidneys. Within tissues, lactate can be exchanged between white and red fibers within a muscle bed and between astrocytes and neurons in the brain. Within cells, lactate can be exchanged between the cytosol and mitochondria and between the cytosol and peroxisomes. Lactate shuttling between driver and recipient cells depends on concentration gradients created by the mitochondrial respiratory apparatus in recipient cells for oxidative disposal of lactate.

Studies using isotope tracer technologies have significantly improved understanding of how lactate, a metabolite produced as fuel during normal metabolism and in response to exercise, moves or ‘shuttles’ throughout the body. George Brooks and colleagues at the University of California, Berkeley, USA, reviewed the history of the understanding of lactate shuttling, which has largely been informed by human studies using isotope tracer infusions during rest and exercise. Such research highlights continuous organ–organ, cell–cell, and intracellular lactate shuttling. Lactate moves between producer cells such as skeletal muscle cells and consumer cells in tissues including the heart and brain, where it is preferred over glucose as an energy source. Shuttling depends on lactate concentration gradients created by mitochondrial networks in recipient cells. Lactate is disposed of via oxidation.

The blood lactate/pyruvate equilibrium affair

The Lactate Shuttle hypothesis is supported by a variety of techniques including mass spectrometry analytics following infusion of carbon-labeled isotopic tracers. However, there has been controversy over whether lactate tracers measure lactate (L) or pyruvate (P) turnover. Here, we review the analytical errors, use of inappropriate tissue and animal models, failure to consider L and P pool sizes in modeling results, inappropriate tracer and blood sampling sites, and failure to anticipate roles of heart and lung parenchyma on L⇔P interactions. With support from magnetic resonance spectroscopy (MRS) and immunocytochemistry, we conclude that carbon-labeled lactate tracers can be used to quantitate lactate fluxes.

Lactate and Myocadiac Energy Metabolism

The myocardium is capable of utilizing different energy substrates, which is referred to as “metabolic flexibility.” This process assures ATP production from fatty acids, glucose, lactate, amino acids, and ketones, in the face of varying metabolic contexts. In the normal physiological state, the oxidation of fatty acids contributes to approximately 60% of energy required, and the oxidation of other substrates provides the rest. The accumulation of lactate in ischemic and hypoxic tissues has traditionally be considered as a by-product, and of little utility. However, recent evidence suggests that lactate may represent an important fuel for the myocardium during exercise or myocadiac stress. This new paradigm drives increasing interest in understanding its role in cardiac metabolism under both physiological and pathological conditions. In recent years, blood lactate has been regarded as a signal of stress in cardiac disease, linking to prognosis in patients with myocardial ischemia or heart failure. In this review, we discuss the importance of lactate as an energy source and its relevance to the progression and management of heart diseases.

Lactate in contemporary biology: a phoenix risen

After a century, it's time to turn the page on understanding of lactate metabolism and appreciate that lactate shuttling is an important component of intermediary metabolism in vivo. Cell‐cell and intracellular lactate shuttles fulfil purposes of energy substrate production and distribution, as well as cell signalling under fully aerobic conditions. Recognition of lactate shuttling came first in studies of physical exercise where the roles of driver (producer) and recipient (consumer) cells and tissues were obvious. Moreover, the presence of lactate shuttling as part of postprandial glucose disposal and satiety signalling has been recognized. Mitochondrial respiration creates the physiological sink for lactate disposal in vivo. Repeated lactate exposure from regular exercise results in adaptive processes such as mitochondrial biogenesis and other healthful circulatory and neurological characteristics such as improved physical work capacity, metabolic flexibility, learning, and memory. The importance of lactate and lactate shuttling in healthful living is further emphasized when lactate signalling and shuttling are dysregulated as occurs in particular illnesses and injuries. Like a phoenix, lactate has risen to major importance in 21st century biology.

Mitochondrial lactate metabolism: history and implications for exercise and disease

Mitochondrial structures were probably observed microscopically in the 1840s, but the idea of oxidative phosphorylation (OXPHOS) within mitochondria did not appear until the 1930s. The foundation for research into energetics arose from Meyerhof's experiments on oxidation of lactate in isolated muscles recovering from electrical contractions in an O2 atmosphere. Today, we know that mitochondria are actually reticula and that the energy released from electron pairs being passed along the electron transport chain from NADH to O2 generates a membrane potential and pH gradient of protons that can enter the molecular machine of ATP synthase to resynthesize ATP. Lactate stands at the crossroads of glycolytic and oxidative energy metabolism. Based on reported research and our own modelling in silico, we contend that lactate is not directly oxidized in the mitochondrial matrix. Instead, the interim glycolytic products (pyruvate and NADH) are held in cytosolic equilibrium with the products of the lactate dehydrogenase (LDH) reaction and the intermediates of the malate-aspartate and glycerol 3-phosphate shuttles. This equilibrium supplies the glycolytic products to the mitochondrial matrix for OXPHOS. LDH in the mitochondrial matrix is not compatible with the cytoplasmic/matrix redox gradient; its presence would drain matrix reducing power and substantially dissipate the proton motive force. OXPHOS requires O2 as the final electron acceptor, but O2 supply is sufficient in most situations, including exercise and often acute illness. Recent studies suggest that atmospheric normoxia may constitute a cellular hyperoxia in mitochondrial disease. As research proceeds appropriate oxygenation levels should be carefully considered.

The tortuous path of lactate shuttle discovery: From cinders and boards to the lab and ICU

Once thought to be a waste product of oxygen limited (anaerobic) metabolism, lactate is now known to form continuously under fully oxygenated (aerobic) conditions. Lactate shuttling between producer (driver) and consumer cells fulfills at least 3 purposes; lactate is: (1) a major energy source, (2) the major gluconeogenic precursor, and (3) a signaling molecule. The Lactate Shuttle theory is applicable to diverse fields such as sports nutrition and hydration, resuscitation from acidosis and Dengue, treatment of traumatic brain injury, maintenance of glycemia, reduction of inflammation, cardiac support in heart failure and following a myocardial infarction, and to improve cognition. Yet, dysregulated lactate shuttling disrupts metabolic flexibility, and worse, supports oncogenesis. Lactate production in cancer (the Warburg effect) is involved in all main sequela for carcinogenesis: angiogenesis, immune escape, cell migration, metastasis, and self-sufficient metabolism. The history of the tortuous path of discovery in lactate metabolism and shuttling was discussed in the 2019 American College of Sports Medicine Joseph B. Wolffe Lecture in Orlando, FL.

Lactate as a fulcrum of metabolism

Mistakenly thought to be the consequence of oxygen lack in contracting skeletal muscle we now know that the L-enantiomer of the lactate anion is formed under fully aerobic conditions and is utilized continuously in diverse cells, tissues, organs and at the whole-body level. By shuttling between producer (driver) and consumer (recipient) cells lactate fulfills at least three purposes: 1] a major energy source for mitochondrial respiration; 2] the major gluconeogenic precursor; and 3] a signaling molecule. Working by mass action, cell redox regulation, allosteric binding, and reprogramming of chromatin by lactylation of lysine residues on histones, lactate has major influences in energy substrate partitioning. The physiological range of tissue [lactate] is 0.5–20 mM and the cellular Lactate/Pyruvate ratio (L/P) can range from 10 to >500; these changes during exercise and other stress-strain responses dwarf other metabolic signals in magnitude and span. Hence, lactate dynamics have rapid and major short- and long-term effects on cell redox and other control systems. By inhibiting lipolysis in adipose via HCAR-1, and muscle mitochondrial fatty acid uptake via malonyl-CoA and CPT1, lactate controls energy substrate partitioning. Repeated lactate exposure from regular exercise results in major effects on the expression of regulatory enzymes of glycolysis and mitochondrial respiration. Lactate is the fulcrum of metabolic regulation in vivo.

Lactate dehydrogenase supports lactate oxidation in mitochondria isolated from different mouse tissues

Research over the past seventy years has established that mitochondrial-l-lactate dehydrogenase (m-L-LDH) is vital for mitochondrial bioenergetics. However, in recent report, Fulghum et al. concluded that lactate is a poor fuel for mitochondrial respiration [1]. In the present study, we have followed up on these findings and conducted an independent investigation to determine if lactate can support mitochondrial bioenergetics. We demonstrate herein that lactate can fuel the bioenergetics of heart, muscle, and liver mitochondria. Lactate was just as effective as pyruvate at stimulating mitochondrial coupling efficiency. Inclusion of LDH (sodium oxamate or GSK 2837808A) and pyruvate dehydrogenase (PDH; CPI-613) inhibitors abolished respiration in mitochondria energized with lactate. Lactate also fueled mitochondrial ROS generation and was just as effective as pyruvate at stimulating H₂O₂ production. Additionally, lactate-induced ROS production was inhibited by both LDH and PDH inhibitors. Enzyme activity measurements conducted on permeabilized mitochondria revealed that LDH is localized in mitochondria. In aggregate, we can conclude that mitochondrial LDH fuels bioenergetics in several tissues by oxidizing lactate.

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Lactate can fuel mitochondrial respiration.

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Lactate serves as a substrate for H₂O₂ production.

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Mitochondria contain LDH.

The Strategic Location of Glycogen and Lactate: From Body Energy Reserve to Brain Plasticity

Brain energy metabolism has been the object of intense research in recent years. Pioneering work has identified the different cell types involved in energy production and use. Recent evidence has demonstrated a key role of L-Lactate in brain energy metabolism, producing a paradigm-shift in our understanding of the neuronal energy metabolism. At the center of this shift, is the identification of a central role of astrocytes in neuroenergetics. Thanks to their morphological characteristics, they are poised to take up glucose from the circulation and deliver energy substrates to neurons. Astrocyte neuron lactate shuttle (ANLS) model, has shown that the main energy substrate that astrocytes deliver to neurons is L-Lactate, to sustain neuronal oxidative metabolism. L-Lactate can also be produced from glycogen, the storage form of glucose, which is exclusively localized in astrocytes. Inhibition of glycogen metabolism and the ensuing inhibition of L-Lactate production leads to cognitive dysfunction. Experimental evidence indicates that the role of lactate in cognitive function relates not only to its role as a metabolic substrate for neurons but also as a signaling molecule for synaptic plasticity. Interestingly, a similar metabolic uncoupling appears to exist in peripheral tissues plasma, whereby glucose provides L-Lactate as the substrate for cellular oxidative metabolism. In this perspective article, we review the known information on the distribution of glycogen and lactate within brain cells, and how this distribution relates to the energy regime of glial vs. neuronal cells.

Transport-exclusion pharmacology to localize lactate dehydrogenase activity within cells

BACKGROUND

Recent in vitro and in vivo work has shown that lactate provides an important source of carbon for metabolic reactions in cancer cell mitochondria. An interesting question is whether lactate is oxidized by lactate dehydrogenase (LDH) in the cytosol and/or in mitochondria. Since metabolic processes in the cytosol and mitochondria are affected by redox balance, the location of LDH may have important regulatory implications in cancer metabolism.

METHODS

Within most mammalian cells, metabolic processes are physically separated by membrane-bound compartments. Our general understanding of this spatial organization and its role in cellular function, however, suffers from the limited number of techniques to localize enzymatic activities within a cell. Here, we describe an approach to assess metabolic compartmentalization by monitoring the activity of pharmacological inhibitors that cannot be transported into specific cellular compartments.

RESULTS

Oxamate, which chemically resembles pyruvate, is transported into mitochondria and inhibits LDH activity in purified mitochondria. GSK-2837808A, in contrast, is a competitive inhibitor of NAD, which cannot cross the inner mitochondrial membrane. GSK-2837808A did not inhibit the LDH activity of intact mitochondria, but GSK-2837808A did inhibit LDH activity after the inner mitochondrial membrane was disrupted.

CONCLUSIONS

Our results are consistent with some mitochondrial LDH that is accessible to oxamate, but inaccessible to GSK-2837808A until mitochondria are homogenized. This strategy of using inhibitors with selective access to subcellular compartments, which we refer to as transport-exclusion pharmacology, is broadly applicable to localize other metabolic reactions within cells.

The Science and Translation of Lactate Shuttle Theory

Once thought to be a waste product of anaerobic metabolism, lactate is now known to form continuously under aerobic conditions. Shuttling between producer and consumer cells fulfills at least three purposes for lactate: (1) a major energy source, (2) the major gluconeogenic precursor, and (3) a signaling molecule. "Lactate shuttle" (LS) concepts describe the roles of lactate in delivery of oxidative and gluconeogenic substrates as well as in cell signaling. In medicine, it has long been recognized that the elevation of blood lactate correlates with illness or injury severity. However, with lactate shuttle theory in mind, some clinicians are now appreciating lactatemia as a "strain" and not a "stress" biomarker. In fact, clinical studies are utilizing lactate to treat pro-inflammatory conditions and to deliver optimal fuel for working muscles in sports medicine. The above, as well as historic and recent studies of lactate metabolism and shuttling, are discussed in the following review.

Divergent lactate dehydrogenase isoenzyme profile in cellular compartments of primate forebrain structures

The compartmentalization and association of lactate dehydrogenase (LDH) with specific cellular structures (e.g., synaptosomal, sarcoplasmic or mitochondrial) may play an important role in brain energy metabolism. Our previous research revealed that LDH in the synaptosomal fraction shifts toward the aerobic isoforms (LDH-B) among the large-brained haplorhine primates compared to strepsirrhines. Here, we further analyzed the subcellular localization of LDH in primate forebrain structures using quantitative Western blotting and ELISA. We show that, in cytosolic and mitochondrial subfractions, LDH-B expression level was relatively elevated and LDH-A declined in haplorhines compared to strepsirrhines. LDH-B expression in mitochondrial fractions of the neocortex was preferentially increased, showing a particularly significant rise in the ratio of LDH-B to LDH-A in chimpanzees and humans. We also found a significant correlation between the protein levels of LDH-B in mitochondrial fractions from haplorhine neocortex and the synaptosomal LDH-B that suggests LDH isoforms shift from a predominance of A-subunits toward B-subunits as part of a system that spatially buffers dynamic energy requirements of brain cells. Our results indicate that there is differential subcellular compartmentalization of LDH isoenzymes that evolved among different primate lineages to meet the energy requirements in neocortical and striatal cells.

The occurrence of l-lactate dehydrogenase in the inner mitochondrial compartment of pig liver

Although pig represents a model species in biomedical research including studies dealing with liver patho-physiology, some aspects of liver metabolism need to be addressed. In particular, whether and how pig mitochondria can metabolize l-lactate remains to be established. We show here that pig liver mitochondria (PLM) possess their own l-lactate dehydrogenase (mL-LDH). This was shown both via immunological analysis and by assaying photometrically the L-LDH reaction in solubilised PLM. The mL-LDH reaction shows hyperbolic dependence on the substrate concentration, it is inhibited by oxamate and proves to differ from the cytosolic activity (cL-LDH), as revealed by the difference found in both pH profiles and temperature dependence of m- and cL-LDH. Titration experiments with digitonin show that mL-LDH is restricted in mitochondrial inner compartment. In agreement with the above findings, three genes in Sus scrofa genome encoded for L-LDH subunits which are predicted to have mitochondrial localization, as investigated by Target P 1.1 and PredSL analysis.

Lactate oxidation at the mitochondria: a lactate-malate-aspartate shuttle at work

Lactate, the conjugate base of lactic acid occurring in aqueous biological fluids, has been derided as a “dead-end” waste product of anaerobic metabolism. Catalyzed by the near-equilibrium enzyme lactate dehydrogenase (LDH), the reduction of pyruvate to lactate is thought to serve to regenerate the NAD⁺ necessary for continued glycolytic flux. Reaction kinetics for LDH imply that lactate oxidation is rarely favored in the tissues of its own production. However, a substantial body of research directly contradicts any notion that LDH invariably operates unidirectionally in vivo. In the current Perspective, a model is forwarded in which the continuous formation and oxidation of lactate serves as a mitochondrial electron shuttle, whereby lactate generated in the cytosol of the cell is oxidized at the mitochondria of the same cell. From this perspective, an intracellular lactate shuttle operates much like the malate-aspartate shuttle (MAS); it is also proposed that the two shuttles are necessarily interconnected in a lactate-MAS. Among the requisite features of such a model, significant compartmentalization of LDH, much like the creatine kinase of the phosphocreatine shuttle, would facilitate net cellular lactate oxidation in a variety of cell types.

Synaptosomal lactate dehydrogenase isoenzyme composition is shifted toward aerobic forms in primate brain evolution

With the evolution of a relatively large brain size in haplorhine primates (i.e. tarsiers, monkeys, apes, and humans), there have been associated changes in the molecular machinery that delivers energy to the neocortex. Here we investigated variation in lactate dehydrogenase (LDH) expression and isoenzyme composition of the neocortex and striatum in primates using quantitative Western blotting and isoenzyme analysis of total homogenates and synaptosomal fractions. Analysis of isoform expression revealed that LDH in synaptosomal fractions from both forebrain regions shifted towards a predominance of the heart-type, aerobic isoform LDH-B among haplorhines as compared to strepsirrhines (i.e. lorises and lemurs), while in the total homogenate of the neocortex and striatum there was no significant difference in LDH isoenzyme composition between the primate suborders. The largest increase occurred in synapse-associated LDH-B expression in the neocortex, with an especially remarkable elevation in the ratio of LDH-B/LDH-A in humans. The phylogenetic variation in the ratio of LDH-B/LDH-A was correlated with species-typical brain mass but not the encephalization quotient. A significant LDH-B increase in the subneuronal fraction from haplorhine neocortex and striatum suggests a relatively higher rate of aerobic glycolysis that is linked to synaptosomal mitochondrial metabolism. Our results indicate that there is a differential composition of LDH isoenzymes and metabolism in synaptic terminals that evolved in primates to meet increased energy requirements in association with brain enlargement.

Lactate kinetics at the lactate threshold in trained and untrained men

To understand the meaning of the lactate threshold (LT) and to test the hypothesis that endurance training augments lactate kinetics [i.e., rates of appearance and disposal (Ra and Rd, respectively, mg·kg−1·min−1) and metabolic clearance rate (MCR, ml·kg−1·min−1)], we studied six untrained (UT) and six trained (T) subjects during 60-min exercise bouts at power outputs (PO) eliciting the LT. Trained subjects performed two additional exercise bouts at a PO 10% lower (LT-10%), one of which involved a lactate clamp (LC) to match blood lactate concentration ([lactate]b) to that achieved during the LT trial. At LT, lactate Ra was higher in T (24.1 ± 2.7) than in UT (14.6 ± 2.4; P < 0.05) subjects, but Ra was not different between UT and T when relative exercise intensities were matched (UT-LT vs. T-LT-10%, 67% V̇o2max). At LT, MCR in T (62.5 ± 5.0) subjects was 34% higher than in UT (46.5 ± 7.0; P < 0.05), and a reduction in PO resulted in a significant increase in MCR by 46% (LT-10%, 91.5 ± 14.9, P < 0.05). At matched relative exercise intensities (67% V̇o2max), MCR in T subjects was 97% higher than in UT ( P < 0.05). During the LC trial, MCR in T subjects was 64% higher than in UT ( P < 0.05), in whom %V̇o2max and [lactate]b were similar. We conclude that 1) lactate MCR reaches an apex below the LT, 2) LT corresponds to a limitation in MCR, and 3) endurance training augments capacities for lactate production, disposal and clearance.

Lactate oxidation in human skeletal muscle mitochondria

Lactate is an important intermediate metabolite in human bioenergetics and is oxidized in many different tissues including the heart, brain, kidney, adipose tissue, liver, and skeletal muscle. The mechanism(s) explaining the metabolism of lactate in these tissues, however, remains unclear. Here, we analyze the ability of skeletal muscle to respire lactate by using an in situ mitochondrial preparation that leaves the native tubular reticulum and subcellular interactions of the organelle unaltered. Skeletal muscle biopsies were obtained from vastus lateralis muscle in 16 human subjects. Samples were chemically permeabilized with saponin, which selectively perforates the sarcolemma and facilitates the loss of cytosolic content without altering mitochondrial membranes, structure, and subcellular interactions. High-resolution respirometry was performed on permeabilized muscle biopsy preparations. By use of four separate and specific substrate titration protocols, the respirometric analysis revealed that mitochondria were capable of oxidizing lactate in the absence of exogenous LDH. The titration of lactate and NAD(+) into the respiration medium stimulated respiration (P ≤ 0.003). The addition of exogenous LDH failed to increase lactate-stimulated respiration (P = 1.0). The results further demonstrate that human skeletal muscle mitochondria cannot directly oxidize lactate within the mitochondrial matrix. Alternately, these data support previous claims that lactate is converted to pyruvate within the mitochondrial intermembrane space with the pyruvate subsequently taken into the mitochondrial matrix where it enters the TCA cycle and is ultimately oxidized.

D-Lactate altered mitochondrial energy production in rat brain and heart but not liver

Background

Substantially elevated blood D-lactate (DLA) concentrations are associated with neurocardiac toxicity in humans and animals. The neurological symptoms are similar to inherited or acquired abnormalities of pyruvate metabolism. We hypothesized that DLA interferes with mitochondrial utilization of L-lactate and pyruvate in brain and heart.

Methods

Respiration rates in rat brain, heart and liver mitochondria were measured using DLA, LLA and pyruvate independently and in combination.

Results

In brain mitochondria, state 3 respiration was 53% and 75% lower with DLA as substrate when compared with LLA and pyruvate, respectively (p < 0.05). Similarly in heart mitochondria, state 3 respiration was 39% and 86% lower with DLA as substrate when compared with LLA or pyruvate, respectively (p < 0.05). However, state 3 respiration rates were similar between DLA, LLA and pyruvate in liver mitochondria. Combined incubation of DLA with LLA or pyruvate markedly impaired state 3 respiration rates in brain and heart mitochondria (p < 0.05) but not in liver mitochondria. DLA dehydrogenase activities were 61% and 51% lower in brain and heart mitochondria compared to liver, respectively, whereas LLA dehydrogenase activities were similar across all three tissues. An LDH inhibitor blocked state 3 respiration with LLA as substrate in all three tissues. A monocarboxylate transporter inhibitor blocked respiration with all three substrates.

Conclusions

DLA was a poor respiratory substrate in brain and heart mitochondria and inhibited LLA and pyruvate usage in these tissues. Further studies are warranted to evaluate whether these findings support, in part, the possible neurological and cardiac toxicity caused by high DLA levels.

Cell-cell and intracellular lactate shuttles

Once thought to be the consequence of oxygen lack in contracting skeletal muscle, the glycolytic product lactate is formed and utilized continuously in diverse cells under fully aerobic conditions. 'Cell-cell' and 'intracellular lactate shuttle' concepts describe the roles of lactate in delivery of oxidative and gluconeogenic substrates as well as in cell signalling. Examples of the cell-cell shuttles include lactate exchanges between between white-glycolytic and red-oxidative fibres within a working muscle bed, and between working skeletal muscle and heart, brain, liver and kidneys. Examples of intracellular lactate shuttles include lactate uptake by mitochondria and pyruvate for lactate exchange in peroxisomes. Lactate for pyruvate exchanges affect cell redox state, and by itself lactate is a ROS generator. In vivo, lactate is a preferred substrate and high blood lactate levels down-regulate the use of glucose and free fatty acids (FFA). As well, lactate binding may affect metabolic regulation, for instance binding to G-protein receptors in adipocytes inhibiting lipolysis, and thus decreasing plasma FFA availability. In vitro lactate accumulation upregulates expression of MCT1 and genes coding for other components of the mitochondrial reticulum in skeletal muscle. The mitochondrial reticulum in muscle and mitochondrial networks in other aerobic tissues function to establish concentration and proton gradients necessary for cells with high mitochondrial densities to oxidize lactate. The presence of lactate shuttles gives rise to the realization that glycolytic and oxidative pathways should be viewed as linked, as opposed to alternative, processes, because lactate, the product of one pathway, is the substrate for the other.

200th anniversary of lactate research in muscle

This year, 2008, marks the bicentennial of research into lactate metabolism in muscle. Berzelius linked lactate accumulation to exercise in 1807/1808 when he noted the presence of lactate in the muscles of "hunted stags." Today, the exact mechanism of intramuscular lactate oxidation and the relationship of lactate dehydrogenase to mitochondria remain unresolved as animated debate surrounds the intracellular lactate shuttle.

A lactatic perspective on metabolism

The cell-to-cell lactate shuttle was introduced in 1984 and has been repeatedly supported by studies using a variety of experimental approaches. Because of its large mass and metabolic capacity, skeletal muscle is probably the major component of the lactate shuttle in terms of both production and consumption. Muscles exercising in a steady state are avid consumers of lactate, using most of the lactate as an oxidative fuel. Cardiac muscle is highly oxidative and readily uses lactate as a fuel. Lactate is a major gluconeogenic substrate for the liver; the use of lactate to form glucose increases when blood lactate concentration is elevated. Illustrative of the widespread shuttling of lactate, even the brain takes up lactate when the blood level is increased. Recently, an intracellular lactate shuttle has also been proposed. Although disagreements abound, current evidence suggests that lactate is the primary end-product of glycolysis at cellular sites remote from mitochondria. This lactate could subsequently diffuse to areas adjacent to mitochondria. Evidence is against lactate oxidation within the mitochondrial matrix, but a viable hypothesis is that lactate could be converted to pyruvate by a lactate oxidation complex with lactate dehydrogenase located on the outer surface of the inner mitochondrial membrane. In another controversial area, the role of lactic acid in acid-base balance has been hotly debated in recent times. Careful analysis reveals that lactate, not lactic acid, is the substrate/product of metabolic reactions. One view is that lactate formation alleviates acidosis, whereas another is that lactate is a causative factor in acidosis. Surprisingly, there is little direct mechanistic evidence regarding cause and effect in acid-base balance. However, there is insufficient evidence to discard the term "lactic acidosis."

Highly Potent and Selective Substrate Analogue Factor Xa Inhibitors ContainingD-Homophenylalanine Analogues as P3 Residue: Part 2

A series of highly potent substrate-analogue factor Xa inhibitors containing D-homophenylalanine analogues as the P3 residue has been identified by systematic optimization of a previously described inhibitor structure. An initial lead, benzylsulfonyl-D-hPhe-Gly-4-amidinobenzylamide (3), inhibits fXa with an inhibition constant of 6.0 nM. Most modifications of the P2 amino acid and P4 benzylsulfonyl group did not improve the affinity and selectivity of the compounds as fXa inhibitors. In contrast, further variation at the P3 position led to inhibitors with significantly enhanced potency and selectivity. Inhibitor 27, benzylsulfonyl-D-homo-2-pyridylalanyl(N-oxide)-Gly-4-amidinobenzylamide, inhibits fXa with a K(i) value of 0.32 nM. The inhibitor has strong anticoagulant activity in plasma and doubles the activated partial thromboplastin time and prothrombin time at concentrations of 280 nM and 170 nM, respectively. Compound 27 inhibits the prothrombinase complex with an IC(50) value of 5 nM and is approximately 50 times more potent than the reference inhibitor DX-9065a in this assay.

Negligible direct lactate oxidation in subsarcolemmal and intermyofibrillar mitochondria obtained from red and white rat skeletal muscle

We examined the controversial notion of whether lactate is directly oxidized by subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria obtained from red and white rat skeletal muscle. Respiratory control ratios were normal in SS and IMF mitochondria. At all concentrations (0.18-10 mm), and in all mitochondria, pyruvate oxidation greatly exceeded lactate oxidation, by 31- to 186-fold. Pyruvate and lactate oxidation were inhibited by alpha-cyano-4-hydroxycinnamate, while lactate oxidation was inhibited by oxamate. Excess pyruvate (10 mm) inhibited the oxidation of palmitate (1.8 mm) as well as lactate (1.8 mm). In contrast, excess lactate (10 mm) failed to inhibit the oxidation of either palmitate (1.8 mm) or pyruvate (1.8 mm). The cell-permeant adenosine analogue, AICAR, increased pyruvate oxidation; in contrast, lactate oxidation was not altered. The monocarboxylate transporters MCT1 and 4 were present on SS mitochondria, but not on IMF mitochondria, whereas, MCT2, a high-affinity pyruvate transporter, was present in both SS and IMF mitochondria. The lactate dehydrogenase (LDH) activity associated with SS and IMF mitochondria was 200- to 240-fold lower than in whole muscle. Addition of LDH increased the rate of lactate oxidation, but not pyruvate oxidation, in a dose-dependent manner, such that lactate oxidation approached the rates of pyruvate oxidation. Collectively, these studies indicate that direct mitochondrial oxidation of lactate (i.e. an intracellular lactate shuttle) does not occur within the matrix in either IMF or SS mitochondria obtained from red or white rat skeletal muscle, because of the very limited quantity of LDH within mitochondria.

Diacetyldiamidoximeester of pentamidine, a prodrug for treatment of protozoal diseases: synthesis, in vitro and in vivo biotransformation

Pentamidine is an effective antimicrobial agent. To increase its poor oral bioavailability due to the strong basic amidine functionality, the less basic O-acetylamidoxime prodrug, the diacetyldiamidoximeester, was used, which has greatly improved lipophilicity. The objectives of this investigation were the synthesis of all potential metabolites of the double prodrug, the conformational analysis of its structure, and to study the in vitro and in vivo biotransformation by ester cleavage and N-reduction to pentamidine via four intermediate metabolites. The biotransformation of diacetyldiamidoximeester to pentamidine involving the reduction of the amidoxime function and the ester cleavage could be demonstrated. The kinetic parameters were determined. Amidoximes were efficiently metabolized by several enzyme systems located in microsomes and mitochondria of different organs including the final formation of the active metabolite pentamidine. The formation of pentamidine after oral administration of the diacetyldiamidoximeester to rats could be demonstrated as well.

Ultracentrifugation studies of the location of the site involved in the interaction of pig heart lactate dehydrogenase with acidic phospholipids at low pH. A comparison with the muscle form of the enzyme

Lactate dehydrogenase (LDH) from the pig heart interacts with liposomes made of acidic phospholipids most effectively at low pH, close to the isoelectric point of the protein (pH = 5.5). This binding is not observed at neutral pH or high ionic strength. LDH-liposome complex formation requires an absence of nicotinamide adenine dinucleotides and adenine nucleotides in the interaction environment. Their presence limits the interaction of LDH with liposomes in a concentration-dependent manner. This phenomenon is not observed for pig skeletal muscle LDH. The heart LDH-liposome complexes formed in the absence of nicotinamide adenine dinucleotides and adenine nucleotides are stable after the addition of these substances even in millimolar concentrations. The LDH substrates and studied nucleotides that inhibit the interaction of pig heart LDH with acidic liposomes can be ordered according to their effectiveness as follows: NADH > NAD > ATP = ADP > AMP > pyruvate. The phosphorylated form of NAD (NADP), nonadenine nucleotides (GTP, CTP, UTP) and lactate are ineffective. Chemically cross-linked pig heart LDH, with a tetrameric structure stable at low pH, behaves analogously to the unmodified enzyme, which excludes the participation of the interfacing parts of subunits in the interaction with acidic phospholipids. The presented results indicate that in lowered pH conditions, the NADH-cofactor binding site of pig heart LDH is strongly involved in the interaction of the enzyme with acidic phospholipids. The contribution of the ATP/ADP binding site to this process can also be considered. In the case of pig skeletal muscle LDH, neither the cofactor binding site nor the subunit interfacing areas seem to be involved in the interaction.

Colocalization of MCT1, CD147, and LDH in mitochondrial inner membrane of L6 muscle cells: evidence of a mitochondrial lactate oxidation complex

Results of previous studies suggested a role of mitochondria in intracellular and cell-cell lactate shuttles. Therefore, by using a rat-derived L6 skeletal muscle cell line and confocal laser-scanning microscopy (CLSM), we examined the cellular locations of mitochondria, lactate dehydrogenase (LDH), the lactate-pyruvate transporter MCT1, and CD147, a purported chaperone protein for MCT1. CLSM showed that LDH, MCT1, and CD147 are colocalized with the mitochondrial reticulum. Western blots showed that cytochrome oxidase (COX), NADH dehydrogenase, LDH, MCT1, and CD147 are abundant in mitochondrial fractions of L6 cells. Interactions among COX, MCT1, and CD147 in mitochondria were confirmed by immunoblotting after immunoprecipitation. These findings support the presence of a mitochondrial lactate oxidation complex associated with the COX end of the electron transport chain that might explain the oxidative catabolism of lactate and, hence, mechanism of the intracellular lactate shuttle.

Oxygen-insensitive enzymatic reduction of oximes to imines

The reduction of oximes to imines under anaerobic and aerobic conditions was studied using (E)- and (Z)-2,4,6-trimethylacetophenone oxime, benzaldoxime and (E)-2,4,6-trimethylbenzaldoxime. Pig and human liver microsomes, pig liver mitochondria and cytosol to a minor extent catalyzed the conversion of both isomeric ketoximes to the corresponding stable imine, the (E)-isomer being the better substrate. All reactions were oxygen-insensitive and required active protein and NADH or NADPH; however, NADH was preferred as cofactor. The reconstituted liver microsomal system of a pig liver CYP2D enzyme (NADH-benzamidoxime reductase), which is known to reduce N-hydroxylated derivatives of strongly basic functional groups, such as amidoximes, is also capable of reducing oximes. As expected, the corresponding imine was detected in relevant amounts when incubating 2,4,6-trimethyl-acetophenone oxime using the reconstituted enzyme system, but reduction rates were significantly lower compared to rates obtained when incubating benzamidoxime. Steric hindrance due to the methyl groups in ortho position to the oxime functionality could be excluded as being responsible for the lower conversion rates according to results obtained in incubations of 2,4,6-trimethylbenzamidoxime. When incubating benzaldoxime, only benzoic acid could be detected as metabolite, since the aldehyde is easily oxidized during incubation procedures, whereas incubations of (E)-2,4,6-trimethylbenzaldoxime also showed the formation of the corresponding aldehyde. These results allow us to postulate that the metabolism of aldoximes like 2,4,6-trimethylbenzaldoxime most likely proceeds through enzymatic reduction of the oxime to yield the intermediate imine, which is subsequently hydrolyzed to the aldehyde and then oxidized to the corresponding benzoic acid.

Lactate: the ultimate cerebral oxidative energy substrate?

Research over the past two decades has renewed the interest in lactate, no longer as a useless end product of anaerobic glycolysis in brain (and other tissues), but as an oxidative substrate for energy metabolism. While this topic would be considered blasphemy only three decades ago, much recent evidence indicates that lactate does play a major role in aerobic energy metabolism in the brain, the heart, skeletal muscle, and possibly in any other tissue and organ. Nevertheless, this concept has challenged the old dogma and ignited a fierce debate, especially among neuroscientists, pitting the supporters of glucose as the major oxidative energy substrate against those who support lactate as a possible alternative to glucose under certain conditions. Meanwhile, researchers working on energy metabolism in skeletal muscle have taken great strides toward bridging between these two extreme positions, while avoiding the high decibels of an emotional debate. Employing their findings along with the existing old and new data on cerebral energy metabolism, it is postulated here that lactate is the only major product of cerebral (and other tissues) glycolysis, whether aerobic or anaerobic, neuronal or astrocytic, under rest or during activation. Consequently, this postulate entails that lactate is a major, if not the only, substrate for the mitochondrial tricarboxylic acid cycle. If proven true, this hypothesis could provide better understanding of the biochemistry and physiology of (cerebral) energy metabolism, while holding important implications in the field of neuroimaging. Concomitantly, it could satisfy both 'glucoseniks' and 'lactatians' in the ongoing debate.

Partial reconstruction of in vitro gluconeogenesis arising from mitochondrial l-lactate uptake/metabolism and oxaloacetate export via novel L-lactate translocators

In the light of the occurrence of L-lactate dehydrogenase inside the mitochondrial matrix, we looked at whether isolated rat liver mitochondria can take up and metabolize L-lactate, and provide oxaloacetate outside mitochondria, thus contributing to a partial reconstruction of gluconeogenesis in vitro. We found that: (1) L-lactate (10 mM), added to mitochondria in the presence of a cocktail of glycolysis/gluconeogenesis enzymes and cofactors, can lead to synthesis of glyceraldehyde-3-phosphate at a rate of about 7 nmol/min per mg mitochondrial protein. (2) Three novel translocators exist to mediate L-lactate traffic across the inner mitochondrial membrane. An L-lactate/H+ symporter was identified by measuring fluorimetrically the rate of endogenous pyridine nucleotide reduction. Consistently, L-lactate oxidation was found to occur with P/O ratio=3 (where P/O ratio is the ratio of mol of ATP synthesized to mol of oxygen atoms reduced to water during oxidative phosphorylation) and with generation of membrane potential. Proton uptake, which occurred as a result of addition of L-lactate to RLM together with electron flow inhibitors, and mitochondrial swelling in ammonium L-lactate solutions were also monitored. L-Lactate/oxaloacetate and L-lactate/pyruvate anti-porters were identified by monitoring photometrically the appearance of L-lactate counter-anions outside mitochondria. These L-lactate translocators, which are distinct from the monocarboxylate carrier, were found to differ from each other in V(max) values and in inhibition and pH profiles, and proved to regulate mitochondrial L-lactate metabolism in vitro. The role of lactate/mitochondria interactions in gluconeogenesis is discussed.

Lactate metabolism: a new paradigm for the third millennium

For much of the 20th century, lactate was largely considered a dead-end waste product of glycolysis due to hypoxia, the primary cause of the O2 debt following exercise, a major cause of muscle fatigue, and a key factor in acidosis-induced tissue damage. Since the 1970s, a 'lactate revolution' has occurred. At present, we are in the midst of a lactate shuttle era; the lactate paradigm has shifted. It now appears that increased lactate production and concentration as a result of anoxia or dysoxia are often the exception rather than the rule. Lactic acidosis is being re-evaluated as a factor in muscle fatigue. Lactate is an important intermediate in the process of wound repair and regeneration. The origin of elevated [lactate] in injury and sepsis is being re-investigated. There is essentially unanimous experimental support for a cell-to-cell lactate shuttle, along with mounting evidence for astrocyte-neuron, lactate-alanine, peroxisomal and spermatogenic lactate shuttles. The bulk of the evidence suggests that lactate is an important intermediary in numerous metabolic processes, a particularly mobile fuel for aerobic metabolism, and perhaps a mediator of redox state among various compartments both within and between cells. Lactate can no longer be considered the usual suspect for metabolic 'crimes', but is instead a central player in cellular, regional and whole body metabolism. Overall, the cell-to-cell lactate shuttle has expanded far beyond its initial conception as an explanation for lactate metabolism during muscle contractions and exercise to now subsume all of the other shuttles as a grand description of the role(s) of lactate in numerous metabolic processes and pathways.

Metabolic response to exercise

At the beginning, the survival of humans was strictly related to their physical capacity. There was the need to resist predators and to provide food and water for life. Achieving these goals required a prompt and efficient energy system capable of sustaining either high intensity or maintaining prolonged physical activity. Energy for skeletal muscle contraction is supplied by anaerobic and aerobic metabolic pathways. The former can allow short bursts of intense physical activity (60-90 sec) and utilizes as energetic source the phosphocreatine shuttle and anaerobic glycolysis. The aerobic system is the most efficient ATP source for skeletal muscle. The oxidative phosporylation of carbohydrates, fats and, to a minor extent, proteins, can sustain physical activity for many hours. Carbohydrates are the most efficient fuel for working muscle and their contribution to total fuel oxidation is positively related to the intensity of exercise. The first metabolic pathways of carbohydrate metabolism to be involved are skeletal muscle glycogenolysis and glycolysis. Later circulating glucose, formed through activated gluconeogenesis, becomes an important energetic source. Among glucose metabolites, lactate plays a primary role as either direct or indirect (gluconeogenesis) energy source for contracting skeletal muscle. Fat oxidation plays a primary role during either low-moderate intensity exercise or protracted physical activity (over 90-120 min). Severe muscle glycogen depletion results in increased rates of muscle proteolysis and branched chain amino acid oxidation. Endurance training ameliorates physical performance by improving cardiopulmonary efficiency and optimizing skeletal muscle supply and oxidation of substrates.

Characterization of in vitro biotransformation of new, orally active, direct thrombin inhibitor ximelagatran, an amidoxime and ester prodrug

N-Hydroxylated amidines (amidoximes) can be used as prodrugs of amidines. The prodrug principle was developed in our laboratory for pentamidine and had been applied to several other drug candidates. One of these compounds is melagatran, a novel, synthetic, low molecular weight, direct thrombin inhibitor. To increase the poor oral bioavailability due to its strong basic amidine functionality selected to fit the arginine side pocket of thrombin, the less basic N-hydroxylated amidine was used in addition to an ethyl ester-protecting residue. The objective of this investigation was to study the reduction and the hydrolytic metabolism of ximelagatran via two mono-prodrugs (N-hydroxy-melagatran and ethyl-melagatran) to melagatran by in vitro experiments. New high-performance liquid chromatography methods were developed to analyze all four compounds. The biotransformation of ximelagatran to melagatran involving the reduction of the amidoxime function and the ester cleavage could be demonstrated in vitro by microsomes and mitochondria from liver and kidney of pig and human, and the kinetic parameters were determined. So far, one enzyme system capable of reducing N-hydroxylated structures has been identified in pig liver microsomes, consisting of cytochrome b(5), NADH-cytochrome b(5) reductase, and a P450 isoenzyme of the subfamily 2D. This enzyme system also reduces ximelagatran and N-hydroxy-melagatran. The participation of recombinant human CYP1A2, 2A6, 2C8, 2C9, 2C19, 2D6, and 3A4 with cytochrome b(5) and b(5) reductase in the reduction can be excluded. In summary, ximelagatran and N-hydroxy-melagatran are easily reduced by several enzyme systems located in microsomes and mitochondria of different organs.

The role of mitochondrial transport in energy metabolism

Since mitochondria are closed spaces in the cell, metabolite traffic across the mitochondrial membrane is needed to accomplish energy metabolism. The mitochondrial carriers play this function by uniport, symport and antiport processes. We give here a survey of about 50 transport processes catalysed by more than 30 carriers with a survey of the methods used to investigate metabolite transport in isolated mammalian mitochondria. The role of mitochondria in metabolic pathways including ammoniogenesis, amino acid metabolism, mitochondrial shuttles etc. is also reported in more detail, mainly in the light of the existence of new transport processes.

Reduction of N-hydroxylated compounds: amidoximes (N-hydroxyamidines) as pro-drugs of amidines

In order to examine the importance of metabolic cycles and in particular of reductions of N-hydroxylated compounds, the reversible metabolism at the amidine, guanidine, and amidinohydrazone nitrogen atoms of various drugs and model compounds was investigated. Many of these N-oxygenated metabolites are very easily reduced back into the starting materials. A comparison of the kinetic data for the N-hydroxylation and reduction suggests that the reduction should predominate in vivo. This could be verified by in vivo studies. Thus, N-hydroxylated amidines (amidoximes) can be used as pro-drugs of amidines. Because of their strong basicity, amidines, guanidines, and amidinohydrazones are protonated under physiological conditions, are very hydrophilic, and are usually not absorbed from the gastrointestinal tract. The N-hydroxylated derivatives of amidines (amidoximes), guanidines (N-hydroxyamidines), and amidinohydrazones (N-hydroxyamidinohydrazones) are less basic because of the introduction of the oxygen atom. They are absorbed from the gastrointestinal tract and then reduced to the active amidines, guanidines, and amidinohydrazones. The pro-drug principle was originally developed in our laboratory for pentamidine and then applied to other amidines such as sibrafiban and melagatran (ximelagatran). The enzymatic basis of N-oxidative processes is very well understood, whereas reductions have been less extensively investigated. We purified an enzyme system from pig and human liver consisting of cytochrome b5, its reductase, and a P450 enzyme, which is involved in the reduction of the N-hydroxylated compounds. Similar activities were found in all species studied so far. Furthermore, comparable reductive reactions could also be demonstrated with microsomal fractions from organs other than liver. In addition, mitochondria are highly capable of performing the reductions of these N-hydroxylated compounds. Thus, several organs and cell organelles are involved in the reduction explaining the extensive reduction of the pro-drugs in vivo underlying the suitability of the concept for drug development.

D-Lactate transport and metabolism in rat liver mitochondria

In the present study we investigated whether isolated rat liver mitochondria can take up and metabolize D-lactate. We found the following: (1) externally added D-lactate causes oxygen uptake by mitochondria [P/O ratio (the ratio of mol of ATP synthesized to mol of oxygen atoms reduced to water during oxidative phosphorylation)=2] and membrane potential (Delta(psi)) generation in processes that are rotenone-insensitive, but inhibited by antimycin A and cyanide, and proton release from coupled mitochondria inhibited by alpha-cyanocinnamate, but not by phenylsuccinate; (2) the activity of the putative flavoprotein (D-lactate dehydrogenase) was detected in inside-out submitochondrial particles, but not in mitochondria and mitoplasts, as it is localized in the matrix phase of the mitochondrial inner membrane; (3) three novel separate translocators exist to mediate D-lactate traffic across the mitochondrial inner membrane: the D-lactate/H(+) symporter, which was investigated by measuring fluorimetrically the rate of endogenous flavin reduction, the D-lactate/oxoacid antiporter (which mediates both the D-lactate/pyruvate and D-lactate/oxaloacetate exchanges) and D-lactate/malate antiporter studied by monitoring photometrically the appearance of the D-lactate counteranions outside mitochondria. The D-lactate translocators, in the light of their different inhibition profiles separate from the monocarboxylate carrier, were found to differ from each other in the V(max) values and in the inhibition and pH profiles and were shown to regulate mitochondrial D-lactate metabolism in vitro. The D-lactate translocators and the D-lactate dehydrogenase could account for the removal of the toxic methylglyoxal from cytosol, as well as for D-lactate-dependent gluconeogenesis.

No evidence of an intracellular lactate shuttle in rat skeletal muscle

The concerted view is that cytosolic pyruvate is transferred into mitochondria and after oxidative decarboxylation further metabolized in the tricarboxylic acid cycle. Recently this view has been challenged. Based on experimental evidence from rat skeletal muscle it has been concluded that mitochondria predominantly oxidize lactate in vivo and that this constitutes part of an 'intracellular lactate shuttle'. This view appears to be gaining acceptance in the scientific community and due to its conceptual importance, confirmation by independent experiments is required. We have repeated the experiments in mitochondria isolated from rat soleus muscle. Contrary to the previously published findings we cannot find any mitochondrial respiration with lactate. Analysis of lactate dehydrogenase (LDH) by spectrophotometry demonstrated that the activity in the mitochondrial fraction was only 0.7 % of total activity. However, even when external LDH was added to mitochondria, there were no signs of respiration with lactate. In the presence of conditions where lactate is converted to pyruvate (external additions of both LDH and NAD(+)), mitochondrial oxygen consumption increased. Furthermore, we provide theoretical evidence that direct mitochondrial lactate oxidation is energetically unlikely. Based on the present data we conclude that direct mitochondrial lactate oxidation does not occur in skeletal muscle. The presence of an 'intracellular lactate shuttle' can therefore be questioned.

Intra- and extra-cellular lactate shuttles

The "lactate shuttle hypothesis" holds that lactate plays a key role in the distribution of carbohydrate potential energy that occurs among various tissue and cellular compartments such as between: cytosol and mitochondria, muscle and blood, blood and muscle, active and inactive muscles, white and red muscles, blood and heart, arterial blood and liver, liver and other tissues such as exercising muscle, intestine and portal blood, portal blood and liver, zones of the liver, and skin and blood. Studies on resting and exercising humans indicate that most lactate (75-80%) is disposed of through oxidation, with much of the remainder converted to glucose and glycogen. Lactate transport across cellular membranes occurs by means of facilitated exchange along pH and concentration gradients involving a family of lactate transport proteins, now called monocarboxylate transporters (MCTs). Current evidence is that muscle and other cell membrane lactate transporters are abundant with characteristics of high Km and Vmax. There appears to be long-term plasticity in the number of cell membrane transporters, but short-term regulation by allosteric modulation or phosphorylation is not known. In addition to cell membranes, mitochondria also contain monocarboxylate transporters (mMCT) and lactic dehydrogenase (mLDH). Therefore, mitochondrial monocarboxylate uptake and oxidation, rather than translocation of transporters to the cell surfaces, probably regulate lactate flux in vivo. Accordingly, the "lactate shuttle" hypothesis has been modified to include a new, intracellular component involving cytosolic to mitochondrial exchange. The intracellular lactate shuttle emphasizes the role of mitochondrial redox in the oxidation and disposal of lactate during exercise and other conditions.

Role of mitochondrial lactate dehydrogenase and lactate oxidation in the intracellular lactate shuttle

To evaluate the potential role of mitochondrial lactate dehydrogenase (LDH) in tissue lactate clearance and oxidation in vivo, isolated rat liver, cardiac, and skeletal muscle mitochondria were incubated with lactate, pyruvate, glutamate, and succinate. As well, alpha-cyano-4-hydroxycinnamate (CINN), a known monocarboxylate transport inhibitor, and oxamate, a known LDH inhibitor were used. Mitochondria readily oxidized pyruvate and lactate, with similar state 3 and 4 respiratory rates, respiratory control (state 3/state 4), and ADP/O ratios. With lactate or pyruvate as substrates, alpha-cyano-4-hydroxycinnamate blocked the respiratory response to added ADP, but the block was bypassed by addition of glutamate (complex I-linked) and succinate (complex II-linked) substrates. Oxamate increased pyruvate (approximately 10-40%), but blocked lactate oxidation. Gel electrophoresis and electron microscopy indicated LDH isoenzyme distribution patterns to display tissue specificity, but the LDH isoenzyme patterns in isolated mitochondria were distinct from those in surrounding cell compartments. In heart, LDH-1 (H4) was concentrated in mitochondria whereas LDH-5 (M4) was present in both mitochondria and surrounding cytosol and organelles. LDH-5 predominated in liver but was more abundant in mitochondria than elsewhere. Because lactate exceeds cytosolic pyruvate concentration by an order of magnitude, we conclude that lactate is the predominant monocarboxylate oxidized by mitochondria in vivo. Mammalian liver and striated muscle mitochondria can oxidize exogenous lactate because of an internal LDH pool that facilitates lactate oxidation.

Mammalian fuel utilization during sustained exercise

The 'crossover' and 'lactate shuttle' concepts of substrate utilization in humans during exercise are extended to describe metabolic responses on other mammalian species. The 'crossover concept' is that lipid plays a predominant role in sustaining efforts requiring half or less aerobic capacity (VO2max); however, greater relative efforts depend increasingly on blood glucose and muscle glycogen as substrates. Thus, as exercise intensity increases from mild to moderate and hard, fuel selection switches (crosses over) from lipid to carbohydrate dependence. Glycogen and glucose catabolic rates are best described as exponential functions of exercise intensity, but with a greater gain in slope of the glycogen than glucose response. In contrast, plasma free fatty acid flux is described as an inverted hyperbola with vertex at approximately 50% VO2max. Both endocrine and intra-cellular factors play critical roles in determining substrate balance during sustained exercise. Moreover, genotypic adaptation for aerobic capacity as well as phenotypic adaptations to short- and long-term chronic activity affect the balance of substrate utilization during exercise. The concept of a 'lactate shuttle' is that during hard exercise, as well as other conditions of accelerated glycolysis, glycolytic flux in muscle involves lactate formation regardless of the state of oxygenation. Further, according to the lactate shuttle concept, lactate represents a major means of distributing carbohydrate potential energy for oxidation and gluconeogenesis. In humans and other mammals, the formation, distribution and disposal of lactate (not pyruvate) represent key steps in the regulation of intermediary metabolism during sustained exercise.

L-lactate dehydrogenase A4- and A3B isoforms are bona fide peroxisomal enzymes in rat liver. Evidence for involvement in intraperoxisomal NADH reoxidation

The subcellular localization of l-lactate dehydrogenase (LDH) in rat hepatocytes has been studied by analytical subcellular fractionation combined with the immunodetection of LDH in isolated subcellular fractions and liver sections by immunoblotting and immunoelectron microscopy. The results clearly demonstrate the presence of LDH in the matrix of peroxisomes in addition to the cytosol. Both cytosolic and peroxisomal LDH subunits have the same molecular mass (35.0 kDa) and show comparable cross-reactivity with an anti-cytosolic LDH antibody. As revealed by activity staining or immunoblotting after isoelectric focussing, both intracellular compartments contain the same liver-specific LDH-isoforms (LDH-A4 > LDH-A3B) with the peroxisomes comprising relatively more LDH-A3B than the cytosol. Selective KCl extraction as well as resistance to proteinase K and immunoelectron microscopy revealed that at least 80% of the LDH activity measured in highly purified peroxisomal fractions is due to LDH as a bona fide peroxisomal matrix enzyme. In combination with the data of cell fractionation, this implies that at least 0.5% of the total LDH activity in hepatocytes is present in peroxisomes. Since no other enzymes of the glycolytic pathway (such as phosphoglucomutase, phosphoglucoisomerase, and glyceraldehyde-3-phosphate dehydrogenase) were found in highly purified peroxisomal fractions, it does not seem that LDH in peroxisomes participates in glycolysis. Instead, the marked elevation of LDH in peroxisomes of rats treated with the hypolipidemic drug bezafibrate, concomitantly to the induction of the peroxisomal beta-oxidation enzymes, strongly suggests that intraperoxisomal LDH may be involved in the reoxidation of NADH generated by the beta-oxidation pathway. The interaction of LDH and the peroxisomal palmitoyl-CoA beta-oxidation system could be verified in a modified beta-oxidation assay by adding increasing amounts of pyruvate to the standard assay mixture and recording the change of NADH production rates. A dose-dependent decrease of NADH produced was simulated with the lowest NADH value found at maximal LDH activity. The addition of oxamic acid, a specific inhibitor of LDH, to the system or inhibition of LDH by high pyruvate levels (up to 20 mm) restored the NADH values to control levels. A direct effect of pyruvate on palmitoyl-CoA oxidase and enoyl-CoA hydratase was excluded by measuring those enzymes individually in separate assays. An LDH-based shuttle across the peroxisomal membrane should provide an efficient system to regulate intraperoxisomal NAD+/NADH levels and maintain the flux of fatty acids through the peroxisomal beta-oxidation spiral.

Quantitative comparison between the gel-film and polyvinyl alcohol methods for dehydrogenase histochemistry reveals different intercellular distribution patterns of glucose-6-phosphate and lactate dehydrogenases in mouse liver

The precise histochemical localization and quantification of the activity of soluble dehydrogenases in unfixed cryostat sections requires the use of tissue protectants. In this study, two protectants, polyvinyl alcohol (PVA) and agarose gel, were compared for assaying the activity of lactate dehydrogenase (LDH) and glucose-6-phosphate dehydrogenase (G6PDH) in normal female mouse liver. Quantification of enzyme activity was determined cytophotometrically in periportal (PP), pericentral (PC) and midzonal (MZ) areas. No coloured reaction product was present in PVA media after the incubation period. In contrast, the agarose gels appeared to be highly coloured after incubation. As a consequence, sections incubated with gel media were less intensely stained than those incubated in PVA-containing media. The specific G6PDH reaction (test minus control) yielded approximately 75% less formazan in sections incubated by the agarose gel method than with the PVA method. Further, the amount of formazan deposits attributable to G6PDH activity was highest in the midzonal and pericentral zones of the liver lobule with PVA media, and Kupffer cells could be discriminated easily because of their high G6PDH activity. Significant zonal differences or Kupffer cells could not be observed when agarose gel films were used for the detection of G6PDH activity. The LDH localization patterns appeared to be more uniform after incubation with both methods: no significant differences in specific test minus control reactions were seen between PP, PC and MZ. However, less formazan production (33%) was detected in sections incubated with agarose gels when compared with those incubated with PVA media.(ABSTRACT TRUNCATED AT 250 WORDS)

The initial reaction velocities of lactate dehydrogenase in various cell types

The initial reaction velocities (vi) of lactate dehydrogenase in hepatocytes, cardiac muscle fibres, skeletal (gastrocnemius) muscle fibres, gastric parietal cells, ductal epithelial and acinar cells of the parotid gland, and oocytes were determined, by computer-assisted image analysis, in unfixed sections of these tissues incubated at 37 degrees C on substrate-containing agarose gel films. They were found to fit the equations vi = a1 zero A (equation 1) and vi-v = a2 zero A (equation 2) reported previously for mouse hepatocytes (Nakae & Stoward, 1993a, b), where v and zero A are, respectively, the gradients (or steady-state velocities) and the intercepts on the absorbance axis of the linear regression lines of the absorbance (A) of the final reaction product on incubation times between 1 and 3 min, and a1 and a2 are constants. Both equations 1 and 2 fitted the observed vi closely for mouse (a1 = 2.7, a2 = 2.2) and human (a1 = 3.0, a2 = 1.9) hepatocytes. However, equation 2 fitted the observed vi better than equation 1 for mouse cardiac muscle fibres (a1 = 1.5), skeletal muscle fibres (a2 = 1.2), gastric parietal cells (a2 = 1.7), acinar (a2 = 1.4) and striated ductal (a2 = 2.2) epithelial cells of the parotid gland, and oocytes (a2 = 1.6). The values of vi calculated from the two equations agreed with the observed vi to within about 11%. They ranged from 105 mumole hydrogen equivalents/cm3 cell/min units in hepatocytes to 24 units in parotid acinar cells, but for other cell types they were between 46 and 61 units. These are all considerably higher than values reported previously.

Evidence for an in vivo modification of mitochondrial proteins by coenzyme A

Following denaturation of mitochondrial proteins by sodium dodecyl sulfate, a [1-14C]pantothenic acid-derived radioactivity proved to be acid precipitable in the outer membrane, the intermembrane space, the inner membrane and in the matrix of rat liver mitochondria, where it had the highest specific radioactivity of 541 +/- 29 cpm/100 micrograms protein. This tightly and/or covalently bound protein radioactivity could be released by incubation in the presence of dithioerythreitol; it was identified as [14C]coenzyme A by its HPLC retention time, its absorption spectrum and its radioactivity. This acid-stable and thiol-labile coenzyme A-binding apparently refers to specific protein binding sites. With the purified, homogeneous mitochondrial matrix enzymes acetyl-CoA acetyltransferase (acetoacetyl-CoA thiolase) (EC 2.3.1.9, acetyl-CoA:acetyl-CoA C-acetyltransferase) and 3-oxoacyl-CoA thiolase (EC 2.3.1.16) coenzyme A was found exclusively, e.g., in the modified, partially-active forms A1 und A2 of acetyl-CoA acetyltransferase and not in the unmodified fully-active enzyme. Thus it is evident that this coenzyme A modification is transient. We suggest that coenzyme A-modification is a signal involved in the assembly or the degradation process of distinct mitochondrial matrix proteins.

L-lactate oxidation by skeletal muscle mitochondria

1. Mitochondria isolated from rat skeletal muscle possess lactate dehydrogenase which is involved in direct oxidation of L-lactate in the presence of external NAD. 2. L-lactate oxidation can be stimulated in a reversible manner by ADP. 3. Mitochondrial lactate oxidation is sensitive to oxamate-inhibitor of LDH, alpha-cyano-3-hydroxy-cinnamate-pyruvate translocase inhibitor and respiratory chain inhibitors (rotenone, antimycin A, KCN). 4. In the same conditions the mitochondria did not oxidize pyruvate in the absence of malate, whereas, oxidize pyruvate plus external NADH in an uncoupling manner.

Citric acid cycle as a "one-step" reaction

One of the most familiar metabolic pathways is the citric acid cycle. This reaction sequence is characterized by intermediates which can be detected in aqueous solution. The recent success in isolating an enzyme cluster with five cycle activities suggests that the cyclic sequence should find a counterpart in a highly organized enzyme system for effective catalysis. In the present article, a new way of looking at this complex reaction sequence is proposed utilizing the hydrophobicity of active sites to define the entrance or exit of substrates as a first or last reaction step. With this view, highly reactive intermediates can be formulated which are able to react in a quasi "one step" mechanism to stable end-products. It is possible to derive the known intermediates of the citric acid cycle by a transition from a proposed hydrophobic catalytic site into the aqueous phase. A variety of biochemical concepts in use ("energy-rich"-bonds, group transfer, substrate-channelling, transporters, etc.) can be understood to be derived from methodology commonly in use. The proposed reaction mechanism demands a specific organization of the proteins forming the enzymatic activity(ies). Isolated and purified enzymes should, in consequence, be seen as isolation products, differing from their functionally organized form in a living cell. In this paper, a new concept is proposed by which a variety of observations and concepts can be unified.