Metabolism and bound water in overwintering insects

The freezing-tolerant gall fly larva, Eurosta solidaginis, provides an excellent model system for the study of metabolic adaptation and metabolic control for low-temperature survival during overwintering. Low-temperature acclimation of the larvae results in dramatic alterations in metabolic flux producing a sequential synthesis of two cryoprotectants, glycerol at warmer temperatures followed by sorbitol when larvae are exposed to 5 degrees C. Regulation of metabolism in the larvae appears to exploit temperature change, temperature effects on enzyme kinetics, and temperature/modulator interactions with enzymes producing the alterations in metabolic flux leading to differential polyol synthesis. For instance, temperature/modulator effects on phospho-fructokinase appear to be the major factor halting carbon flow into glycerol synthesis at low temperatures and diverting flux instead into the pathway of sorbitol synthesis. Alterations in the cellular content of bound water and the metabolic pools of free versus bound soluble metabolites may also have important regulatory consequences for low-temperature metabolism. Bound water content of the larvae increases with low-temperature acclimation and is attributable to changes in water binding by both low-molecular-weight (polyols) and high-molecular-weight (proteins, glycogen) subcellular components. A restrictive effect of high bound water content may be one factor causing the strong depression of metabolic activity seen in the larvae as a result of extracellular freezing. In addition, bound water may have a more subtle effect in determining the relative pool sizes of bound versus free metabolites in the cell. 31P-NMR studies of whole larvae show that the content of free phosphorylated intermediates in the cell diminishes with decreasing temperatures despite a measured constancy in the total pool size of these intermediates. An increase in the content of bound metabolites with low temperature may restrict metabolism by limiting the availability of substrates and effectors of enzyme reactions.

Regulation of coenzyme utilization by bovine liver glutamate dehydrogenase: investigations using thionicotinamide analogues of NAD and NADP in a dual wavelength assay

1. The coenzyme preference of bovine liver glutamate dehydrogenase (GDH) was probed using dual wavelength spectroscopy and pairing the thionicotinamide analogues, S-NAD or S-NADP (which have absorbance maxima at 400 nm), with the natural coenzymes, NADP or NAD. 2. S-NAD and S-NADP were found to be good alternate substrates for GDH: the apparent Km's for the thioderivatives were similar to those of the corresponding natural coenzymes, the apparent Km's for glutamate were unaltered by the substitution of the thioderivatives, and the effects of inhibitors and activators on S-NAD or S-NADP kinetics were qualitatively the same as those found for NAD or NADP, respectively. 3. Dual wavelength assays paired NAD and S-NADP or S-NAD and NADP to study the simultaneous reduction of the two coenzymes. Conditions of increasing glutamate concentrations produced differential effects on the rates of the NAD vs NADP reactions, the result, with either nucleotide pair, promoting the NADP linked reaction. 4. Activators and inhibitors of the GDH reaction also showed differential effects upon the NAD vs NADP linked reaction rates in the dual wavelength assay. ADP and leucine, which activate both the NAD and the NADP linked reactions in single coenzyme assays, preferentially activate the NADP or S-NADP linked reactions in the dual nucleotide assays. GTP produced greater inhibition of the NAD or S-NAD linked reactions than of the NADP or S-NADP reactions while ATP inhibited NAD or S-NAD reactions and activated NADP or S-NADP reactions. The net effect of all metabolite modulators was to promote the NADP linked reaction by decreasing the activity ratios, v(NAD)/v(S-NADP) or v(S-NAD)/v(NADP). 5. The results are consistent with the suggestion that NADP is the preferred coenzyme for the oxidative deamination of glutamate by GDH even though the enzyme is capable of utilizing either coenzyme in vitro.

Hydrogen shuttles in gills of water versus air breathing osteoglossids

1. Using subcellular preparations of gills from Arapaima, an obligate air breather, and aruana, a related osteoglossid that is an obligate water breather, a comparison was made of the relative roles of the malate-aspartate cycle and the alpha-glycerophosphate (alpha-GP) cycle in transferring reducing equivalents from the cytosol to the mitochondria. 2. In aruana gill preparations, the alpha-GP cycle could be most clearly demonstrated by reconstructing it with purified isolated mitochondria, using the oxidation rate of exogenous NADH as a measure of the cycling activity. 3. Subcellular preparations of Arapaima gill, in contrast to the aruana gill, were not responsive to exogenous alpha-glycerophosphate, but a glutamate-malate stimulated O2 uptake was sensitive to aminooxyacetate, an aminotransferase inhibitor, a result that would be expected if the respiration were based on malate-aspartate cycling. 4. It was concluded that, compared to the alpha-glycerophosphate cycle, the malate-aspartate cycle was relatively more active in Arapaima gill than in aruana gill, and possible implications were discussed.

Hydrogen shuttles in air versus water breathing fishes

1. The malate-aspartate cycle was demonstrable in subcellular preparations of hearts from Arapaima, Lepidosiren, and Synbranchus (obligate air breathers), Hoplerythriunus (facultative air breather), and Osteoglossum and Hoplias (obligate water breathers). 2. Although no respiratory evidence for significant alpha-glycerophosphate cycle participation could be shown in the air breathers, this cycle was demonstrable in hearts of water breathers. 3. In agreement with the O2 uptake studies, it was possible to reconstruct the malate-aspartate, but not the alpha-glycerophosphate cycle, in isolated mitochondria from air breathers, while both shuttles could be reconstructed with heart mitochondria in the case of water breathing fishes.

Purification and properties of glutamate dehydrogenase from the mantle muscle of the squid, Loligo pealeii. Role of the enzyme in energy production from amino acids

1. The activity of glutamate dehydrogenase was measured in the tissues of the squid, Loligo pealeii. The enzyme occurs in high activity in digestive pouch, systemic heart, and all muscle tissues. 2. Glutamate dehydrogenase from mantle muscle is located intra-mitochondrially, has a molecular weight of 310,000, and is electrophoretically similar to the enzyme from all other squid tissues. 3. The enzyme from mantle muscle was purified 40-fold by elution from DEAE-cellulose and used for kinetic studies. The enzyme is NAD+-specific, activated by ADP, AMP, and leucine, and inhibited by GTP, GDP, ATP, and reaction products (in particular NADH). 4. Squid glutamate dehydrogenase shows an almost absolute dependence on ADP. The purified enzyme is activated over 100-fold by saturating concentrations of ADP (Ka = 0,75 7M); The pH optima are also altered significantly by ADP. 5. The enzyme appears to be kinetically adapted to favour glutamate oxidation in comparison to glutamate dehydrogenase from other resources. The evidence indicates that the primary role of glutamate dehydrogenase in squid mantle muscle is in regulating the catabolism of amino acids for energy production.

Metabolic biochemistry of water- vs. air-breathing fishes: muscle enzymes and ultrastructure

To delineate what modifications in muscle metabolic biochemistry correlate with transition to air breathing in fishes, the myotomal muscles of aruana, an obligate water breather, and Arapaima, a related obligate air breather, were compared using electron microscopy and enzyme methods. White muscle in both species maintained a rather similar ultrastructure, characterized by large-diameter fibers, very few mitochondria, and few capillaries. However, aruana white muscle displayed nearly fivefold higher levels of pyruvate kinase, threefold higher levels of muscle-type lactate dehydrogenase, and a fourfold higher ratio of fructose diphosphatase –phosphofructokinase activity; at the same time, enzymes in aerobic metabolism occurred at about one-half the levels in Arapaima. Red muscle was never found in the myotomal mass of aruana, but in Arapaima, red muscle was present and seemed fueled by glycogen, lipid droplets never being observed. From these and other data, it was concluded that in myotomal muscle two processes correlate with the transition to air breathing in Amazon osteoglossids: firstly, an emphasis in oxidative metabolism, and secondly, a retention of red muscle. However, compared with more active water-breathing species, Arapaima sustains an overall dampening of enzyme activities in its myotomal muscle, which because of the large myotome mass explains why its overall metabolic rate is relatively low. By keeping the oxidative capacity of its myotomal muscle low, Arapaima automatically conserves O2 either for a longer time or for other more O2-requiring organs in the body, a perfectly understandable strategy for an air-breathing, diving fish, comparable with that observed in other diving vertebrates. A similar comparison was also made of two erythrinid fishes, one that skimmed the O2-rich surface layers of water and one that obtained three quarters of its O2 from water, one quarter from air. Ultrastructural and enzyme data led to the unexpected conclusion that the surface skimmer sustained a higher oxidative capacity in its myotomal muscles than did the facultative air breather.

Metabolic biochemistry of water- vs. air-breathing osteoglossids: heart enzymes and ultrastructure

The ultrastructure of the inner myocardium of aruana, an osteoglossid water breather, and Arapaima, an air-breathing Amazon relative, was compared. The aruana heart was laden with glycogen granules while Arapaima heart was fat loaded. Associated with air breathing in Arapaima, the ultrastructure of the inner myocardium displayed abundant mitochondria, clearly differentiated into myofibrillar and peripheral populations. As many of the mitochondrial characteristics of Arapaima resembled those in the mammalian heart, it was postulated that the inner myofibrillar mitochondria are probably specialized for oxidative metabolism as is the case also in the mammalian heart, while subsarcolemmal mitochondria were specialized for the exchange of materials with the blood. In contrast with the heart of the air breather, in aruana the inner myocardium contained two cell types. Type I cells like those in Arapaima myocardium were specialized for aerobic metabolism, displaying abundant mitochondria mostly myofibrillar in location and ample glycogen granules as a potential substrate source. Type II cells by comparison contained fewer mitochondria, but were rich in glycogen granules, and appeared specialized for anaerobic metabolism.

Control of ammoniagenesis in the kidney of water- and air-breathing osteoglossids: characterization of glutamate dehydrogenase

Glutamate dehydrogenases (EC 1.4.1.2) from the kidney of Osteoglossum bicirrhosum (called aruana) and Arapaima gigas were kinetically characterized. The two enzymes exhibited several common characteristics including Vmax activity ratio, pH optimum, affinity for cofactors, a marked preference for NAD(H) over NADP(H), and a very low affinity for NH4+. A variety of regulatory metabolites affected both enzymes. GTP and GDP were inhibitory while ADP, ATP, AMP, and leucine activated the enzymes. Both enzymes displayed potent product inhibition which was partially reversed by low levels of ADP. Arapaima kidney glutamate dehydrogenase was tightly regulated by the adenylate and guanylate nucleotides, inhibition by GTP and GDP and deinhibition by ADP and AMP being much stronger for this enzyme than for the aruana enzyme. Aruana glutamate dehydrogenase, however, was more responsive to NAD–NADH control. The enzyme was more sensitive to NAD(H) product inhibition and this inhibition was poorly reversed by ADP. From these data, it was concluded that both fish kidney glutamate dehydrogenases could function in glutamate oxidation in vivo. However, the Arapaima enzyme appeared most clearly adapted to a catabolic role, activity being more tightly linked to the energy status of the mitochondrion. Conversely, the aruana enzyme displayed regulatory properties allowing it the potential to function in NADH oxidation during periods of hypoxic stress.

Purification and properties of squid mantle adenylate kinase. Role of NADH in control of the enzyme

Adenylate kinase (ATP:AMP phosphotransferase, EC 2.7.4.3) from the mantle muscle of the squid, Loligo pealeii, was purified over 170-fold to homogeneity as judged by polyacrylamide and starch gel electrophoresis. The tissue contains a single isozyme of adenylate kinase, the enzyme from cytoplasmic and mitochondrial compartments (90 and 10% of total activity, respectively) being identical in physical and kinetic properties. Molecular weight was found to be 27,000 +/- 400. The enzyme shows a pH optimum of 8.2 in the forward (APD utilizing) and 7.4 in the reverse direction. Michaelis constants for ADP, ATP, and AMP are 0.70, 0.13, and 0.15 mM, respectively, with optimal Mg2+:adenylate ratios being 1:2 for ADP and 1:1 for ATP. A comparison of mass action ratios with the equilibrium constant indicated that squid adenylate kinase is held out of equilibrium in resting, but not active, muscle. A search for metabolic modulators of adenylate kinase revealed that NADH (Ki of 0.1 mM) was the only modulator which exerted a significant effect within its in vivo concentration range. The data presented indicate that NADH inhibition is the factor maintaining adenylate kinase in a nonequilibrium state in resting muscle and that release of this inhibition can serve to integrate adenylate kinase into the known scheme of intermediary metabolism in this tissue. A sharp drop in NADH levels at the onset on muscular work co-ordinates that activation of aerobic metabolism in this tissue and allows adenylate kinase to return to equilibrium function. At equilibrium, the enzyme can function to ampligy the concentration of AMP, a potent activator and deinhibitor of key glycolytic and Krebs cycle enzymes. The effect of modulators of adenylate kinase in preventing denaturation by heat or proteolysis revealed that NADH and substrates induced conformational changes in the enzyme which rendered it less susceptible to denaturation. The conformation state induced by NADH differed from that induced by substrate.

Purification and properties of adductor muscle phosphofructokinase from the oyster, Crassostrea virginica. The aerobic/anaerobic transition: role of arginine phosphate in enzyme control

Phosphofructokinase from oyster (Crassostrea virginica) adductor muscle occurs in a single electrophorectic form at an activity of 8.1 mumol of product formed per minute per gram wet weight. The enzyme was purified to homogeneity by a novel method involving extraction in dilute ethanol and subsequent precipitation with polyethylene glycol. Oyster adductor phosphofructokinase has a molecular weight of 3400000 +/- 20000 as measured by Sephadex gel chromatography. Mg2+ or Mn2+ can satisfy the divalent ion requirement while ATP, GTP, or ITP can serve as phosphate donors for the reaction. Oyster adductor phosphofructokinase displays hyperbolic saturation kinetics with respect to all substrates (fructose 6-phosphate, ATP, and Mg2+) at either pH 7.9 OR PH 6.8. The Michaelis constant for fructose 6 phosphate at pH 6.8, the cellular pH of anoxic oyster tissues, is 3.5 mM. In the presence of AMP, by far the most potent activator and deinhibitor of the enzyme, this drops to 0.70 mM. Many traditional effectors of phosphofructokinase including citrate, NAD(P)H,Ca2+, fructose 1,6-bisphosphate, 3-phosphoglycerate, ADP, and phosphoenolpyruvate do not alter enzyme activity when tested at their physiological concentrations. Monovalent ions (K +, NH4+) are activators of the enzyme. ATP and arginine phosphate are the only compounds found to inhibit the adductor enzyme. The inhibitory action of both can be reversed by physiological concentrations of AMP(0.2- 1.0mM) and to a lesser extent by high concentrations of Pi (20 mM) and adenosine 3' :5'-monophosphate (0.1 mM). The two inhibitors exhibit very different pH versus inhibition profiles. The Ki (ATP) decreases from 5.0 mM to 1.3 mM as the pH decreases from 7.9 to 6.8, whereas the Ki for arginine phosphate increases from 1.3 mM to 4.5 mM for the same pH drop. Of all compounds tested, only AMP, within its physiological range, activated adductor phosphofructokinase significantly at low pH values. The kinetic data support the proposal that arginine phosphate, not ATP or citrate, is the most likely regulator of adductor phosphofructokinase in vivo under aerobic, high tissue pH, conditions. In anoxia, the depletion of arginine phosphate reserves and the increase in AMP concentrations in the tissue, coupled with the increase in the Ki for arginine phosphate brought about by low pH conditions, serves to activate phosphofructokinase to aid maintenance of anaerobic energy production.

Octopus mantle citrate synthase

Citrate synthase (EC 4.1.3.7) in mantle muscle of the octopus, Octopus cyanea, occurs in relatively low specific activity and is largely independent of pH between 7.5 and 9.0. Catalytic activity is regulated by the adenylate energy charge and by at least two Krebs cycle intermediates, α-ketoglutarate and citrate. Of the adenylates, ATP is by far the most potent inhibitor, at near-physiological concentrations (4 mM), causing almost a 20-fold increase in the Michaelis constant for acetyl-CoA. Citrate and α-ketoglutarate, on the other hand, are competitive with respect to oxaloacetate, rather than acetyl-CoA, and bring about large increases in the Michaelis constant for oxaloacetate. The regulatory properties of citrate synthase allow a curtailment of carbon flow into the Krebs cycle during periods of burst muscle work, when mantle anaerobic glycolysis is strongly activated.

Catalytic and regulatory properties of pyruvate kinase isozymes from octopus mantle muscle and liver

The octopus is a relatively slow-moving animal that relies upon burst swimming to power its predatory activities. Pyruvate kinase (EC, 2.7.1.40), as one of the major glycolytic control sites, must be regulated in such a fashion to allow the increased glycolytic rate characteristic of burst metabolism. The mantle enzyme is regulated by the concerted action of ATP, arginine phosphate, and citrate. The Km for ADP was 0.28 mM and that for phosphoenolpyruvate (PEP), 0.25 mM. In contrast to many other invertebrate muscle pyruvate kinases, the enzyme is insensitive to fructose-1,6-diphosphate (FDP) activation.The pyruvate kinase from the liver is kinetically and electrophoretically distinct from the mantle enzyme. The liver isozyme has a considerably lower affinity for PEP (Km = 0.85 mM), is inhibited by ATP, citrate, and arginine phosphate, and is subject to a strong activation by FDP (Ka = 1 × 10−6). These differences between the pyruvate kinases from catabolic and synthetic tissues are reminiscent of the distinctions between mammalian muscle and liver pyruvate kinases.

The pyruvate branch point in squid brain: competition between octopine dehydrogenase and lactate dehydrogenase

Octopine dehydrogenase (EC 1.5.1.11) and lactate dehydrogenase (EC 1.1.1.27) from the brain of the squid, Symplectoteuthis oualaniensis, were studied kinetically. Lactate dehydrogenase had a higher affinity for pyruvate (Km = 0.37 mM) than octopine dehydrogenase (Km = 0.86 mM), whereas their affinities for NADH were similar (Km values for NADH were about 0.01 mM). The affinity of lactate dehydrogenase for lactate was low (Km = 14 mM), whereas the affinity of octopine dehydrogenase for octopine was high (Km = 0.2 mM). NAD+ strongly inhibited octopine dehydrogenase (Ki = 0.04 mM), and had a less pronounced effect on lactate dehydrogenase (Ki = 0.5 mM). The kinetic characteristics of the enzymes suggest that brain lactate dehydrogenase could function to maintain redox balance under stress conditions, whereas brain octopine dehydrogenase would function mainly in the oxidation of exogenous octopine to arginine and pyruvate.