The oxidation--reduction state of cytochrome oxidase in freeze trapped gerbil brains

In situ measurements of brain tissue hemoglobin saturation and blood volume by reflectance spectrophotometry in the visible spectrum

Before the development of near-infrared spectroscopy (NIRS) for monitoring of hemoglobin and cytochromes in situ, the Jobsis laboratory designed a visible light reflectance spectrophotometer. The method was not as useful for cytochrome oxidase measurements, which stimulated the search for a better method that culminated in NIRS. Visible light reflectance spectrophotomery was, however, usefully applied in several experimental applications, such as the study of brain capillary hemoglobin saturation during changes in inspired gas mixtures in awake and anesthetized animals, and to record transient increases in total hemoglobin (blood volume) after local neuronal activation by direct cortical electrical stimulation, demonstrating a response that is fundamental to functional magnetic resonance imaging blood oxygen level-dependent methods. A third application of the instrumentation was for brain capillary red cell mean transit time analysis, estimated by recording the passage of a red cell-free bolus through the cerebral cortical optical monitoring field. Taken together with his previous application of fluorescence detection of nicotinamide adenine dinucleotide, the visible and near-infrared spectroscopy demonstrate that Frans Jobsis was a pioneer in the application of optical techniques to the study of intact organs in situ. These methods have been used to illuminate the basic function of the cerebrovascular and metabolic pathways in both physiological and pathological conditions.

Effects of nitric oxide and peroxynitrite on the cytochrome oxidase K(m) for oxygen: implications for mitochondrial pathology

This review summarises current knowledge about the effect of oxygen on cytochrome oxidase activity in vitro and in vivo. Cytochrome oxidase normally operates above its K(m) for oxygen in vivo. However, decreases in the intracellular oxygen concentration (hypoxia) under physiological extremes, or during pathophysiology, can cause mitochondrial respiration to become oxygen limited. Inhibitors that raise the enzyme's K(m) will induce oxygen limitation under apparently normoxic conditions. It is known that the concentrations of nitric oxide and peroxynitrite are raised in a number of pathophysiological conditions. These compounds are capable of reversibly and irreversibly raising the cytochrome oxidase K(m) for oxygen. Therefore, measurements of cell and mitochondrial respiration in vitro that fail to systematically vary oxygen through the range of physiological concentrations are likely to underestimate the effects of nitric oxide and peroxynitrite in vivo.

Oxidation and reduction of cytochrome oxidase in the neonatal brain observed by in vivo near-infrared spectroscopy

Near-infrared spectroscopy was used to determine the relationship between the redox state of mitochondrial cytochrome oxidase CuA and haemoglobin oxygenation in the isoflurane-anaesthetized neonatal pig brain. Adding 7% CO2 to the inspired gases increased the total haemoglobin concentration by 8 microM and oxidized CuA by 0.2 microM. Decreasing the inspired oxygen fraction to zero for 90 s dropped the oxyhaemoglobin concentration by 27 microM and reduced CuA by 1.8 microM. However, no change in the CuA redox state was observed until oxyhaemoglobin had decreased by more than 10 microM. The response of the CuA redox state to these stimuli was very similar following 80% replacement of the haemoglobin by a perfluorocarbon blood substitute; this demonstrates that the results in the normal haematocrit were not a spectral artefact due to the high haemoglobin/cytochrome oxidase ratio. We conclude that the large reductions in the CuA redox state during anoxia are caused by a decrease in the rate of oxygen delivery to the cytochrome oxidase oxygen binding site; the small oxidations, however, are likely to reflect the effects of metabolic changes on the redox state of CuA, rather than increases in the rate of oxygen delivery.

Measurement of cytochrome oxidase and mitochondrial energetics by near-infrared spectroscopy

Cytochrome oxidase is the terminal electron acceptor of the mitochondrial respiratory chain. It is responsible for the vast majority of oxygen consumption in the body and essential for the efficient generation of cellular ATP. The enzyme contains four redox active metal centres; one of these, the binuclear CuA centre, has a strong absorbance in the near-infrared that enables it to be detectable in vivo by near-infrared spectroscopy. However, the fact that the concentration of this centre is less than 10% of that of haemoglobin means that its detection is not a trivial matter. Unlike the case with deoxyhaemoglobin and oxyhaemoglobin, concentration changes of the total cytochrome oxidase protein occur very slowly (over days) and are therefore not easily detectable by near-infrared spectroscopy. However, the copper centre rapidly accepts and donates an electron, and can thus change its redox state quickly; this redox change is detectable by near-infrared spectroscopy. Many factors can affect the CuA redox state in vivo (Cooper et al. 1994), but most significant is likely to be the molecular oxygen concentration (at low oxygen tensions, electrons build up on CuA as reduction of oxygen by the enzyme starts to limit the steady-state rate of electron transfer). The factors underlying haemoglobin oxygenation, deoxygenation and blood volume changes are, in general, well understood by the clinicians and physiologists who perform near-infrared spectroscopy measurements. In contrast, the factors that control the steady-state redox level of CuA in cytochrome oxidase are still a matter of active debate, even amongst biochemists studying the isolated enzyme and mitochondria. Coupled with the difficulties of accurate in vivo measurements it is perhaps not surprising that the field of cytochrome oxidase near-infrared spectroscopy has a somewhat chequered past. Too often papers have been written with insufficient information to enable the measurements to be repeated and few attempts have been made to test the algorithms in vivo. In recent years a number of research groups and commercial spectrometer manufacturers have made a concerted attempt to not only say how they are attempting to measure cytochrome oxidase by near-infrared spectroscopy but also to demonstrate that they are really doing so. We applaud these attempts, which in general fall into three areas: first, modelling of data can be performed to determine what problems are likely to derail cytochrome oxidase detection algorithms (Matcher et al. 1995); secondly haemoglobin concentration changes can be made by haemodilution (using saline or artificial blood substitutes) in animals (Tamura 1993) or patients (Skov & Greisen 1994); and thirdly, the cytochrome oxidase redox state can be fixed by the use of mitochondrial inhibitors and then attempts make to cause spurious cytochrome changes by dramatically varying haemoglobin oxygenation, haemoglobin concentration and light scattering (Cooper et al. 1997). We have previously written reviews covering the difficulties of measuring the cytochrome near-infrared spectroscopy signal in vivo (Cooper et al. 1997) and the factors affecting the oxidation state of cytochrome oxidase CuA (Cooper et al. 1994). In this article we would like to strike a somewhat more optimistic note--we will stress the usefulness this measurement may have in the clinical environment, as well as describing conditions under which we can have confidence that we are measuring real changes in the CuA redox state.

The detection of cytochrome oxidase heme iron and copper absorption in the blood-perfused and blood-free brain in normoxia and hypoxia

The resolution of cytochrome and hemoglobin changes in in vivo rat and cat brains has required studies over wide wavelength ranges (580-1100 nm) with a novel spectroscopic technique using blood-free and blood-perfused brains. Tissue oxygen was varied from physiological levels to 0 and hematocrits were varied from normal to less than 1%. The experimental results were subjected to a multicomponent analysis using the Beer-Lambert law. At normal hematocrits, the oxygen saturation of hemoglobin in the brain was found to be 30-50% in rats and cats, indicating that the optical method responded primarily to the saturation of the venous ends of the capillary beds. With low hematocrits, both brains showed the absorption band of reduced cytochrome c, the iron component of cytochrome aa3, plus the absorption band of the oxidized copper component. In cat brains, the background absorption changed at all wavelengths. Thus, no isosbestic points were observed in the spectra. In rat brains, however, they were readily observed. The "overtones" of water absorption in the NIR region were found to be significant in the difference spectra of the cat brain, but not in the rat brain. Parallel absorbance changes in the heme and copper components of cytochrome aa3 were obtained in rat and cat brains during the normoxic-hypoxic transition. The ratio of the iron absorbance at 605 nm to the copper absorbance at 830 nm is much smaller in both brains than the in vitro value due to the shorter path length of photon migration at the shorter wavelengths.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of oxidative phosphorylation in the mammalian cell

The cell is capable of maintaining a steady-state flux of energy from mitochondrial oxidative phosphorylation, producing ATP, to the cytosolic adenosinetriphosphatases (ATPases), performing work. Considerable effort has been devoted to investigating the individual mechanisms involved in these two processes. However, less effort has been directed toward learning how these reactions of energy metabolism interact through the cytosol to maintain the observed steady state in the intact cell. The "classical" model for the cytosolic interaction of these two processes involves the feedback of ATP hydrolysis products, ADP and Pi, from the ATPases to oxidative phosphorylation. This model is based on data from isolated mitochondria in which the rate of oxidative phosphorylation is controlled by the concentration of ADP and Pi. Yet, recent data from intact tissues with high oxidative phosphorylation capacities (i.e., heart, brain, and kidney) indicate that the cytosolic concentration of ADP and Pi do not change significantly with work. These data imply that this simple feedback model is not adequate to explain the regulation of energy metabolism in these tissues. Other sites within the oxidative phosphorylation process must be playing a regulatory role or the kinetics of ATP synthesis must be very different than currently believed to establish the steady state. This review covers the potential sites within oxidative phosphorylation which may be regulated through cytosolic transducers to result in the necessary feedback network regulating the steady-state flow of energy in the cell. These sites will include substrate delivery to the cytochrome chain, the processes involved in the phosphorylation of ADP to ATP, and the delivery of oxygen.

The polyphasic nature of the respiratory process at the mitochondrial level

The kinetics of oxygen consumption by rat liver mitochondria, respiring under a variety of metabolic conditions, have been studied. Respiration was initiated by injecting oxygen into anaerobic suspensions of mitochondria. It was found that, irrespective of the metabolic state of the mitochondria and the nature of the respiratory substrate, the rates of electron flow and oxygen consumption follow the pattern of a polyphasic reaction. The rates of oxygen uptake during the first phase are extremely fast and depend on oxygen concentration. The second phase represents a transition in which net oxidation of cytochrome-c oxidase stops and the rates of oxygen consumption suddenly decrease. The third phase is characterized by its changeability. Depending on initial conditions the rates may increase, decrease, or remain constant, although the reaction is not one of zero order. During the last phase, the rates decrease and the oxidase becomes increasingly reduced. It is postulated that the mitochondrial respiratory process is basically a cyclic event in which the redox state of the membrane and the rates of oxygen consumption oscillate with amplitudes and frequencies conditioned by the energy demand and energy-yielding capacity of the cell.

Spectrophotometry of b-type cytochromes in rat brain in vivo and in vitro

Terminal oxidase inhibitors such as cyanide (CN) and carbon monoxide (CO) produce different absorption changes in the intact brain, suggesting different mitochondrial responses to the inhibitors. In the present study, the nature of the cytochromes involved in CO and CN responses in vivo was investigated by low-temperature spectroscopy of rat brain, frozen in situ, and of preparations of brain homogenate and isolated mitochondria. Comparison of the spectra from different preparations at the high resolution afforded by low-temperature spectroscopy indicated that absorption responses to CO in vivo originated from mitochondrial b cytochromes. Further detailed spectral analysis of mitochondrial preparations revealed three CN-insensitive b cytochromes in nonsynaptic brain mitochondria; one cytochrome could be reduced by succinate in the presence of CN, the second could be reduced by succinate plus ATP, and the third could be reduced only by anaerobiosis. The spectral characteristics of the mitochondrial b cytochromes, when compared with spectra from CO-exposed brain tissue frozen in situ, strongly implicated the energy-dependent cytochrome b in the oxidation-reduction (redox) responses caused by CO in vivo.

Mitochondrial function in normal and genetically altered cells and tissues

The impact upon oxidative metabolism of normal and pathological variations of oxidative capability is just beginning to be understood, based upon the few examples of human and animal subject survivals and the relatively few cell systems in which the impact of molecular pathologies on function has been studied. On the one hand, difficulties of isolation of systems containing altered oxidases are significant because of ineffective assembly or small amounts of surviving isoenzymes, and on the other hand, unexpected fragilities of the oxidase system may lead to low yields when subjected to the preparative stresses appropriate to the wild types. To circumvent these problems, this paper describes the application, in vivo, of noninvasive, nondestructive techniques to study the function of cytochrome oxidase and other components of the respiratory chain, particularly cytochromes b-c1 in human subjects on the one hand, and in isolated cells on the other, principally mutants of Saccharomyces cerevisiae in which the subunit content is varied. Two principal spectroscopic approaches are employed: optical and phosphorus magnetic resonance spectroscopy (P MRS). Optical spectroscopy of the near red region of the spectrum provides effective analysis of brain and muscle, as does the surface coil of space-resolved phosphorus magnetic resonance. Both techniques are applicable to suspensions of single cells such as yeast. The optical method yields essential information on oxygen delivery to tissues by hemoglobin and myoglobin and oxygen utilization by cytochrome oxidase. P MRS affords essential information on the efficiency of ATP generation and the extent to which oxidative metabolism meets the needs of cell function in terms of the ratio of phosphocreatine to inorganic phosphate (PCr/Pi). This in turn enables the calculation of the velocity of oxidative metabolism, V, in relation to its maximum capability, Vm, according to a Michaelis-Menten relationship that involves control not only by ADP (Pi/PCr) and Pi, but also by oxygen and substrate deliveries. Thus, an overview of the functionality of mitochondria in cells and tissues is uniquely provided by this combined approach and thereby deficiencies of components of the respiratory chain are quantified.

Detection of an oxidizable fraction of cytochrome oxidase in intact rat brain

Rapid-scanning reflectance spectrophotometry was used to evaluate the reduction-oxidation state of cytochrome oxidase in normoxic rat brain. Reflectance spectra were recorded from intact blood-perfused cerebral cortices after increased oxidative metabolic activity induced by direct cortical stimulation. Reflectance spectra taken from blood-free rat brain and from the rat ear vascular bed were used to identify cytochrome and hemoglobin components of spectra taken from intact brain. Cortical stimulation provoked shifts toward increased oxidation of cytochrome oxidase that were detectable in reflectance spectra. Observations of an oxidizable fraction of cytochrome oxidase demonstrate that a fraction of the cytochrome oxidase pool exists in a reduced state in normoxic brain. The presence of reduced cytochrome oxidase suggests that oxygen delivery to the brain is restricted by microvascular control mechanisms either as a function of brain metabolic physiology or as protection against oxygen toxicity.

A rapid-scanning spectrophotometer designed for biological tissues in vitro or in vivo

Rapid and sensitive detection of optical signals from biological materials has been very useful in studies of cell physiology and biochemistry. A rapid-scanning spectrophotometer is described here that has the following advantages: (i) it can be used in transmission or reflection modes, (ii) it can rapidly accumulate spectra and simultaneous kinetic data, (iii) it has high accuracy and sensitivity, and (iv) it can analyze and store large amounts of spectral information. Evaluations described here are aimed toward the measurement of reduction/oxidation shifts of mitochondrial cytochromes in tissues, but the flexibility of the optical components makes this spectrophotometer adaptable to the study of light absorption changes of intrinsic or extrinsic optically active molecules in a variety of light scattering preparations, tissues, and organs in vitro or in vivo.

Optical measurements of oxygen delivery and consumption in gerbil cerebral cortex

Oxygen metabolism of the cerebral cortex of anesthetized gerbils was monitored by surface fluorescence and reflectance spectrophotometry both in vivo and in specimens freeze-trapped by the surface-freezing technique. Fiber optic light guides were used for optical coupling for determining mitochondrial flavoprotein and pyridine nucleotide fluorescence and for recording dual-wavelength reflectance spectra that indicated the levels of ferrocytochromes aa3 and c + c1 in the cortex and the oxygen saturation of cortical hemoglobin. During anoxic episodes increases in ferrocytochromes aa3 and c + c1 and in reduced pyridine nucleotides and flavoproteins were observed only when the cortical hemoglobin was more than 85% disoxygenated. Indeed the steady-state levels of oxidation-reduction of mitochondrial respiratory-chain components remained constant over a wide range of oxygen delivery, increased reduction being observed only when the fraction of inspired oxygen fell below 6%; these properties of mitochondria in vivo resemble those found in vitro. However, in normoxic animals the absorbance at 605 nm that may arise from ferri- or ferrocytochrome aa3 is larger than would be expected of purified mitochondria and may represent a pool of mitochondria that remains reduced at all levels of tissue oxygenation or a pigment not involved in oxygen metabolism.