State of Oxidation-Reduction and State of Binding in the Cytosolic NADH-System as Disclosed by Equilibration with Extracellular Lactate/Pyruvate in Hemoglobin-Free Perfused Rat Liver

Abstract

where subscripts e and c refer to extracellular and cytosolic spaces, respectively, several lines of evidence support the assumption that near-equilibrium conditions exist for processes 1 through 3 during the normoxic steady state of rat liver [l-31.Since 1965, in studies with the isolated hemoglobin free perfused organ [4,5], extensive use was made of this system in the direction from right to left, using the ratio interest was focussed on the other direction in order to obtain detailed information on the NADH system of liver oytosol which was previously not accessible. Extracellular [lactate]/[pyruvate] was used as experimental variable, and dependent changes in cytosolic NADH were observed by a variety of methods, in particular by continuous measurement of fluorescence and absorbance of perfused liver a t pyridine-nucleotide-specific wavelengths.By means of mathematical extrapolation of titration results in a double reciprocal plot derived on the basis of the mass action law, the maximal

Energy‐dependent regulation of the steady‐state concentrations of the components of the lactate dehydrogenase reaction in liver

“…It has been known for some years that the components of the lactate dehydrogenase reaction in liver are maintained in a steady-state believed to be close to thermodynamic equilibrium [ 1,2]. The ratio, [lactate]/[pyruvate] has thus been considered to reflect the 'redox state' (the ratio of 'free' [NAD]/ [NADH]) within the cytoplasmic compartment of the hepatic cell [3].…”

The glutamate dehydrogenase system and the redox state of mitochondrial free nicotinamide adenine dinucleotide in myocardium

“…This could indicate that the lactate dehydrogenase reaction is not at equilibrium with the total bound NADH/NAD' pool in heart muscle. This situation is different from that in the liver where an equilibrium between the total bound NADH/NAD' pool and extracellular lactate and pyruvate has been demonstrated [20]. Another line of evidence also points to the lack of equilibrium.…”

“…Moreover, when the flavoprotein fluorescence changes were plotted against the changes in the glutamate dehydrogenase system in a double reciprocal manner [20], a straight-line was obtained ( fig.3). Similar plots of the flavin fluorescence against the lactate/pyruvate ratio were not linear, in contrast to observations on rat liver [21], and neither was the plot of the tissue NADH fluorescence against the lactate/pyruvate ratio (not shown).…”

Linear relationships between mitochondrial forces and cytoplasmic flows argue for the organized energy‐coupled nature of cellular metabolism

“…Furthermore, they accommodate well the numerous observations of apparent nearequilibrium between cytoplasmic and mitochondrial redox and phosphorylation states [19,41]. However, it would seem that such relationships should be interpreted in terms of irreversible thermodynamics [10] rather than on the basis of the classical equilibrium model proposed [19,41,42].…”