Relationship between Oxygen Uptake of Perifused Rat‐Liver Cells and the Cytosolic Phosphorylation State Calculated from Indicator Metabolites and a Redetermined Equilibrium Constant

Abstract

The relationship between the rate of oxygen uptake in isolated rat‐liver cells and the cytosolic phosphorylation state was studied under steady‐state conditions. The cells were perifused with different concentrations of the gluconeogenic substrates lactate + pyruvate in order to obtain different steady‐state rates of oxygen consumption and glucose production. The cytosolic phosphorylation state was calculated by the metabolite method. For this calculation, the apparent equilibrium constant of the combined glyceraldehyde‐3‐phosphate dehydrogenase and 3‐phosphoglycerate kinase reactions was redetermined taking into account the strong Mg2+ dependence of the overall reactions. At the free Mg2+ concentration in the cytosol (0.4 mM) the value of Kapp is 912 M−1. Using this value, a cytosolic phosphorylation state of 2700 M−1 can be calculated for cells incubated in the presence of 10 mM alanine. This value agrees very closely with that obtained by direct measurement after fractionation of the cells by the digitonin procedure. It is possible to describe the regulation of the rate of O2 uptake in perifused liver cells in terms of the principles of irreversible thermodynamics. It is concluded that the transition from one steadystate to another is accompanied by a thermodynamics regulation of the rate of O2 consumption by the cytosolic phosphorylation state, the mitochondrial redox level and the partial pressure of oxygen. Copyright © 1978, Wiley Blackwell. All rights reserved