Cellular respiration during hypoxia. Role of cytochrome oxidase as the oxygen sensor in hepatocytes.
We previously reported that hepatocytes exhibit a reversible suppression of respiration during prolonged hypoxia (PO2 = 20 torr for 3-5 h). Also, isolated bovine heart cytochrome c oxidase undergoes a reversible decrease in apparent Vmax when incubated under similar conditions. This study sought to link the hypoxia-induced changes in cytochrome oxidase to the inhibition of respiration seen in intact cells. Hepatocytes incubated at PO2 = 20 torr exhibited decreases in respiration and increases in [NAD(P)H] after 2-3 h that were reversed upon reoxygenation (PO2 = 100 torr). Respiration during hypoxia was also inhibited when N,N,N',N'-tetramethyl-p-phenylenediamine (0.5 mM) and ascorbate (5 mM) were used to reduce cytochrome c, suggesting that cytochrome oxidase was partially inhibited. Similarly, liver submitochondrial particles revealed a 44% decrease in the apparent Vmax of cytochrome oxidase after hypoxic incubation. In hepatocytes loaded with tetramethylrhodamine ethyl ester (10 nM) to quantify mitochondrial membrane potential, acute hypoxia (<30 min) produced no change in fluorescence, consistent with the absence of an acute change in respiration. However, fluorescence increased during acute reoxygenation after prolonged hypoxia, suggesting an increase in potential. The control exhibited by NADH over mitochondrial respiration was not altered during hypoxia. Thus, changes in the Vmax of cytochrome oxidase during prolonged hypoxia correlate with the changes in respiration and mitochondrial potential. This suggests that the oxidase functions as an oxygen sensor in the intact hepatocyte.
Chandel
N S
NS
Department of Medicine, Section of Pulmonary and Critical Care, the University of Chicago, Chicago, Illinois 60637, USA.
Budinger
G R
GR
Choe
S H
SH
Schumacker
P T
PT
D025181:ATP Synthetase Complexes; D000818:Animals; D002384:Catalysis; D002417:Cattle; D015687:Cell Hypoxia; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006096:Gramicidin / Q000494:pharmacology; D066298:In Vitro Techniques; D007202:Indicators and Reagents / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008564:Membrane Potentials; D008930:Mitochondria, Liver / Q000201:enzymology; D009097:Multienzyme Complexes / Q000378:metabolism; D009206:Myocardium / Q000201:enzymology; D009243:NAD / Q000378:metabolism; D009249:NADP / Q000378:metabolism; D010042:Ouabain / Q000494:pharmacology; D010101:Oxygen Consumption; D017856:Phosphotransferases (Phosphate Group Acceptor) / Q000378:metabolism; D013772:Tetramethylphenylenediamine / Q000378:metabolism; D013997:Time Factors
D004791:Enzyme Inhibitors; D007202:Indicators and Reagents; D009097:Multienzyme Complexes; D009243:NAD; D006096:Gramicidin; D009249:NADP; D010042:Ouabain; D003576:Electron Transport Complex IV; D025181:ATP Synthetase Complexes; D017856:Phosphotransferases (Phosphate Group Acceptor); D013772:Tetramethylphenylenediamine