Belinsky S A, Kauffman F C, Thurman R G
Department of Pharmacology, University of North Carolina, Chapel Hill 27599-7365.
Mol Pharmacol. 1989 Apr;35(4):512-8.
Interaction between glycolysis and mitochondrial oxidations to supply reducing equivalents at high rates for mixed function oxidation was evaluated in the perfused liver after treatment of rats with beta-naphthoflavone. Livers from fasted beta-naphthoflavone-treated rats were employed because rates of 7-ethoxycoumarin O-deethylation were constant (16 mumol/g/hr) for at least 1 hr of perfusion. Preinfusion with KCN, an inhibitor of oxidative phosphorylation, caused the rate of 7-ethoxycoumarin O-deethylation to decline by 60% over 30 min of perfusion. The decline in rates of mixed function oxidation in the intact liver was not due to a direct effect of KCN on cytochrome P-450, inasmuch as cyanide did not diminish rates of 7-ethoxycoumarin O-deethylation by isolated microsomes. Cyanide rapidly decreased hepatic oxygen uptake by 70% and increased rates of glycolysis (lactate plus pyruvate production) from less than 10 to over 60 mumol/g/hr. Rates of glycolysis and mixed function oxidation subsequently declined in parallel during infusion of KCN. Infusion of ethanol (20 mM), a known inhibitor of glycolysis, decreased the stimulation of glycolysis caused by KCN to 20 mumol/g/hr and lowered maximal rates of 7-hydroxycoumarin production to about 6 mumol/g/hr. Both mixed function oxidation and glycolysis also declined in parallel over 30 min of perfusion in the presence of ethanol and KCN. When cyanide infusion was terminated, rates of oxygen uptake returned rapidly to basal values; however, rates of mixed function oxidation remained low. In contrast, infusion of ethanol in the absence of cyanide had no effect on rates of mixed function oxidation. Infusion of glucose (30 mM) or pyruvate (1 mM) after KCN restored maximal rates of mixed function oxidation in parallel with increases in rates of glycolysis. In contrast to results obtained in livers from fasted rats, cyanide and ethanol had little effect on 7-ethoxycoumarin O-deethylation in livers from fed rats. Taken together, these results argue strongly that rates of mixed function oxidation in the intact livers of fasted rats are sustained by reducing equivalents derived from mitochondrial oxidations. Glycolysis can supply substrates needed for the transport of reducing equivalents from the mitochondria into the cytosol for mixed function oxidation. Because glycogen reserves are minimal in the fasted state, rates of glycolysis and mixed function oxidation declined in parallel during the infusion of cyanide, because reducing equivalents derived from mitochondria are not available.
在用β-萘黄酮处理大鼠后，对灌注肝脏中糖酵解与线粒体氧化之间的相互作用进行了评估，以高速率提供还原当量用于混合功能氧化。使用来自禁食的β-萘黄酮处理大鼠的肝脏，因为7-乙氧基香豆素O-脱乙基化速率在至少1小时的灌注过程中保持恒定（16 μmol/g/小时）。预先注入氧化磷酸化抑制剂KCN，导致7-乙氧基香豆素O-脱乙基化速率在30分钟的灌注过程中下降60%。完整肝脏中混合功能氧化速率的下降并非由于KCN对细胞色素P-450的直接作用，因为氰化物并未降低分离的微粒体中7-乙氧基香豆素O-脱乙基化的速率。氰化物迅速使肝脏氧摄取量降低70%，并使糖酵解速率（乳酸加丙酮酸生成速率）从低于10 μmol/g/小时增加到超过60 μmol/g/小时。在注入KCN期间，糖酵解速率和混合功能氧化速率随后平行下降。注入乙醇（20 mM），一种已知的糖酵解抑制剂，将KCN引起的糖酵解刺激降低到20 μmol/g/小时，并将7-羟基香豆素的最大生成速率降低到约6 μmol/g/小时。在乙醇和KCN存在的情况下，混合功能氧化和糖酵解在30分钟的灌注过程中也平行下降。当终止氰化物注入时，氧摄取速率迅速恢复到基础值；然而，混合功能氧化速率仍然很低。相比之下，在不存在氰化物的情况下注入乙醇对混合功能氧化速率没有影响。在KCN之后注入葡萄糖（30 mM）或丙酮酸（1 mM）可使混合功能氧化的最大速率恢复，同时糖酵解速率增加。与禁食大鼠肝脏中获得的结果相反，氰化物和乙醇对喂食大鼠肝脏中的7-乙氧基香豆素O-脱乙基化几乎没有影响。综上所述，这些结果有力地表明，禁食大鼠完整肝脏中的混合功能氧化速率由线粒体氧化产生的还原当量维持。糖酵解可以提供将还原当量从线粒体转运到细胞质中用于混合功能氧化所需的底物。由于禁食状态下糖原储备极少，在注入氰化物期间，糖酵解速率和混合功能氧化速率平行下降，因为无法获得来自线粒体的还原当量。
