Depression by ethionine of phosphorylating oxidation in hepatic mitochondria

Effects of 2-mercaptoacetate in isolated liver mitochondria in vitro. Competitive inhibition of 3-hydroxybutyrate dehydrogenase and depression of the beta-oxidation pathway

The effects of 2-mercaptoacetate on the respiration rates induced by different substrates were studied in vitro in isolated liver mitochondria. With palmitoyl-L-carnitine or 2-oxoglutarate as the substrate, the ADP-stimulated respiration (State 3) was dose-dependently inhibited by 2-mercaptoacetate. with glutamate or succinate as the substrate. State-3 respiration was only slightly inhibited by 2-mercaptoacetate. In contrast, the oxidation rate of 3-hydroxybutyrate was competitively inhibited by 2-mercaptoacetate in both isolated mitochondria and submitochondrial particles. In uncoupled mitochondria and in mitochondria in which ATP- and GTP-dependent acyl-CoA biosynthesis was inhibited, the inhibitory effect of 2-mercaptoacetate on palmitoyl-L-carnitine oxidation was abolished; under the same conditions, however, inhibition of 3-hydroxybutyrate oxidation by 2-mercaptoacetate still persisted. These results led to the following conclusions: 2-mercaptoacetate itself enters the mitochondrial matrix, inhibits fatty acid oxidation through a mechanism requiring an energy-dependent activation of 2-mercaptoacetate and itself inhibits 3-hydroxybutyrate oxidation through a competitive inhibition of the membrane-bound 3-hydroxybutyrate dehydrogenase. This study also strongly suggests that the compound responsible for the inhibition of fatty acid oxidation is 2-mercaptoacetyl-CoA.

2-Mercaptoacetate administration depresses the beta-oxidation pathway through an inhibition of long-chain acyl-CoA dehydrogenase activity

To elucidate the mechanisms through which 2-mercaptoacetate administration inhibits fatty acid oxidation in the liver, the respiration rates induced by different substrates were studied polarographically in rat hepatic mitochondria isolated 3 h after 2-mercaptoacetate administration. Palmitoyl-L-carnitine oxidation was almost completely inhibited in either the absence or presence of malonate. Octanoate oxidation was also inhibited, and the intramitochondrial acyl-CoA content was markedly increased. The oxidation rate of pyruvate and 2-oxoglutarate on the one hand and of 3-hydroxybutyrate, succinate and glutamate on the other was either normal or only slightly decreased. In the presence of 2,4-dinitrophenol, the extent of the inhibition of palmitoyl-L-carnitine oxidation was unchanged. All these results are consistent with the hypothesis that the 2-mercaptoacetate inhibition of fatty acid oxidation is due to an inhibition of the beta-oxidation pathway itself. Finally, the mitochondrial defect responsible for this inhibition was shown to be an inhibition of palmitoyl-CoA dehydrogenase activity (EC 1.3.99.3).