Hepatic Organelle Interaction

The metabolism of lipid-soluble drugs and chemicals by the endoplasmic reticulum mixed function oxidase in liver slices or hepatic microsomes, the latter in the presence of mitochondria, was stimulated up to 80% by succinate and was inhibited up to 30% by oxalacetate. When the combined organelles were used, maximal effects were obtained at 4 to 5 times more mitochondrial protein than microsomal protein. Combination of mitochondria with microsomes also caused a supra-additive effect on respiration in the presence of succinate, and an enhanced inhibition of respiration when oxalacetate was present. Respiration by the combined organelles, in the presence of levels of cyanide which inhibit mitochondrial respiration 90 %, left respiration 5 times higher than mitochondrial and 8 times higher than microsomal respiration in the presence of succinate. The stimulatory effect of succinate occurred even with a large excess of NADPH, and succinate could not support drug metabolism when NADP was used instead of NADPH, indicating that stimulation is not the result of regeneration of NADPH. Stimulation required direct organelle contact; dilution of liver homogenate or containment of the mitochondria in a dialysis sac prevented the succinate effect. Both succinate and oxalacetate were without effect on NADPH-cytochrome P-450 reductase in media containing mitochondria plus microsomes, indicating that the control is probably after this step. Both effects were observed, even on addition of NADH to the NADPH-supported reaction, suggesting that the regulation is due to either an alteration of the effectiveness of the input of a second electron or an effect other than provision of reducing equivalents. Electron microscopic examination of liver slices indicated that the mitochondrial control may be due to structural stabilization of the endoplasmic reticulum membrane; during drug metabolism by liver slices, the rough endoplasmic reticulum lost