STUDIES ON 3 MICROSOMAL ELECTRON-TRANSFER ENZYME-SYSTEMS .2. SPECIFICITY OF ELECTRON FLOW PATHWAYS

The routes of microsomal [rat liver] electron flow to the 3 terminal oxidative enzymes, the mixed function oxidase, the fatty acyl CoA desaturase and the lipid peroxidase were examined by the use of specific antibodies, by alteration of electron transfer enzyme levels and with the inhibitor NADP+. From these studies a number of conclusions are drawn: NADH-supported lipid peroxidation utilizes NADH-cytochrome b5 reductase, but electron flow does not go via cytochrome b5. The positive modifier effect of type I substrates on NADPH-driven cytochrome P-450 reduction is seen also with NADH-supported cytochrome P-450 reductase activity. The latter reaction proceeds via cytochrome b5 while the former does not. Cross-reactivity can occur between NADH-cytochrome b5 reductase and NADPH-cytochrome c reductase, but at a rate too slow to support most reactions. Cytochrome b5 appears to exist in 2 pools; 1 pool is readily inhibited by antibody and the other pool is either inaccessible to or incompletely inhibited by antibody. The various cytochrome b5-dependent reactions show different abilities to use the noninhibited hemoprotein. NADH-cytochrome c reductase activity and NADH-synergism appear to utilize only the former pool and are completely inhibitable by antibody. Other NADH-supported reactions (.DELTA.9-desaturation and mixed-function oxidation) utilize the total cytochrome b5 population. Fortification studies show that the extra bound cytochrome b5 is distributed in the same manner as the endogenous cytochrome b5.