Inhibition of human placental oestradiol-17beta dehydrogenase by adenosine triphosphate: the importance of the coenzyme.

adult liver microsomal preparations; neonatal liver microsomal preparations were also active. Kidney microsomal preparations were about 12 % as active, but microsomal suspensions from other tissues had less than 5 % of the adult liver microsomal activity (Table 1). In experiments involving mixing liver and brain microsomal preparations, or supernatants, there was no evidence for an inhibitory effect of brain microsomal fraction or supernatant on the activity of liver microsomal fraction. Our results show that NADPH-cytochrome c reductase and cytochrome b, are widely distributed in microsomal fractions obtained from rat tissues; cytochrome P-450 and NADPH-ADP/FeZ+-linked lipid peroxidation are much more restricted in their distribution. The NADPH-linked lipid peroxidation has been postulated to involve not only NADPH-cytochrome c reductase, but also a thiol-group-dependent protein possibly containing a non-haem iron group. The low activity of NADPH-linked lipid peroxidation in tissues containing relatively high NADPH-cytochrome c reductase activity may reflect the oxidation of essential thiol groups during microsomal-fraction preparation, or a tissue deficiency in the thiol-group-dependent protein required for coupling the NADPH-flavoprotein to peroxidative electron flow. Preliminary experiments have shown that a low concentration of cysteine stimulates lipid peroxidation in brain microsomal preparations fivefold, although having no significant effect on liver microsomal activity. The role of such thiol-group-containing substances and of nonhaem iron in regulating microsomal NADPH-linked lipid peroxidation, and thereby the production of reactive aldehydes, thus seems of importance.