Age-dependent changes in rat liver microsomal NADPH cytochrome C (P-450) reductase: a kinetic analysis.

Rat liver microsomal NADPH cytochrome c (P-450) reductase (EC 1.6.2.4) exhibits several marked age-dependent changes, including a decline in specific activity, reduced inducibility and enhanced thermostability, which are indicative of an alteration in the quality of this enzyme. The present study examined the kinetic profile of the microsomal-bound enzyme in an effort to further define the effects of aging on the hepatic mixed function oxidase system. Intact microsomes isolated from young adult (3 months), mature (16 months) and senescent (27 months) rats were subjected to an extensive double reciprocal kinetic analysis employing NADPH and cytochrome c as substrates. The Km values obtained with the initial substrate (NADPH) remained unchanged with animal age, whereas there was a decline in this parameter for the artificial acceptor substrate, cytochrome c. The Vmax values for both substrates were reduced as a function of increasing age, perhaps reflecting a concomitant decline in the relative amount(s) of efficient reductase in the microsomes.