Temperature Dependence of Cytochrome P-450 Reduction

The NADPH-dependent reduction of rat hepatic microsomal cytochrome P-450 has been studied as a function of temperature. In the temperature range 4-37’ the reduction reaction was found to be biphasic and composed of two concurrent first order processes. This phenomenon was observed with microsornes from untreated and phenobarbital-induced animals in the presence or absence of exogenous Type I substrates. The amount of cytochrome P-450 reduced in the fast phase comprised approximately 70% of the total cytochrome P-450 at temperatures above 20”. The temperature dependence of the fast phase was unusual for a membrane-bound enzyme system in that it lacked a discontinuity in the Arrhenius plot at a presumed phase transition temperature for the microsomal membrane. The slow phase of reduction behaved in a normal fashion for a membrane-bound enzyme system with a break in the Arrhenius plot at about 20”. The data presented here combined wit,h previous observations which include (a) the ratio of cytochrome P-450 to NADPH cytochrome P-450 reductase (NADPH:ferricytochrome oxidoreductase, EC 1.6.2.4) is 20:1, (b) the catalytic portion of the reductase molecule probably protrudes above the surface of the membrane, and (c) the cytochrome P-450 molecules are presumably embedded in the membrane support the hypothesis that the hepatic microsomal drug-metabolizing system exists as clusters with most of the cytochrome P-450 molecules arranged about a central reductase molecule. This central flavoprotein reductase is able to randomly reduce those cytochrome P-450 molecules within the cluster without translational motion through the microsomal membrane. The slow phase of reduction represents the reduction of those molecules not directly associated with the clusters.