Abstract
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 degrees the reduction reaction was found to be biphasic and composed of two concurrent first order processes. This phenomenon was observed with microsomes 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 degrees. 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 degrees. The data presented here combined with 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.
Highlights
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
These data were analyzed using the nonlinear estimation procedure, and over the concentration range employed the fraction of cytochrome P-450 reduced in the fast phase is constant as are the first order rate constants for the fast and slow phases of reduction (Fig. 2)
The reductase could reduce several intermediate electron carriers which could in turn reduce the cytochromes P-450
Summary
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. 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.
Sign-in/Register to view highlights & summary 
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call