Thermotropic transitions in fluidity of bovine adrenocortical microsomal membrane and substrate-cytochrome P-450 binding reaction.

Temperature dependence of the substrate-cytochrome P-450 binding reaction is investigated in the adrenocortical microsomes using 17-hydroxyprogesterone as the substrate. The apparent substrate dissociation constant ("Ks") increases with increase in temperature. Van't Hoff's plot of "Ks" shows breaks at 21 and 31 degrees C. These temperatures correlate well with the lipid phase transition temperatures as determined by the technique of fluorescence polarization using 1,6-diphenyl-1,3,5-hexatriene as the probe. The results indicate that the affinity of the substrate depends upon the temperature. In addition the results suggest that changes in the physical state of the lipids is a factor responsible for the temperature dependency of the reaction.