Drug-induction decreases the mobility of cytochrome P-450 in rat liver microsomes: protein rotation study

Abstract

Effect of drug-induction on the rotation of cytochrome P-450 and on lipid fluidity in rat liver microsomes was examined. Rotational diffusion of cytochrome P-450 was examined by observing the decay of absorption anisotropy, r(t), after photolysis of the heme.CO complex by a vertically polarized laser flash. Analysis of r(t) was based on a "rotation-about-membrane normal" model. Microsomal lipid fluidity was measured by observing fluorescence anisotropy of DPH incorporated in the lipid bilayer. The absorption anisotropy decayed within 2 ms to a time-independent value. Rotational diffusion of cytochrome P-450 was dependent on the drug-induction with PB, MC, and PCB when compared with non-induced CON-microsomes. The observed values for the normalized time-independent anisotropy r(infinity)/r(0) are r(infinity)/r(0) = 0.41 (CON-microsomes), 0.54 (PB-microsomes), 0.52 (MC-microsomes), and 0.57 (PCB-microsomes). The average rotational relaxation time phi = 580-690 microseconds was almost unchanged over all microsomes presently examined. A significantly high value of r(infinity)/r(0) = 0.41-0.57 implies the co-existence of mobile and immobile populations of cytochrome P-450. Based on the assumption that the heme tilts about 55 degrees from the membrane plane for all species of P-450s besides P-450PB, 59% (CON-microsomes), 46% (PB-microsomes), 48% (MC-microsomes), and 43% (PCB-microsomes), respectively, of the cytochrome P-450 in microsomes is calculated to be mobile. Upon drug-induction the microsomal membrane was fluidized to some extent as judged by the steady-state fluorescence anisotropy of 0.156 for CON-microsomes and 0.139-0.148 for drug-induced microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)