DRUG-INDUCTION DECREASES THE MOBILITY OF CYTOCHROME-P-450 IN RAT-LIVER MICROSOMES - PROTEIN ROTATION STUDY

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-?? (PB-microsomes), 0.52 (MC-microsomes), and 0.57 (PCB-microsomes). The average rotational relaxation time phi = 580-690-mu-s 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-450(PB), 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. The present observations imply that the drug-inductions with PB, MC, and PCB cause an immobilization of cytochrome P-450 molecules which might be due to increased protein-protein interactions of the drug-induced P-450 molecules, because no rigidification in the lipid due to the induction was observed.