Rat liver cytochrome P-450IA1 and/or yeast NADPH-ytochrome P-450 reductase was expressed genetically in yeast microsomes. The ratio of P-450IA1 to the reductase was about 17:l and 1:2 without and with coexpression of the reductase, respectively. Rotational diffusion of P-450IA1 was examined by observing the flash-induced absorption anisotropy, r(t), of the heme420 complex. In only P-450IAl-expressed microsomes, 28% of P-450IA1 was rotating with a rotational relaxation time (4) of about 1200 ~s

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... The mobile population was increased to 43% by the presence of the coexpressed reductase, while 4 was not changed significantly. Increased concentration of KCl from 0 to 1000 mM caused considerable mobilization of I Abbreviations: DPH, 1,6-diphenyl-l,3,5-hexatriene; L / P , lipid-toprotein weight ratio; MC, 3-methylcholanthrene; PB, phenobarbital; PCB, polychlorinated biphenyl; cytochrome P-450IA1, major type of rat liver P-450 in 3-methylcholanthrene-induced microsomes; cytochromes P-45OPB and P-45OMC, major types of rat liver P-450s in PB-and MCinduced microsomes, respectively (P-45hB is a mixture of P-450IIB1 and P-45011B2; P-45OMC is a mixture of P-450IAl and P-450IA2); P-450, and P-45OI1" cytochrome P-450s in adrenal cortex, catalyzing cholesterol side-chain cleavage and steroid 1 lj3-hydroxylation, respectively.