The hepatic microsomes of rat and mice were subfractionated by the procedure of Dallner. When a 1.3 M sucrose lower layer was used for the two-step discontinuous gradient, no differences in spectral characteristics were noted between subfractions, though the smooth fractions (SER) had higher oxidative activity towards the substrates tested. When lower layers of 1.05, 1.1 or 1.15 M sucrose were used, the SER isolated contained cytochrome P-450 with significantly different spectral characteristics from that of the rough fraction (RER). The SER cytochrome P-450 had a wavelength maximum in the carbon-monoxide reduced difference spectrum that was significantly lower (ca. 1.0 nm) than that in the RER. In addition, the type I:CO-reduced spectral ratio of these fractions is significantly elevated. These data indicate that liver microsomes from untreated rats and mice contain more than one cytochrome P-450 and that these cytochromes may be located in different parts of the endoplasmic reticulum.
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