THE ASSOCIATION OF CHOLESTEROL, 5'-NUCLEOTIDASE, AND ALKALINE PHOSPHODIESTERASE I WITH A DISTINCT GROUP OF MICROSOMAL PARTICLES

Abstract

5,aminopyrine demethylase, cytochrome P450 ),and 40-50% of the homogenate content in cholesterol, 5'-nucleotidase, and alkaline phosphodiesterase I (acting on p-nitrophenyl 5'-thymidylate) . The latter three components exhibit a nucleo-microsomal distribution in the sense that the amount recovered in the nuclear fraction is much higher than that expected for particles of microsome size or observed for typical microsomal constituents . Since the high activity of 5'-nucleotidase and the abundance of cholesterol are characteristic properties of purified plasma membrane preparations isolated from low speed sediments (for a review see ref . 5), the obvious interpretation for the presence of these components in the nuclear fraction is that they belong to plasma membrane fragments of large size that sediment together with the nuclei and large cell debris . In line with this interpretation, which is widely accepted, it is tempting to assume that small fragments of plasma membrane sedimenting together with the fragments of the endoplasmic reticulum are responsible for the microsomal localization of most of the remaining cholesterol, 5'-nucleotidase, and alkaline phosphodiesterase I