Asymmetric Localization of Alkyldihydroxyacetone-P Synthase and Acyldihydroxyacetone-P Acyltransferase in Microsomal Vesicles

A number of investigations have made it clear that proteases and phospholipases can be used effectively in combination with detergents to determine the topographical distribution of microsomal membrane components. For example, experiments of this type have shown that glucose-6-phosphatase and nucleoside diphosphatase are located on the lumenal side of microsomal vesicles, whereas cytochrome b 5 resides at the surface [1]. Similar asymmetry is seen with lipids. Studies done with phospholipase A2 indicate phosphatidylserine and phosphatidylethanolamine are localized on the cytoplasmic side of microsomal membranes, in contrast to phosphatidylinositol, sphingomyelin, and cardiolipin, which are at the inner surface; on the other hand, phosphatidylcholine appears to be equally distributed between the two microsomal surfaces [2]. These types of membrane-mapping experiments are based on the fact that microsomes prepared from tissue homogenates are vesicular in nature, with their cisternal content preserved [3]. Furthermore, low detergent concentrations render the vesicles permeable to macromolecules [4,5] or ions [6,7] without causing significant loss of phospholipids or detachment of ribosomes [4,5]. Under these conditions, the effect of detergents is reversible, since the membranes reseal after the detergents are removed so that macromolecules can no longer penetrate to the cisternal side [6,7].