Dolichyl phosphate phosphatase in rat liver microsomes. Avoidance of the use of detergent in testing the effect of phospholipids on dolichyl phosphate phosphatase

Abstract

A system was developed for testing the effect of phospholipids on dolichyl phosphate phosphatase, a membrane-associated enzyme. This enzyme was solubilized, delipidated, stabilized and concentrated in such a way that minimal quantities of Triton X-100 were carried by enzyme extracts to the incubation mixture. Its substrate, dolichyl phosphate, could be kept in aqueous medium as suspended particles without addition of detergent. When dolichyl phosphate phosphatase was assayed using the substrate in this detergent-free form, values for Km, pH optimum and temperature optimum were different from those obtained with detergent-solubilized substrate. This assay of dolichyl phosphate phosphatase almost free of detergent allowed testing of the effect of specific phospholipids on enzyme activity with minimal interference produced by endogenous phospholipids or exogenous detergent. Sphingomyelin, phosphatidylethanolamine or phosphatidylcholine (zwitterionic phospholipids) acted as activators, whereas phosphatidic acid and phosphatidylinositol, negatively-charged phospholipids, were inhibitors of dolichyl phosphate phosphatase.