The regulation and cellular functions of phosphatidylcholine hydrolysis

Abstract

PHOSPHODIESTERASES THAT HYDROLYSE PtdChoPhosphodiesteratic cleavage of PtdCho occurs by two distinct types of phospholipases.A phospholipase C activity produces DG and phosphocholine, whereas a phospholipase D activity generates PtdOH and choline.In addition to hydrolysis, phospholipase D catalyses a unique transfer reaction whereby the phosphatidyl moiety of the phospholipid substrate is trans- ferred to primary alcohols to produce phosphatidylalcohols [20,21].This transphosphatidylation may be considered the general reaction, with hydrolysis representing a specific case in which the acceptor is water. Mammalian phospholipase CCell-free preparations from a variety of rat tissues exhibit phospholipase C activities that degrade PtdCho [22-26].Phospholipase C activities of lysosomal origin with acid pH optima also hydrolyse phosphatidylethanolamine (PtdEtn), phosphatidylinositol and phosphatidylglycerol [22].PtdCho- cleaving phospholipases C with alkaline pH optima have been detected in rat brain cytosol [23] and rat liver membranes [24].Cell-free preparations from endothelial cells also degrade exo- genous PtdCho [25,26].More recently, phospholipases C that utilize PtdCho as substrate have been partially purified from dog heart cytosol [27], bull seminal plasma [28] and promonocytic U937 cells [29].Utilizing exogenous phospholipids as substrates, these activities exhibit neutral pH optima and do not hydrolyse Vol.269