Purification and properties of a monoacylglycerol lipase in human erythrocytes.

A membrane-bound monoacylglycerol lipase (MAGL) activity, previously demonstrated in intact human erythrocytes [Boyer, Somma, Vérine, L'Hôte, Finidori, Merger and Arnaud (1981) J. Clin. Endocrinol. Metab. 53, 143-148], has now been purified to apparent homogeneity by a five-step procedure involving solubilization in CHAPS and sequential chromatographies on Sephacryl S-400, DEAE-Trisacryl, Zn(2+)-chelating Sepharose and Superose 12 columns. The purified protein has a molecular mass of 68 +/- 2 kDa, as determined by SDS/PAGE and gel filtration, suggesting that the enzyme behaves as a monomer. The concentration-dependence of MAGL activity with monooleoylglycerol, the preferred substrate showed kinetics typical of an interfacial lipolytic enzyme displaying optimal activity on emulsified substrate particles; apparent Km values were 0.27 mM and 0.49 mM for the sn-1(3)- and sn-2-isomers respectively. MAGL had no, or negligible, activity towards tri-oleoylglycerol, di-oleoylglycerol, oleoylcholesterol, oleoyl-CoA and phosphatidylcholine; it was inhibited by di-isopropylfluorophosphate, PMSF and diethyl p-nitrophenyl phosphate, suggesting that MAGL is a serine hydrolase. MAGL activity was not modified by bile salt or apolipoprotein C-II, whereas a dose-dependent inhibition was observed with apolipoprotein A-I.

Triacylglycerol in biomembranes.

A better knowledge of the biochemical and biophysical properties of cell membranes has revealed fundamental concepts concerning the regulation of cell functions by intrinsic components of the lipid matrix. Membrane lipids exhibit high chemical heterogeneity, with hundreds of distinct chemical species; studies of structure-function relationships have unraveled new roles for an increasing number of these lipids as determinants of membrane structure, anchors for membrane-associated proteins or signalling agents. Recent observations have confirmed triacylglycerol (TG) as a quantitatively minor intrinsic membrane component which seems to play a specific role in important metabolic events such as cell stimulation or transformation and metastatic processes. The rapid turnover of the acyl chains into TG of cell membranes suggests an active metabolism. In the plasma membrane, TG appears to be implicated in the generation of transient non-bilayer domains suspected to be associated with specific cellular events. This paper summarizes the current information on TG metabolism and focuses on the potential role of this neutral lipid species on the structure and function of cell membranes.