Monoglyceride and diglyceride lipases from human platelet microsomes

In the present study, we have characterized the properties of both diglyceride lipase (lipoprotein lipase, EC 3.1.1.24) and monoglyceride lipases (acylglycerol lipase, EC 3.1.1.23) in an attempt to assess the potential roles of these two enzymes in the release of arachidonate in activated human platelets. Diglyceride lipase exhibited maximal activity at pH 3.5, whereas monoglyceride lipase showed optimal activity at pH 7.0. Neither of the lipases were inhibited by EDTA or stimulated by Ca2+, Mg2+ or Mn2+. Both enzymes, however, were strongly inhibited by Hg2+ and Cu2+, indicating the involvement of sulfhydryl groups in catalytic activity. This suggestion was further supported by their sensitivity toward sulfhydryl inhibitors, with monoglyceride lipase being more susceptible to inhibition. Both lipases were found to be inhibited to a different degree by a variety of antiplatelet drugs blocking aggregation and arachidonate release. Kinetic studies indicated that dichotomous metabolism of diacylglycerol to monoacylglycerol and to phosphatidic acid could occur concurrently, since the apparent Km values for diglyceride lipase and for diglyceride kinase were comparable. Further studies showed that the specific activity of monoglyceride lipase was at least 100-fold higher than that of diglyceride lipase, indicating that the rate-limiting step in the release of arachidonate was the reaction catalyzed by diglyceride lipase.