SPHINGOSINE INHIBITS RAT HEPATIC MONOACYLGLYCEROL ACYLTRANSFERASE IN TRITON X-100 MIXED MICELLES AND ISOLATED HEPATOCYTES

Hepatic monoacylglycerol acyltransferase (MGAT), a developmentally-reg ulated microsomal activity that catalyzes the synthesis of sn-1,2-diac ylglycerol, is regulated by anionic phospholipids and sn-1,2-diacylgly cerol in Triton X-100 mixed micelles. Sphingomyelin stimulated MGAT ac tivity, whereas sphingosine, sphinganine, phytosphingosine, and steary lamine were inhibitors (IC50 of 9, 5.5, 5, and 6 mol %, respectively). Since ceramide and octylamine had relatively little effect, inhibitio n appears to require a free amino group and a long-chain hydrocarbon. Inhibition by sphingosine was competitive with respect to phosphatidic acid, phosphatidylinositol, or phosphatidylserine, suggesting that an ionic phospholipids may activate MGAT at a specific site that is compe titively blocked by sphingolipids. Both sphingosine and sphinganine in hibited MGAT activity in cultured hepatocytes from 10-day-old rats in a dose-dependent manner. Stimulation of MGAT activity by diacylglycero l was specific for sn-1,2-stereoisomers that contained two long fatty acyl chains. The diacylglycerol analogs phorbol 12-myristyl 13-acetate and ceramide had no effect. The highly cooperative activation of MGAT by sn-1,2-diacylglycerol was also inhibited by sphingosine. It is unl ikely that activation of MGAT by low molar concentrations of anionic p hospholipids is solely due to electrostatic interactions between the e nzyme and negatively charged lipids because high ionic strength, neomy cin, and Ca2+ had similar effects on enzyme activity irrespective of t he presence or absence of phosphatidic acid. These data suggest that M GAT activity may be regulated physiologically by specific intermediate s of glycerolipid metabolism and that, in neonatal rat liver, signal t ransduction may be linked to the synthesis of complex lipids via the m onoacylglycerol pathway.