Bg. Bhat et al., SPHINGOSINE INHIBITS RAT HEPATIC MONOACYLGLYCEROL ACYLTRANSFERASE IN TRITON X-100 MIXED MICELLES AND ISOLATED HEPATOCYTES, Biochemistry, 34(35), 1995, pp. 11237-11244
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.