Ra. Coleman et al., DIRADYLGLYCEROLS ALTER FATTY-ACID INHIBITION OF MONOACYLGLYCEROL ACYLTRANSFERASE ACTIVITY IN TRITON X-100 MIXED MICELLES, Biochemistry, 37(17), 1998, pp. 5916-5922
The activity of hepatic monoacylglycerol acyltransferase (MGAT) (EC 2.
3.1.22), a developmentally expressed microsomal enzyme, is inhibited b
y long-chain fatty acids, and stimulated by its product 1,2-diacyl-sn-
glycerol. Because the quantities of fatty acids and diacylglycerols ar
e likely to vary in membranes during different physiological condition
s and could thereby alter MGAT activity, we examined their combined ef
fects on MGAT in Triton X-100/phospholipid mixed micelles, MGBT's prod
uct, 1,2-diC18:1-sn-glycerol, which is also normally a cooperative act
ivator of the activity, reversed the 50% inhibition caused by 10 mol %
oleic acid. The presence of oleic acid also allowed low concentration
s (<10 mol %) of 1,2-diC18: 1-sn-glycerol to stimulate MGAT activity w
ithout the lag that is observed in the absence of fatty acid. At 12.6
mol %, 1,2-monoC18:1-sn-glycerol ether, which alone has no effect on M
GAT activity, became an activator in the presence of 10 mol % oleic ac
id. Kinetic studies revealed that in the presence of 15 mol % oleic ac
id, 1,2-diC18:1-sn-glycerol ether increased the apparent V-max by 3.8-
fold while minimally altering the apparent K-m for palmitoyl-CoA. Othe
r neutral lipids including triC18:1-glycerol, ceramide, and cholestero
l oleate did not stimulate MGAT in either the presence or the absence
of fatty acid. Assay conditions altered MGAT's apparent relative prefe
rences for potential monoradylglycerol substrates. The presence of pho
spholipids and of MGAT's 1,2-diacyl-sn-glycerol product increased the
enzyme's apparent preference for its 2-monoacyl-sn-glycerol substrate
by selectively increasing the apparent V-max 2.7-fold only when 2-mono
C18:1-sn-glycerol was the substrate. Thus, in addition to previously r
eported regulation of MGAT by phospholipids and intracellular lipid se
cond messengers, these studies lend additional support to the hypothes
is that changes in other membrane-associated lipids, such as long-chai
n fatty acids and diradylglycerols, may also profoundly alter the acti
vity of MGAT.