Coleman R A, Wang P, Bhat B G
Department of Nutrition, The University of North Carolina, Chapel Hill 27599-7400, USA.
Biochemistry. 1998 Apr 28;37(17):5916-22. doi: 10.1021/bi9802972.
The activity of hepatic monoacylglycerol acyltransferase (MGAT) (EC 2.3.1.22), a developmentally expressed microsomal enzyme, is inhibited by long-chain fatty acids, and stimulated by its product 1, 2-diacyl-sn-glycerol. Because the quantities of fatty acids and diacylglycerols are likely to vary in membranes during different physiological conditions and could thereby alter MGAT activity, we examined their combined effects on MGAT in Triton X-100/phospholipid mixed micelles. MGAT's product, 1,2-diC18:1-sn-glycerol, which is also normally a cooperative activator of the activity, reversed the 50% inhibition caused by 10 mol % oleic acid. The presence of oleic acid also allowed low concentrations (<10 mol %) of 1, 2-diC18:1-sn-glycerol to stimulate MGAT activity without 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 MGAT activity, became an activator in the presence of 10 mol % oleic acid. Kinetic studies revealed that in the presence of 15 mol % oleic acid, 1,2-diC18:1-sn-glycerol ether increased the apparent Vmax by 3. 8-fold while minimally altering the apparent Km for palmitoyl-CoA. Other neutral lipids including tri-C18:1-glycerol, ceramide, and cholesterol oleate did not stimulate MGAT in either the presence or the absence of fatty acid. Assay conditions altered MGAT's apparent relative preferences for potential monoradylglycerol substrates. The presence of phospholipids 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 Vmax 2.7-fold only when 2-monoC18:1-sn-glycerol was the substrate. Thus, in addition to previously reported regulation of MGAT by phospholipids and intracellular lipid second messengers, these studies lend additional support to the hypothesis that changes in other membrane-associated lipids, such as long-chain fatty acids and diradylglycerols, may also profoundly alter the activity of MGAT.
肝脏单酰甘油酰基转移酶（MGAT）（EC 2.3.1.22）是一种在发育过程中表达的微粒体酶，其活性受长链脂肪酸抑制，受其产物1,2 - 二酰基 - sn - 甘油刺激。由于在不同生理条件下，膜中脂肪酸和二酰甘油的量可能会发生变化，从而可能改变MGAT活性，因此我们在Triton X - 100/磷脂混合胶束中研究了它们对MGAT的联合作用。MGAT的产物1,2 - 二C18:1 - sn - 甘油，通常也是该活性的协同激活剂，可逆转由10 mol％油酸引起的50％抑制作用。油酸的存在还使得低浓度（<10 mol％）的1,2 - 二C18:1 - sn - 甘油能够刺激MGAT活性，而不会出现无脂肪酸时所观察到的延迟。在12.6 mol％时，单独对MGAT活性无影响的1,2 - 单C18:1 - sn - 甘油醚在存在10 mol％油酸时成为激活剂。动力学研究表明，在存在15 mol％油酸的情况下，1,2 - 二C18:1 - sn - 甘油醚使表观Vmax增加了3.8倍，而对棕榈酰辅酶A的表观Km影响最小。其他中性脂质，包括三 - C18:1 - 甘油、神经酰胺和胆固醇油酸酯，在有或无脂肪酸的情况下均不刺激MGAT。测定条件改变了MGAT对潜在单酰甘油底物的表观相对偏好。磷脂和MGAT的1,2 - 二酰基 - sn - 甘油产物的存在通过仅在2 - 单C18:1 - sn - 甘油作为底物时选择性地将表观Vmax提高2.7倍，增加了该酶对其2 - 单酰 - sn - 甘油底物的表观偏好。因此，除了先前报道的磷脂和细胞内脂质第二信使对MGAT的调节作用外，这些研究进一步支持了以下假设：其他与膜相关的脂质，如长链脂肪酸和二酰甘油的变化，也可能深刻改变MGAT的活性。
