Igal R A, Wang P, Coleman R A
Departments of Nutrition and Pediatrics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7400, USA.
Biochem J. 1997 Jun 1;324 ( Pt 2)(Pt 2):529-34. doi: 10.1042/bj3240529.
The trafficking of acyl-CoAs within cells is poorly understood. In order to determine whether newly synthesized acyl-CoAs are equally available for the synthesis of all glycerolipids and cholesterol esters, we incubated human fibroblasts with [14C]oleate, [3H]arachidonate or [3H]glycerol in the presence or absence of triacsin C, a fungal metabolite that is a competitive inhibitor of acyl-CoA synthetase. Triacsin C inhibited de novo synthesis from glycerol of triacylglycerol, diacylglycerol and cholesterol esters by more than 93%, and the synthesis of phospholipid by 83%. However, the incorporation of oleate or arachidonate into phospholipids appeared to be relatively unimpaired when triacsin was present. Diacylglycerol acyltransferase and lysophosphatidylcholine acyltransferase had similar dependences on palmitoyl-CoA in both liver and fibroblasts; thus it did not appear that acyl-CoAs, when present at low concentrations, would be preferentially used to acylate lysophospholipids. We interpret these data to mean that, when fatty acid is not limiting, triacsin blocks the acylation of glycerol 3-phosphate and diacylglycerol, but not the reacylation of lysophospholipids. Two explanations are possible: (1) different acyl-CoA synthetases exist that vary in their sensitivity to triacsin; (2) an independent mechanism channels acyl-CoA towards phospholipid synthesis when little acyl-CoA is available. In either case, the acyl-CoAs available to acylate cholesterol, glycerol 3-phosphate, lysophosphatidic acid and diacylglycerol and those acyl-CoAs that are used by lysophospholipid acyltransferases and by ceramide N-acyltransferase must reside in two non-mixing acyl-CoA pools or, when acyl-CoAs are limiting, they must be selectively channelled towards specific acyltransferase reactions.
细胞内酰基辅酶A的转运机制目前还知之甚少。为了确定新合成的酰基辅酶A是否能同等程度地用于合成所有甘油脂和胆固醇酯，我们在有或没有三辛素C（一种真菌代谢产物，是酰基辅酶A合成酶的竞争性抑制剂）存在的情况下，用[14C]油酸、[3H]花生四烯酸或[3H]甘油孵育人成纤维细胞。三辛素C抑制了甘油三酯、二酰基甘油和胆固醇酯从甘油的从头合成，抑制率超过93%，对磷脂合成的抑制率为83%。然而，当存在三辛素时，油酸或花生四烯酸掺入磷脂的过程似乎相对未受影响。二酰基甘油酰基转移酶和溶血磷脂酰胆碱酰基转移酶在肝脏和成纤维细胞中对棕榈酰辅酶A的依赖性相似；因此，当酰基辅酶A浓度较低时，似乎不会优先用于酰化溶血磷脂。我们对这些数据的解释是，当脂肪酸不存在限制时，三辛素会阻断3-磷酸甘油和二酰基甘油的酰化，但不会阻断溶血磷脂的再酰化。有两种可能的解释：（1）存在对三辛素敏感性不同的不同酰基辅酶A合成酶；（2）当酰基辅酶A供应不足时，一种独立的机制会引导酰基辅酶A用于磷脂合成。在任何一种情况下，可用于酰化胆固醇、3-磷酸甘油、溶血磷脂酸和二酰基甘油的酰基辅酶A，以及溶血磷脂酰基转移酶和神经酰胺N-酰基转移酶所使用的那些酰基辅酶A，必定存在于两个不混合的酰基辅酶A池中，或者当酰基辅酶A受到限制时，它们必须被选择性地引导至特定的酰基转移酶反应中。
