Lankester D L, Brown A M, Zammit V A
Hannah Research Institute, Ayr, Scotland, UK.
J Lipid Res. 1998 Sep;39(9):1889-95.
The fatty acyl moieties incorporated into triacylglycerol (TAG) secreted by rat hepatocytes are derived from diacylglycerol (DAG) that is synthesized de novo through the phosphatidate pathway or derived from endogenous, cytosolic TAG after hydrolysis to DAG and re-esterification. We have used a dual-labeling technique (overnight labeling of cell TAG with [3H]oleate, followed by 3 h incubation with [14C]oleate) to quantify the contributions of the two sources towards TAG secretion by cultured rat hepatocytes. A wide range of TAG secretion rates was achieved by short-term incubation of the cells under a variety of conditions. There was no correlation between the overall amount of exogenous 14C-labeled fatty acid metabolized and the rate of either [14C]- or [3H]TAG secretion. By contrast, there was a strong positive correlation between the fraction of newly synthesized [14C]TAG that was secreted (the fractional secretion rate, FSR) and the absolute rate of TAG secretion. This suggests that the partitioning of DAG between (re)synthesis of cytosolic TAG and synthesis of secreted TAG is an important locus for the control of the rate of TAG secretion. Comparison of the ratio: oxidation/TAG secretion for 3H- and 14C-labeled acyl moieties indicated that, for all conditions studied, approximately half the acyl moieties already esterified to the glyceroyl backbone within cytosolic TAG remain unavailable for oxidation when this pool of TAG is mobilized for the synthesis of secreted TAG. The data provide evidence that hydrolysis of cytosolic triacylglycerol (TAG) does not proceed fully to the constituent fatty acids and glycerol, but only to the level of diacylglycerol, followed by re-modelling of approximately half of its acyl chains, before re-esterification to form secretory TAG.
大鼠肝细胞分泌的三酰甘油（TAG）中所含的脂肪酰基部分，来源于通过磷脂酸途径从头合成的二酰甘油（DAG），或来源于内源性胞质TAG水解为DAG后再酯化。我们采用了一种双标记技术（先用[3H]油酸对细胞TAG进行过夜标记，然后用[14C]油酸孵育3小时）来量化这两种来源对培养的大鼠肝细胞TAG分泌的贡献。通过在各种条件下对细胞进行短期孵育，实现了广泛的TAG分泌率。外源14C标记脂肪酸代谢的总量与[14C] - 或[3H]TAG分泌率之间没有相关性。相比之下，分泌的新合成[14C]TAG的比例（分数分泌率，FSR）与TAG分泌的绝对速率之间存在很强的正相关。这表明胞质TAG的（再）合成与分泌TAG的合成之间DAG的分配是控制TAG分泌速率的一个重要位点。对3H和14C标记的酰基部分的氧化/TAG分泌比率的比较表明，在所研究的所有条件下，当这部分TAG被动员用于分泌TAG的合成时，已经酯化到胞质TAG甘油主链上的酰基部分中约一半不能用于氧化。数据提供了证据，表明胞质三酰甘油（TAG）的水解并没有完全进行到组成脂肪酸和甘油，而只进行到二酰甘油水平，随后其约一半的酰基链进行重塑，然后再酯化形成分泌性TAG。
