Burkhardt R, Von Wichert P, Batenburg J J, Van Golde L M
Department of Internal Medicine, Philipps University, Marburg, Federal Republic of Germany.
Biochem J. 1988 Sep 1;254(2):495-500. doi: 10.1042/bj2540495.
The regulation by cyclic AMP and fatty acids of phosphatidylcholine (PC) synthesis in rat alveolar type II cells was studied. In contrast with results with hepatocytes, cyclic AMP and its potent chlorophenylthio analogue had no inhibitory effect on [Me-14C]choline incorporation into PC in pulse-chase studies with alveolar type II cells. The inclusion of the fatty acids palmitate, oleate or linoleate in the chase incubation medium stimulated the incorporation of [Me-14C]choline into PC by type II cells. The effect of palmitate, which was more pronounced than that of the other fatty acids, appeared to be concentration-dependent. Increased [Me-14C]choline incorporation into PC was paralleled by an accelerated disappearance of the radiolabel from choline phosphate, which is consistent with an activation of CTP:choline-phosphate cytidylyltransferase. This enzyme is considered to be rate-limiting in the synthesis of PC de novo by type II cells. As fatty acids are also substrate for PC synthesis, their effect could also be due to compensation for a fatty acid deficiency. To test this possibility, fatty acid synthesis in the type II cells was stimulated by addition of lactate. Even then, an additional stimulation of PC synthesis by palmitate was observed, which supports the regulatory influence of exogenous fatty acids. Incubation of type II cells in the presence of 0.2 mM-palmitate resulted in a 45% increase in the membrane-bound CTP:choline-phosphate cytidylyltransferase activity, whereas the soluble activity remained unchanged. Choline kinase activity in the soluble fraction increased by 48%. However, the increase in choline kinase is unlikely to be responsible for the increased metabolic flux through the choline phosphate pathway, because there is a relatively large pool of choline phosphate in type II cells. Therefore it is suggested that the microsomal CTP:choline-phosphate cytidylyltransferase is the form of this enzyme which is active in surfactant PC synthesis, and possibly has a regulatory role in this pathway.
研究了环磷酸腺苷（cAMP）和脂肪酸对大鼠肺泡Ⅱ型细胞中磷脂酰胆碱（PC）合成的调节作用。与肝细胞的结果相反，在对肺泡Ⅱ型细胞进行脉冲追踪研究时，cAMP及其有效的氯苯硫代类似物对[甲基-14C]胆碱掺入PC没有抑制作用。在追踪孵育培养基中加入脂肪酸棕榈酸、油酸或亚油酸可刺激Ⅱ型细胞将[甲基-14C]胆碱掺入PC。棕榈酸的作用比其他脂肪酸更明显，似乎具有浓度依赖性。[甲基-14C]胆碱掺入PC的增加与放射性标记从磷酸胆碱中加速消失平行，这与CTP：胆碱磷酸胞苷转移酶的激活一致。该酶被认为是Ⅱ型细胞从头合成PC的限速酶。由于脂肪酸也是PC合成的底物，它们的作用也可能是由于补偿脂肪酸缺乏。为了检验这种可能性，通过添加乳酸刺激Ⅱ型细胞中的脂肪酸合成。即便如此，仍观察到棕榈酸对PC合成有额外的刺激作用，这支持了外源性脂肪酸的调节作用。在0.2 mM棕榈酸存在下孵育Ⅱ型细胞，导致膜结合的CTP：胆碱磷酸胞苷转移酶活性增加45%，而可溶性活性保持不变。可溶性部分中的胆碱激酶活性增加了48%。然而，胆碱激酶的增加不太可能是磷酸胆碱途径代谢通量增加的原因，因为Ⅱ型细胞中有相对大量的磷酸胆碱池。因此，有人认为微粒体CTP：胆碱磷酸胞苷转移酶是该酶在表面活性物质PC合成中具有活性的形式，并且可能在该途径中起调节作用。
