Tesan M, Anceschi M M, Bleasdale J E
Biochem J. 1985 Dec 15;232(3):705-13. doi: 10.1042/bj2320705.
Phosphatidylcholine synthesis by rat type II pneumonocytes was altered either by depleting the cells of choline or by exposing the cells to extracellular lung surfactant. Effects of these experimental treatments on the activity of a regulatory enzyme, CTP:phosphocholine cytidylyltransferase, were investigated. Although choline depletion of type II pneumonocytes resulted in inhibition of phosphatidylcholine synthesis, cytidylyltransferase activity (measured in cell homogenates in either the absence or presence of added lipids) was greatly increased. Activation of cytidylyltransferase in choline-depleted cells was rapid and specific, and was quickly and completely reversed when choline-depleted cells were exposed to choline (but not ethanolamine). Choline-dependent changes in enzymic activity were apparently not a result of direct actions of choline on cytidylyltransferase and they were largely unaffected by cyclic AMP analogues, oleic acid, linoleic acid or cycloheximide. The Km value of cytidylyltransferase for CTP (but not phosphocholine) was lower in choline-depleted cells than in choline-repleted cells. Subcellular redistribution of cytidylyltransferase also was associated with activation of the enzyme in choline-depleted cells. When measured in the presence of added lipids, 66.5 +/- 5.0% of recovered cytidylyltransferase activity was particulate in choline-depleted cells but only 34.1 +/- 4.5% was particulate in choline-repleted cells. An increase in particulate cytidylyltransferase also occurred in type II pneumonocytes that were exposed to extracellular surfactant. This latter subcellular redistribution, however, was not accompanied by a change in cytidylyltransferase activity even though incorporation of [3H]choline into phosphatidylcholine was inhibited by approx. 50%. Subcellular redistribution of cytidylyltransferase, therefore, is associated with changes in enzymic activity under some conditions, but can also occur without a resultant alteration in enzymic activity.
通过耗尽胆碱或使细胞暴露于细胞外肺表面活性物质来改变大鼠II型肺细胞的磷脂酰胆碱合成。研究了这些实验处理对一种调节酶CTP：磷酸胆碱胞苷转移酶活性的影响。虽然II型肺细胞的胆碱耗尽导致磷脂酰胆碱合成受到抑制，但胞苷转移酶活性（在添加脂质或不添加脂质的情况下在细胞匀浆中测量）却大大增加。胆碱耗尽细胞中胞苷转移酶的激活迅速且具有特异性，当胆碱耗尽细胞暴露于胆碱（而非乙醇胺）时，这种激活会迅速且完全逆转。酶活性的胆碱依赖性变化显然不是胆碱对胞苷转移酶直接作用的结果，并且它们在很大程度上不受环磷酸腺苷类似物、油酸、亚油酸或环己酰亚胺的影响。胆碱耗尽细胞中胞苷转移酶对CTP（而非磷酸胆碱）的Km值低于胆碱充足的细胞。胞苷转移酶的亚细胞重新分布也与胆碱耗尽细胞中该酶的激活有关。在添加脂质的情况下测量时，回收的胞苷转移酶活性中66.5±5.0%在胆碱耗尽细胞中是颗粒状的，但在胆碱充足的细胞中只有34.1±4.5%是颗粒状的。暴露于细胞外表面活性物质的II型肺细胞中也出现了颗粒状胞苷转移酶的增加。然而，尽管[3H]胆碱掺入磷脂酰胆碱受到约50%的抑制，但后一种亚细胞重新分布并未伴随胞苷转移酶活性的变化。因此，胞苷转移酶的亚细胞重新分布在某些情况下与酶活性的变化有关，但也可能在酶活性无相应改变的情况下发生。
