Zablocki K, Miller S P, Garcia-Perez A, Burg M B
Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD 20892.
Proc Natl Acad Sci U S A. 1991 Sep 1;88(17):7820-4. doi: 10.1073/pnas.88.17.7820.
Although GPC has long been recognized as a degradation product of phosphatidylcholine, only recently is there wide appreciation of its role as a compatible and counteracting osmolyte that protects cells from osmotic stress. GPC is osmotically regulated in renal cells. Its level varies directly with extracellular osmolality. Cells in the kidney medulla in vivo and in renal epithelial cell cultures (MDCK) accumulate large amounts of GPC when exposed to high concentrations of NaCl and urea. Osmotic regulation of GPC requires choline in the medium, presumably as a precursor for synthesis of GPC. Choline transport into the cells, however, is not osmoregulated. The purpose of the present studies was to use MDCK cell cultures as a defined model to distinguish whether osmotically induced accumulation of GPC results from increased GPC synthesis or decreased GPC disappearance. The rate of incorporation of 14C from [14C]choline into GPC, the steady-state GPC synthesis rate, and the activity of phospholipase A2 (which can catalyze a step in the synthesis of GPC from phosphatidylcholine) are not increased by high NaCl and urea. In fact all are decreased by approximately one-third. Therefore, we find no evidence that high NaCl and urea increases the GPC synthesis rate. On the other hand, the rate coefficient for cellular GPC disappearance and the activity of GPC:choline phosphodiesterase (EC 3.1.4.2), which catalyzes degradation of GPC, are decreased by approximately two-thirds by high NaCl and urea. We conclude that high NaCl and urea increase the level of GPC by inhibiting its enzymatic degradation.
尽管甘油磷酰胆碱（GPC）长期以来一直被认为是磷脂酰胆碱的降解产物，但直到最近人们才广泛认识到它作为一种相容性和对抗性渗透溶质的作用，这种作用能保护细胞免受渗透压应激。GPC在肾细胞中受到渗透压调节。其水平与细胞外渗透压直接相关。体内肾髓质细胞以及肾上皮细胞培养物（MDCK）在暴露于高浓度氯化钠和尿素时会积累大量GPC。GPC的渗透压调节需要培养基中的胆碱，推测胆碱是GPC合成的前体。然而，胆碱向细胞内的转运不受渗透压调节。本研究的目的是使用MDCK细胞培养物作为一个明确的模型，以区分渗透压诱导的GPC积累是由于GPC合成增加还是GPC消失减少所致。高浓度氯化钠和尿素并不会增加[14C]胆碱中14C掺入GPC的速率、GPC的稳态合成速率以及磷脂酶A2（可催化从磷脂酰胆碱合成GPC的一个步骤）的活性。事实上，所有这些都降低了约三分之一。因此，我们没有发现高浓度氯化钠和尿素会增加GPC合成速率的证据。另一方面，细胞内GPC消失的速率系数以及催化GPC降解的GPC：胆碱磷酸二酯酶（EC 3.1.4.2）的活性，在高浓度氯化钠和尿素作用下降低了约三分之二。我们得出结论，高浓度氯化钠和尿素通过抑制GPC的酶促降解来提高其水平。
