Biological functions of mammalian non-pacreatic type Phospholipase A_2

已知哺乳动物细胞具有几种类型的磷脂酶A2。Ⅱ组磷脂酶A2检测到相当数量的大鼠腹膜肥大细胞。发现在细胞刺激时，该酶分泌到培养基中。肥大细胞的脱粒和类花生酸的产生被II组磷脂酶A2的抑制剂显著抑制。这些观察结果可能表明，这种类型的磷脂酶A2在脱粒过程的进展中发挥作用。分子量为85 kDa的胞质磷脂酶A2被认为是类花生酸和白三烯产生的限速酶。我们发现该酶不仅具有磷脂酶A2活性，而且具有溶血磷脂酶活性。与花生四烯酸的释放同时产生的溶血磷脂由于其洗涤剂样性质而对细胞有毒。细胞质中的85 kDa磷脂酶A_2可能通过其内在的溶血磷脂酶活性来分解这些脂质，我们在哺乳动物脑中发现了非钙依赖型磷脂酶A_2，而不是上述的钙依赖型磷脂酶A_2。此外，我们还成功地进行了酶的纯化和cDNA克隆。出乎意料的是，编码该亚基的基因与Miller-Dieker无脑畸形的致病基因相同。这种遗传性综合征可导致光滑脑的形成和早期死亡。这些结果提示，细胞内磷脂酶可能在中枢神经系统发育中起重要作用。

Mammalian cells are known to posseces several type of phospholipase A2. Group II phospholipase A2 was detected in appreciable amounts in rat perioneal mast cells. It was found that the enzyme was is secreted into medium upon stimulation of the cells. The degranulation and eicosanoid production by mast cells are significantly suppressed by the inhibitors of group II phospholipase A2. These observation may suggest that this type of phospholipase A2 play an role in the progression of the degranulation process. A cytosolic phospholipiase A2 with a molecular weight of 85kDa is thought to be a rate limiting enzyme for the production of eicosanoids and leukotrienes. We have discovered that the enzyme has not only phospholipase A2 activity but also lysophospholipase activity. The lysophospholipids produced concomitant with the relase of arachidonaic acid is toxic to the cells because of its detergent-like nature. The cytosolic 85kDa phospholipase A2 may scavenge such lipids by its intrinsic lysophospholipiase activity.Instead of Ca2+-dependent phospholipase A2 as mentioned above, we have discovered Ca2+-independent phospholipase A2 from mammalian brain. Moreover, we have succeeded in purification and cDNA cloning of the enzyme Unexpectedly, the gene encoding fothe one of the subunit is identical with the causative gene for the Miller-Dieker lissencephaly. This inherited syndrome caused a formation of smooth brain and a death in early age These results suggest that intrascellular phospholipase may play an essental role in the development of central nervous system.