The degradation of platelet-activating factor and related lipids: susceptibility to phospholipases C and D.
The degradation of platelet-activating factor and related lipids: susceptibility to phospholipases C and D.
复制标题
血小板激活因子和相关脂质的降解：对磷脂酶 C 和 D 的敏感性。
作者：
Wilcox,RW;Wykle,RL;Schmitt,JD;Daniel,LW
1-O-Octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) is an ether-linked lipid that exhibits selective cytotoxicity toward several types of tumor cells and is relatively inactive toward normal cells under the same conditions of treatment. The mechanism of this selective cytotoxicity is unknown. We conducted studies to determine whether this compound is metabolized by phospholipases C and D and, if so, whether sensitive and resistant cells differ in their ability to degrade ET-18-OCH3by these enzymes. We have examined the metabolism of the L-isomer of ET-18-OCH3, 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (L-ET-18-OCH3), by lysophospholipase D of rat liver microsomes and by a phospholipase D from the marine bacteriumVibrio damsela. The metabolism of L-ET-18-OCH3was also examined in cell culture using Madin-Darby canine kidney cells, human promyelocytic leukemia cells and human myelocytic leukemia cells. In these studies, L-ET-18-OCH3and related 1-O-alkyl-linked phosphocholine analogs radiolabeled with3H in the 1-O-alkyl chain were used.L-ET-18-OCH3was not hydrolyzed by lysophospholipase D from rat liver microsomes under conditions where cleavage of 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine was observed. However, phospholipase D from the marine bacteriumV. damselareadily hydrolyzed L-ET-18-OCH3to 1-O-[3H]octadecyl-2-O-methyl-sn-glycero-3-phosphate, demonstrating that L-ET-18-OCH3can be degraded by a phospholipase D. Platelet-activating factor (PAF) and lyso-PAF were also substrates for the bacterial phospholipase D.When intact cells were incubated with radiolabeled L-ET-18-OCH3a product was formed that was identified as 1-O-[3H]octadecyl-2-O-methyl-sn-glycerol. There are two mechanisms that could account for the appearance of this product. The first involves cleavage of the compound by a phospholipase C, resulting in direct release of the diglyceride. The second possible mechanism involves cleavage by a phospholipase D to form the phosphatidic acid analog with subsequent hydrolysis to the diglyceride by a phosphohydrolase. Preliminary data support the phospholipase C-type mechanism. Regardless of which mechanism operates in intact cells, the metabolic degradation of L-ET-18-OCH3does not appear to be a significant factor in the selective cytotoxicity of this antitumor agent.
