Phosphatidylserine biosynthesis in cultured Chinese hamster ovary cells. III. Genetic evidence for utilization of phosphatidylcholine and phosphatidylethanolamine as precursors

In the preceding paper, we reported that Chinese hamster ovary (CHO) cells contain two different serine-exchange enzymes (I and II) which catalyze the base-exchange reaction of phospholipid(s) with serine and that a phosphatidylserine-requiring mutant (strain PSA-3) of CHO cells is defective in serine-exchange enzyme I and lacks the ability to synthesize phosphatidylserine. In this study, we examined precursor phospholipids for phosphatidylserine biosynthesis in CHO cells. When mutant PSA-3 and parent (CHO-K1) cells were cultured with [³²P]phosphatidylcholine, phosphatidylserine in the parent accumulated radioactivity while that in the mutant was not labeled significantly. On the contrary, when cultured with [³²P]phosphatidylethanolamine, the mutant incorporated the label into phosphatidylserine more efficiently than the parent. Furthermore, we found that mutant PSA-3 grew normally in growth medium supplemented with 30 μM phosphatidylethanolamine as well as phosphatidylserine and that the biosynthesis of phosphatidylserine in the mutant was normal when cells were cultured in the presence of exogenous phosphatidylethanolamine. The simplest interpretation of these findings is that phosphatidylserine in CHO cells is biosynthesized through the following sequential reactions: phosphatidylcholine → phosphatidylserine → phosphatidylethanolamine → phosphatidylserine. The three reactions are catalyzed by serine-exchange enzyme I, phosphatidylserine decarboxylase, and serine-exchange enzyme II, respectively.