4.F.1 The Choline/Ethanolaminephosphotransferase 1 (CEPT1) Family
These phosphotransfer enzymes catalyze both phosphatidylcholine (PC) and phosphatidylethanolamine (PE) biosynthesis from CDP-choline and CDP-ethanolamine, respectively, plus diglycerides, in a process coupled to transmembrane phospholipid transport to distribute phosphatidyl choline to the lumenal surface of the endoplasmic reticulum (Henneberry et al. 2000).
The human CEPT1 uses diradylglycerols (diacylglycerols) as substrates, and the fatty acyl specificities of CEPT1 for these molecules in a mixed micellar assay have been studied (Wright and McMaster 2002). CEPT1 displayed an apparent Km of 36 microM for CDP-choline and 4.2 mol% for di-18:1 DAG with a Vmax of 14.3 nmol/min/mg. When CDP-ethanolamine was used as substrate, the apparent Km was 98 microM for CDP-ethanolamine and 4.3 mol% for di-18:1 DAG with a Vmax of 8.2 nmol/min/mg. The preferred diradylglycerol substrates used by CEPT1 with CDP-choline as the phosphobase donor were di-18:1 DAG, di-16:1 DAG, and 16:0/18:1 DAG. When CDP-ethanolamine was used as the phosphobase donor, 16:0/18:1 DAG, di-18:1 DAG, and di-16:1 DAG were the preferred substrates. The mixed micelle assay allowed the lipid activation of CEPT to be measured, and both the cholinephosphotransferase and ethanolaminephosphotransferase activities displayed the unusual property of product activation. The protein kinase C inhibitor chelerythrine and the human DAG kinase inhibitor R59949 both inhibited CEPT1 activity with IC50 values of 40 microM (Wright and McMaster 2002).