Metabolism of 1-acyl-2-oleoyl-sn-glycero-3-phosphoethanolamine in castor oil biosynthesis.

We have examined the role of 2-oleoyl-PE (phosphatidylethanolamine) in the biosynthesis of triacylglycerols (TAG) by castor microsomes. In castor microsomal incubation, the label from 14C-oleate of 1-palmitoyl-2-[1-(14)C]oleoyl-sn-glycero-3-phosphoethanolamine is incorporated into TAG containing ricinoleate. The enzyme characteristics, such as optimal pH, and the effect of incubation components of the oleoyl-12-hydroxylase using 2-oleoyl-PE as incubation substrate are similar to those for 2-oleoyl-PC (phosphatidylcholine). However, compared to 2-oleoyl-PC, 2-oleoyl-PE is a less efficient incubation substrate of oleoyl-12-hydroxylase in castor microsomes. Unlike 2-oleoyl-PC, 2-oleoyl-PE is not hydroxylated to 2-ricinoleoyl-PE by oleoyl-12-hydroxylase and is not desaturated to 2-linoleoyl-PE by oleoyl-12-desaturase. We have demonstrated the conversion of 2-oleoyl-PE to 2-oleoyl-PC and vice versa. The incorporation of label from 2-[14C]oleoyl-PE into TAG occurs after its conversion to 2-oleoyl-PC, which can then be hydroxylated or desaturated. We detected neither PE-N-monomethyl nor PE-N,N-dimethyl, the intermediates from PE to PC by N-methylation. The conversion of 2-oleoyl-PE to 2-oleoyl-PC likely occurs via hydrolysis to 1,2-diacyl-sn-glycerol by phospholipase C and then by cholinephosphotransferase. This conversion does not appear to play a key role in driving ricinoleate into TAG.