Molecular species of PC and PE formed during castor oil biosynthesis.

As part of a program to elucidate castor oil biosynthesis, we have identified 36 molecular species of PC and 35 molecular species of PE isolated from castor microsomes after incubations with [14C]-labeled FA. The six [14C]FA studied were ricinoleate, stearate, oleate, linoleate, linolenate, and palmitate, which were the only FA identified in castor microsomal incubations. The incorporation of each of the six FA into PC was better than that into PE. The [14C]FA were incorporated almost exclusively into the sn-2 position of both PC and PE. The incorporation of [14C]stearate and [14C]palmitate into 2-acyl-PC was slower compared to the other four [14C]FA. The incorporation does not show any selectivity for the various lysoPC molecular species. The level of incorporation of [14C]FA in PC was in the order of: oleate > linolenate > palmitate > linoleate > stearate > ricinoleate, and in PE: linoleate > linolenate > oleate > palmitate > stearate > ricinoleate. In general, at the sn-1 position of both PC and PE, linoleate was the most abundant FA, palmitate was the next, and oleate, linolenate, stearate, and ricinoleate were minor FA. The activities of oleoyl-12-hydroxylase, oleoyl-12-desaturase seem unaffected by the FA at the sn-1 position of 2-oleoyl-PC. The FA in the sn-1 position of PC does not significantly affect the activity of phospholipase A2, whereas ricinoleate is preferentially removed from the sn-2 position of PC. The results show that (i) [14C]oleate is most actively incorporated to form 2-oleoyl-PC, the immediate substrate of oleoyl-12-hydroxylase; (ii) 2-ricinoleoyl-PC is formed mostly by the hydroxylation of 2-oleoyl-PC, not from the incorporation of ricinoleate into 2-ricinoleoyl-PC; and (iii) 2-oleoyl-PE is less actively formed than 2-oleoyl-PC.