Biosynthesis of triacylglycerols containing ricinoleate in castor microsomes using 1-acyl-2-oleoyl-sn-glycero-3-phosphocholine as the substrate of oleoyl-12-hydroxylase |
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| Authors: | Jiann-Tsyh Lin Carol L Woodruff Olivier J Lagouche Thomas A McKeon Allan E Stafford Marta Goodrich-Tanrikulu John A Singleton Carol A Haney |
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| Affiliation: | (1) USDA, ARS, Western Regional Research Center, Albany, California 94710;(2) USDA, ARS, Biological and Agricultural Engineering Department, 27695 Raleigh, North Carolina;(3) Department of Chemistry, North Carolina State University, 27695 Raleigh, North Carolina;(4) Present address: 59 Avenue Didier, 94210 La Varenne St. Hilaire, France;(5) U. S. Department of Agriculture, 800 Buchanan St., 94710 Albany, CA |
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| Abstract: | We have examined the biosynthetic pathway of triacylglycerols containing ricinoleate to determine the steps in the pathway
that lead to the high levels of ricinoleate incorporation in castor oil. The biosynthetic pathway was studied by analysis
of products resulting from castor microsomal incubation of 1-palmitoyl-2-14C]oleoyl-sn-glycero-3-phosphocholine, the substrate of oleoyl-12-hydroxylase, using high-performance liquid chromatography, gas chromatography,
mass spectrometry, and/or thin-layer chromatography. In addition to formation of the immediate and major metabolite, 1-palmitoyl-2-14C]rici-noleoyl-sn-glycero-3-phosphocholine, 14C-labeled 2-linoleoyl-phosphatidylcholine (PC), and 14C-labeled phosphatidylethanolamine were also identified as the metabolites. In addition, the four triacylglycerols that constitute
castor oil, triricinolein, 1,2-diricinoleoyl-3-oleoyl-sn-glycerol, 1,2-diricinoleoyl-3-linoleoyl-sn-glycerol, 1,2-diricinoleoyl-3-linolenoyl-sn-glycerol, were also identified as labeled metabolites in the incubation along with labeled fatty acids: ricinoleate, oleate,
and linoleate. The conversion of PC to free fatty acids by phospholipase A2 strongly favored ricinoleate among the fatty acids on the sn-2 position of PC. A major metabolite, 1-palmitoyl-2-oleoyl-sn-glycerol, was identified as the phospholipase C hydrolyte of the substrate; however, its conversion to triacylglycerols was
blocked. In the separate incubations of 2-14C]ricinoleoyl-PC and 14C]ricinoleate plus CoA, the metabolites were free ricinoleate and the same triacylglycerols that result from incubation with
2-oleoyl-PC. Our results demonstrate the proposed pathway: 2-oleoyl-PC. Out results demonstrate the proposed pathway: 2-oleoyl-PC→2-ricinoleoyl-PC→ricinoleate
→triacylglycerols. The first two steps as well as the step of diacylglycerol acyltransferase show preference for producing
ricinoleate and incorporating it in triacylglycerols over oleate and linoleate. Thus, the productions of these triacylglycerols
in this relatively short incubation (30 min), as well as the availability of 2-oleoyl-PC in vivo, reflect the in vivo drive to produce triricinolein in castor bean. |
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