Phosphatidic Acid Synthesis in Castor Bean Endosperm

Abstract
Enzyme assays on organelles isolated from the endosperm of castor bean (Ricinus communis cv. ''Hale'') by sucrose density gradient centrifugation showed that palmitoyl-CoA:sn-glycerol 3-phosphate acyltransferase (EC 2.3.1.15) was localized in the membranes of the endoplasmic reticulum. Mn2+ was required for activity, but Ca2+ and Mg2+ could substitute for Mn2+ at higher concentrations. The apparent Km was 170 .mu.M for sn-glycerol 3-phosphate and approximately 8 .mu.M for palmitoyl-CoA. The optimum pH range was 7-7.5, and the principal reaction product was diacyl-sn-glycerol 3-phosphate (phosphatidic acid). Monoacyl-sn-glycerol 3-phosphate (lysophosphatidic acid) was not released as a free intermediate in the reaction. The maximum activity of the enzyme occurred immediately after imbibition, preceding the development of mitochondria and glyoxysomes.

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