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Phosphatidic Acid Synthesis in Castor Bean Endosperm ¹

^a Thimann Laboratories, University of California, Santa Cruz, California 95064

Enzyme assays on organelles isolated from the endosperm of castor bean (Ricinus communis var. 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 µM for sn-glycerol 3-phosphate and approximately 8 µM for palmitoyl-CoA. The optimum pH range was 7 to 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.