Phosphatidic Acid synthesis in castor bean endosperm

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. Mn(2+) was required for activity, but Ca(2+) and Mg(2+) could substitute for Mn(2+) at higher concentrations. The apparent Km was 170 mum for sn-glycerol 3-phosphate and approximately 8 mum 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.