Microsomal phosphatidate phosphatase in maturing safflower seeds.

An assay system comprising sodium phosphatidate, phosphatidylcholine, and bovine serum albumin has been developed for the reproducible determination of phosphatidate phosphatase activity in maturing seeds of safflower (Carthamus tinctorius L.). The activity was detected in both membrane and soluble fractions, and the microsomal phosphatidate phosphatase was characterized. The optimum pH for Pi release was 6.7, and the activity depended on the concentration of Mg(2+). Phosphatidylcholine and bovine serum albumin stimulated the phosphatase reaction. This phosphatase was highly specific for phosphatidate; lysophosphatidate, and water-soluble phosphate esters did not serve as substrate. The specific activity was approximately 20 nanomoles per minute per milligram of protein, which was close to that of glycerol-phosphate acyltransferase and higher than that of diacylglycerol acyltransferase. Furthermore, the activity per seed was enough to account for the rate of triacylglycerol accumulation in vivo. The step of diacylglycerol formation by phosphatidate phosphatase does not appear to be rate-limiting for triacylglycerol synthesis during seed maturation.