Evidence for a /B-Aspartyl Phosphate Residue in the Phosphorylated Intermediate of the Red Beet Plasma Membrane

A borohydride reduction method was used to identify the phosphorylated amino acid in the phospho-enzyme of the red beet (Beta vulgani L.) plasma membrane ATPase. Plasma membrane fractions were phosphorylated with unlabeled ATP in the presence of MgSO4 at pH 6.5 and then treated with sodium l3Hlborohydride. The borohydride-treated samples were subjected to hydrolysis in 6 normal HCI at 110°C for 22 hours and then analyzed by high voltage paper electrophoresis and thin layer chromatography. This analysis demonstrated the formation of labeled homoserine as the major reduction product when phosphorylated membrane samples were treated with sodium [3Hiborohydride. This suggests that the phosphoryl group in the plasma membrane ATPase of red beet storage tissue is attached to the ,-carboxyl side chain of an aspartic acid residue in the active site of the