Evidence for Histidine Residues on Plasma Membrane Phosphatidate Phosphohydrolase from Rat Liver

Phosphatidate phosphohydrolase (PAP) catalyzes the dephosphorylation of phosphatidic acid to yield Pi and diacylglycerol. Two different forms of PAP in rat hepatocyte have been reported. PAP1 is located in cytosolic and microsomal fractions and participates in the synthesis of triacylglycerols, phosphatidylcholine, and phosphatidylethanolamine, whereas the other form of phosphatidate phosphohydrolase (PAP2) is primarily involved in lipid signaling pathways. In rat liver,PAP2 has two isoforms; one PAP2a and another PAP2b. In this study, essentialhistidine residues were investigatedin native form of rat purified PAP2b withdiethylpyrocarbonate.PAP2b purified from rat liver plasma membrane by solubilizing with n-octyle glucoside and several chromatography steps. Gel electrophoresis (SDS-PAGE) performed on purified enzyme in order to evaluate its purity and to measure the molecular weight of the enzyme subunit. The enzyme inactivated with diethylpyrocarbonate (DEPC) and the number of moles of histidine residues modified per mol of enzyme determined. The specific activity of purified enzyme was 7350mU/mg protein and it showed only a single band on SDS-PAGE with a MW of about 33.8 kDa. The PAP2b inactivated by DEPC. The maximum 6 moles of histidine residues modified per mole of PAP2b, when about 90% of enzyme activity is lost with DEPC.The data showed that the incubation of PAP2b by DEPC can inhibit enzyme activity. Our findings also, revealed the presence of essential histidines in the structure of PAP2b which involve in its activity. This enzyme is likely to have a similar hydrolysis catalytic mechanism as its super family through a phosphohistidine intermediate.