Evidence for histidine residues on plasma membrane phosphatidate phosphohydrolase from rat liver

ABSTRACT

Phosphatidate phosphohydrolase [PAP] catalyzes the dephosphorylation of phosphatidic acid to yield P[i], and diacylglycerol. Two different forms of PAP in rat hepatocyte have been reported. PAP[1] is located in cytosolic and microsomal fractions and participates in the synthesis of triacylglycerols, phosphatidylcholine, and phosphatidylethanolamine, whereas the other form of phosphatidate phosphohydrolase [PAP[2]] is primarily involved in lipid signaling pathways. In rat liver, PAP2 has two isoforms; one PAP[2a] and another PAP2b- In this study, essential histidine residues were investigated in native form of rat purified PAP[2b] with diethylpyrocarbonate. PAP[2b] 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 PAP[2b] inactivated by DEPC. The maximum 6 moles of histidine residues modified per mole of PAP[2b], when about 90% of enzyme activity is lost with DEPC. The data showed that the incubation of PAP[2b] by DEPC can inhibit enzyme activity. Our findings also, revealed the presence of essential histidines in the structure of PAP[2b] which involve in its activity. This enzyme is likely to have a similar hydrolysis catalytic mechanism as its super family through a phosphohistidine intermediate