Phosphoserine phosphatase of human brain : partial purification, characterization, regional distribution, and effect of certain modulators including psychoactive drugs

Phosphoserine phosphatase (PSPase), a cytosolic enzyme has been purified 106 fold from human brain, by employing conventional protein purification techniques. The use of MgCl2 (10 mM) and chloroform treatment, during purification enabled the removal of non-specific proteins. The final enzyme preparation exhibited a broad pH optimum of 5.6-6.6 and could dephosphorylate both L and D enantiomers of the phosphoserine, but with different Km values for O-P-L serine (3.6 x 10(-5) M) and O-P-D serine (1 x 10(-4) M). Enzyme activity was found to be specific for phosphoserine, whereas other phosphoesters including phosphothreonine and phosphoproteins such as casein and phosvitin were found to be poor substrates. The enzyme activity was uncompetitively inhibited by L-serine. Further the PSPase activity was inhibited by vanadate, (41%), trifluoperazine (23%), chlorpromazine (34%) at an equimolar concentration of 1 mM, whereas lithium and ethanol did not influence the enzyme activity. Minor tranquilizers such as diazepam and chlordiazepoxide activated the enzyme activity to an extent of 13% and 59% respectively. In addition, species and regionwise heterogeneity was observed with respect to distribution of enzyme activity in six major areas of human, rabbit and rat brains.