IDENTITY OF ZINC ION-DEPENDENT ACID-PHOSPHATASE FROM BOVINE BRAIN ANDMYOINOSITOL 1-PHOSPHATASE

A 62 kDa Zn2+-dependent acid phosphatase has been purified from bovine brain. The protein was carboxymethylated and then cleaved by endoprot einase Glu-C, trypsin and CNBr. Several fragments were subjected to st ructural analysis either by using mass spectrometry or automated pepti de sequencing. The four sequenced peptides were compared with the know n protein sequences contained in the EMBL Data Bank. All four peptide sequences were identical to the corresponding amino-acid sequences pre sent in myo-inositol I-phosphatase from bovine brain. Furthermore we f ound that the amino-acid composition of Zn2+-dependent acid phosphatas e purified in our laboratory is very similar to that of myo-inositol I -phosphatase, and that several peptide fragments have molecular weight s (measured by mass spectrometry techniques) identical to those expect ed for cleavage-fragments originated from the authentic myo-inositol l -phosphatase. This is one of the key enzymes in the receptor-stimulate d inositol phospholipid metabolism and it has been considered as the p robable target of Li+ ion during LiCl therapy in manic-depressive pati ents. The comparison of the Zn2+-dependent acid phosphatase and the Mg 2+-dependent myo-inositol-1-phosphatase activities, measured at differ ent purification steps, shows that the ratio between the two activitie s was remarkably constant during enzyme purification. We also demonstr ated that in the presence of Mg2+ this enzyme efficiently catalyses th e hydrolysis of myo-inositol I-phosphate, and that the Li+ ion inhibit s this activity. Furthermore, the thermal treatment of the enzyme caus es a time-dependent parallel decrease of both Zn-dependent p-nitrophen yl phosphatase (assayed at pH 5.5) and Mg2+-dependent myo-inositol-1-p hosphatase (assayed at pH 8.0) activities, suggesting the hypothesis t hat the same protein possesses both these activities.