A COMPARATIVE-STUDY OF (CA2-MG2+)-ATPASE ON THE LYSOSOMAL MEMBRANE AND ECTO-ATPASE ON THE PLASMA-MEMBRANE FROM RAT-LIVER()

We showed that rat liver lysosomes possess at least two types of ATPas e besides H+-translocating ATPase (H+-ATPase); namely, N-ethylmaleimid e (NEM)-sensitive and bafilomycin A(1)-insensitive Mg2+-ATPase [ATPase I] and NEM- and bafilomycin A(1)-insensitive (Ca2+-Mg2+)-ATPase [ATPa se II] [Hayashi H., et al. Chem. Pharm. Bull., 37, 2783-2786 (1992)]. Subcellular fractionation showed the presence of similar activity of ( Ca2+-Mg2+)-ATPase not only in the Golgi but also in the plasma membran e fractions, of which plasma membrane had the highest activity, most p robably due to the ecto-ATPase. In this study, we partially purified t he (Ca2+-Mg2+)-ATPases from both lysosomal and plasma membranes and co mpared their properties, Both enzymes had quite similar characteristic s including: (1) broad pH-activity profiles with two pH optima at 4.5 and 7.0, (2) K-m values for ATP being 21-27 mu M (at pH 4.5) and 18-14 mu M (at pH 7.0) (in the presence of CaCl2), (3) hydrolysis of both n ucleoside triphosphates and diphosphate (ADP), (4) inhibition only by vanadate and 4,4'-diisothiocyanatostilbene 2,2'-disulfonic acid (DIDS) at pH 4.0 (but not at pH 7.0), (5) acidic pi values that were shifted by neuraminidase digestion, and (6) a positive reaction against an an tibody to the N-terminal peptide of ecto-ATPase.