Escherichia coli S-adenosylhomocysteine/5'-methylthioadenosine nucleosidase. Purification, substrate specificity and mechanism of action.

Ca2+-activated protein phosphatase activity was demonstrated in mouse pancreatic acinar cytosol with alpha-casein and skeletal-muscle phosphorylase kinase as substrates. This phosphatase activity preferentially dephosphorylated the alpha subunit of phosphorylase kinase. After DEAE-cellulose chromatography, the Ca2+-activated phosphatase activity became dependent on exogenous calmodulin for maximal activity. Half-maximal activation was achieved at 0.5 +/- 0.1 microM-Ca2+. Trifluoperazine completely inhibited Ca2+-activated phosphatase activity, with half-maximal inhibition occurring at 8.5 +/- 0.6 microM. Mn2+, but not Mg2+, at 1 mM concentration could substitute for Ca2+ in eliciting full enzyme activation. The apparent Mr of the phosphatase as determined by Sephadex G-150 chromatography was 93000 +/- 1000. Submitting active fractions obtained after Sephadex chromatography to calmodulin affinity chromatography resulted in the resolution of a major protein of Mr 55500 +/- 300. In conclusion, Ca2+-activated protein phosphatase activity has been identified in exocrine pancreas and has several features in common with Ca2+-activated calmodulin-dependent protein phosphatases previously isolated from brain and skeletal muscle. It is possible that this Ca2+-activated phosphatase may utilize as substrates certain acinar-cell phosphoproteins previously shown to undergo dephosphorylation in response to Ca2+-mediated secretagogues.