title: Regulation of smooth muscle phosphatase-II by divalent cations.
abstract: Smooth Muscle Phosphatases II (SMP-II) which has been purified from turkey gizzards and previously classified as protein phosphatase 2C, is inactive in the absence of divalent cations. Study of the activation of SMP-II by Mg2+ and Mn2+ revealed differences in the modes of activation by these cations. The maximal activation elicited by Mg2+ is 1.5-2.5-fold higher than the maximal Mn2+ activation. However, the latter is achieved at a lower concentration than the maximal Mg2(+)-activation. Furthermore, at low cation concentrations (less than or equal to 2 mM), the Mn2(+)-activated activity is higher than the Mg2(+)-activated activity. In the presence of both cations, the effect of Mn2+ predominates suggesting that the affinity of the enzyme for Mn2+ is greater than for Mg2+. In contrast to Mg2+ and Mn2+, Ca2+ does not activate SMP-II but it was observed to antagonize the effects of Mg2+ and Mn2+. Ca2+ acts as a competitive inhibitor of Mg2+. However, the inhibitory effect at high Ca2+ concentrations is not completely reversed by increasing the Mg2+ concentration. Mn2+ activation is also inhibited by Ca2+ but to a lesser extent. Ca2+ cannot completely inhibit Mn2(+)-activation suggesting that SMP-II has greater affinity for Mn2+ than for Ca2+. The finding that Ca2+ inhibits the activation of SMP-II raises the possibility that Ca2+ may be a regulator of SMP-II in vivo.
authors.affiliation: Department of Biochemistry, University of Saskatchewan, Saskatoon, Canada.
authors.forename: M D
authors.initials: MD
authors.lastname: Pato
authors.forename: E
authors.initials: E
authors.lastname: Kerc
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002413:Cations, Divalent / Q000378:metabolism / Q000494:pharmacology*; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D008345:Manganese / Q000378:metabolism / Q000494:pharmacology; D009130:Muscle, Smooth / Q000201:enzymology*; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D014422:Turkeys
chemical_list: D002413:Cations, Divalent; D008345:Manganese; D010749:Phosphoprotein Phosphatases; D008274:Magnesium; D002118:Calcium