title: Characterisation of a high affinity Ca2+-stimulated, Mg2+-dependent ATPase in the rat parotid plasma membrane.
abstract: Two Ca2+-stimulated ATPase activities have been identified in the plasma membrane of rat parotid: (a) a (Ca2+ + Mg2+)-ATPase with high affinity for free Ca2+ (apparent Km = 208 nM, Vmax = 188 nmol/min per mg) and requiring micromolar concentration of Mg2+ and (b) a (Ca2+ or Mg2+)-ATPase with relatively low affinity for free Ca2+ (K0.5 = 23 microM) or free Mg2+ (K0.5 = 26 microM). The low-affinity (Ca2+ or Mg2+)-ATPase can be maximally stimulated by Ca2+ alone or Mg2+ alone. The high-affinity (Ca2+ + Mg2+)-ATPase exhibits sigmoidal kinetics with respect to ATP concentration with K0.5 = 0.4 mM and a Hill coefficient of 1.91. It displays low substrate specificity with respect to nucleotide triphosphates. Although trifluoperazine inhibits the activity of the high affinity (Ca2+ + Mg2+)-ATPase only slightly, it inhibits the activity of the low-affinity (Ca2+ or Mg2+)-ATPase quite potently with 22 microM trifluoperazine inhibiting the enzymic activity by 50%. Vanadate, inositol 1,4,5-trisphosphate, phosphatidylinositol 4,5-bisphosphate, Na+,K+ and ouabain had no effect on the activities of both ATPases. Calmodulin added to the plasma membranes does not stimulate the activities of both ATPases. The properties of the high-affinity (Ca2+ + Mg2+)-ATPase are distinctly different from those of the previously reported Ca2+-pump activity of the rat parotid plasma membrane.
authors.affiliation: Department of Biochemistry, National University of Singapore, Kent Ridge.
authors.forename: T S
authors.initials: TS
authors.lastname: Teo
authors.forename: P
authors.initials: P
authors.lastname: Thiyagarajah
authors.forename: M K
authors.initials: MK
authors.lastname: Lee
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000494:pharmacology; D002462:Cell Membrane / Q000201:enzymology; D007700:Kinetics; D008274:Magnesium / Q000502:physiology; D009711:Nucleotides / Q000378:metabolism; D010306:Parotid Gland / Q000201:enzymology*; D051381:Rats; D014268:Trifluoperazine / Q000494:pharmacology
chemical_list: D002147:Calmodulin; D009711:Nucleotides; D014268:Trifluoperazine; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium