title: ATP regulation of calcium transport in back-inhibited sarcoplasmic reticulum vesicles.
abstract: At high concentrations of ATP, ATP hydrolysis and Ca2+ transport by the (Ca2+ + MG2+)-ATPase of intact sarcoplasmic reticulum vesicles exhibit a secondary activation that varies with the extent of back-inhibition by Ca2+ accumulated within the vesicles. When the internal ionized Ca2+ is clamped at low and intermediate levels by the use of Ca-precipitating anions, the apparent Km values for activation by ATP are lower than in fully back-inhibited vesicles (high internal Ca2+). In leaky vesicles unable to accumulate Ca2+, raising Ca2+ in the assay medium from 20-30 microM to 5 mM abolishes the activation of hydrolysis by high concentrations of ATP. The level of [32P]phosphoenzyme formed during ATP hydrolysis from [32P]phosphate added to the medium also varies with the extent of back-inhibition; it is highest when Ca2+ is raised to a level that saturates the internal, low-affinity Ca2+ binding sites. In intact vesicles, increasing the ATP concentration from 10 to 400 microM competitively inhibits the reaction of inorganic phosphate with the enzyme but does not change the rate of hydrolysis. In a previous report (De Meis, L., Gomez-Puyou, M.T. and Gomez-Puyou, A. (1988) Eur. J. Biochem. 171, 343-349), it has been shown that the hydrophobic molecules trifluoperazine and iron bathophenanthroline compete for the catalytic site of the Pi-reactive form of the enzyme. Here it is shown that inhibition of ATP hydrolysis by these compounds is reduced or abolished when Ca2+ binds to the low-affinity Ca2+ binding sites of the enzyme. Since inhibition by these agents is indifferent to activation of hydrolysis by high concentrations of ATP, it is suggested that the second Km for ATP and the inhibition by hydrophobic molecules involve two different Ca-free forms of the enzyme.
authors.affiliation: Departmento de Bioquímica, Universidade Federal do Rio de Janeiro, Cidade Universitária, Brazil.
authors.forename: L
authors.initials: L
authors.lastname: de Meis
authors.forename: M M
authors.initials: MM
authors.lastname: Sorenson
mesh_terms: D000255:Adenosine Triphosphate / Q000502:physiology*; D000818:Animals; D001693:Biological Transport, Active; D017301:Ca(2+) Mg(2+)-ATPase / Q000037:antagonists & inhibitors* / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors* / Q000502:physiology; D066298:In Vitro Techniques; D007700:Kinetics; D010618:Phenanthrolines / Q000494:pharmacology; D010710:Phosphates / Q000378:metabolism; D010766:Phosphorylation; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D014268:Trifluoperazine / Q000494:pharmacology
chemical_list: D010618:Phenanthrolines; D010710:Phosphates; D014268:Trifluoperazine; C006686:bathophenanthroline; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium