Modulation of the Ca2+,Mg(2+)-ATPase of sarcoplasmic reticulum by the hypothalamic hypophyseal inhibitory factor.
We have studied the effect of the endogenous inhibitor of the Na+ and Ca2+ pumps, HHIF, on sarcoplasmic reticulum (SR) vesicles. The effect of HHIF on the SR Ca2+,Mg(2+)-ATPase activity shows a biphasic pattern. Low HHIF concentrations activate the Ca2+,Mg(2+)-ATPase by dissipation of Ca2+ gradient across the SR membrane. Higher concentrations irreversibly inhibit this activity following a slow kinetic process both in intact SR membranes and in purified Ca2+,Mg(2+)-ATPase. Differential scanning calorimetry shows that the Ca2+,Mg(2+)-ATPase is denatured after incubation with HHIF concentrations which produced full inhibition of its activity. Micromolar Ca2+ and millimolar Mg2+ ADP protect against the irreversible inhibition of the Ca2+,Mg(2+)-ATPase by HHIF. The concentration of HHIF which produces 50% inhibition depends upon SR membrane concentration and upon the lipid:protein ratio in purified Ca2+,Mg(2+)-ATPase. From this we have obtained a partition coefficient for binding of HHIF to SR membranes of 0.6 (microgram SR protein/ml)-1.
Ricote
M
M
Departamento de Bioquímica y Biología Molecular y Genética, Facultad de Ciencias, Badajoz, Spain.
García-Martín
E
E
Sancho
J
J
Gutiérrez-Merino
C
C
D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002152:Calorimetry, Differential Scanning; D002417:Cattle; D004791:Enzyme Inhibitors / Q000494:pharmacology; D017136:Ion Transport / Q000187:drug effects; D007476:Ionophores / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology*; D011489:Protein Denaturation; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
D004791:Enzyme Inhibitors; D007476:Ionophores; C083016:sodium-potassium ATPase inhibitory factor; D000001:Calcimycin; D010042:Ouabain; D017301:Ca(2+) Mg(2+)-ATPase; D008274:Magnesium; D002118:Calcium