Modification of ATP regulatory function in sarcoplasmic reticulum Ca2(+)-ATPase by hydrophobic molecules.
The effects of the three hydrophobic molecules triphenylphosphine, trifluoperazine and 3-nitrophenol on Ca2+ uptake and ATPase activity in sarcoplasmic reticulum vesicles was investigated. When ATP was the substrate, triphenylphosphine (3 microM) increased the amount of Ca2+ accumulated by the vesicles. At high concentrations triphenylphosphine inhibited Ca2+ uptake. This effect varied depending on the ATP concentration and the type of nucleotide used. With ITP there was only inhibition and no activation of Ca2+ uptake by triphenylphosphine. On the other hand, trifluoperazine inhibited Ca2+ accumulation regardless of whether ATP or ITP was used as substrate. When 5 mM oxalate was included in the medium in order to avoid binding of Ca2+ to the low-affinity Ca2(+)-binding sites of the enzyme, both stimulation by triphenylphosphine and inhibition by trifluoperazine were reduced. In leaky vesicles at low Ca2+ concentrations, triphenylphosphine and 3-nitrophenol were competitive inhibitors of ATPase activity at the regulatory site of the enzyme (0.1-1 mM ATP). A striking difference was observed when both the high- and low-affinity Ca2(+)-binding sites were saturated. In this condition, triphenylphosphine and 3-nitrophenol promoted a 3-4-fold increase in the apparent affinity for ATP at its regulatory site.
Wolosker
H
H
Departamento de Bioquimica, Universidade Federal do Rio de Janeiro, Brazil.
Petretski
J H
JH
De Meis
L
L
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*; D007700:Kinetics; D008962:Models, Theoretical; D009132:Muscles / Q000201:enzymology; D009596:Nitrophenols / Q000494:pharmacology*; D009943:Organophosphorus Compounds / Q000494:pharmacology*; D010070:Oxalates / Q000494:pharmacology; D011485:Protein Binding; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D014268:Trifluoperazine / Q000494:pharmacology*
D009596:Nitrophenols; D009943:Organophosphorus Compounds; D010070:Oxalates; D014268:Trifluoperazine; C061896:triphenylphosphine; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium; C049017:3-nitrophenol