title: The slippage of the Ca2+ pump and its control by anions and curcumin in skeletal and cardiac sarcoplasmic reticulum.
abstract: Ca(2+) transport by sarcoplasmic reticulum (SR) ATPase occurs with an optimal coupling ratio of 2 Ca(2+) per ATP in pre-steady state. However, slippage of the pump and lower coupling ratios are observed in steady state. Slippage depends on the presence of high Ca(2+) in the lumen of SR vesicles and high nucleotide in the medium. Thereby, Ca(2+) and/or nucleotide-bound phosphoenzyme intermediates accumulate and undergo uncoupled cleavage, before vectorial translocation of bound Ca(2+) in the forward direction of the cycle or before productive reversal to ATP synthesis. Transport efficiency and coupling ratios are improved by reduction of nucleotide concentration in the presence of ATP regenerating systems and/or complexation of luminal Ca(2+) with phosphate or oxalate. Curcumin (1-5 microm) lowers the concentration of phosphate or oxalate required to reduce slippage of the Ca(2+) pump. Thereby, under appropriate conditions, curcumin favors kinetic flow, completion of productive cycles, and improvement of coupling ratios. The findings obtained with isolated SR vesicles suggest that slippage is an important phenomenon under prevailing conditions of muscle fibers in situ. Ca(2+) transport and its slippage can be improved by curcumin in cardiac as well as in skeletal SR, raising the possibility of pharmacological interventions to correct defective Ca(2+) homeostasis. Higher curcumin concentrations (5-30 microm), however, inhibit overall ATPase activity and Ca(2+) transport by interfering with phosphoenzyme formation with ATP or P(i).
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA.
authors.forename: Carlota
authors.initials: C
authors.lastname: Sumbilla
authors.forename: David
authors.initials: D
authors.lastname: Lewis
authors.forename: Tina
authors.initials: T
authors.lastname: Hammerschmidt
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Inesi
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000838:Anions; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002384:Catalysis; D003474:Curcumin / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007700:Kinetics; D008956:Models, Chemical; D018482:Muscle, Skeletal / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism*; D051381:Rats; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D013997:Time Factors
chemical_list: D000838:Anions; D004791:Enzyme Inhibitors; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D000252:Calcium-Transporting ATPases; D003474:Curcumin; D002118:Calcium