The slippage of the Ca2+ pump and its control by anions and curcumin in skeletal and cardiac sarcoplasmic reticulum

Abstract

Ca2+ transport by sarcoplasmic reticulum (SR) ATPase occurs with an optimal coupling ratio of 2 Ca2+ 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 Ca2+ in the lumen of SR vesicles and high nucleotide in the medium. Thereby, Ca2+ and/or nucleotide-bound phosphoenzyme intermediates accumulate and undergo uncoupled cleavage, before vectorial translocation of bound Ca2+ 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 Ca2+ with phosphate or oxalate. Curcumin (1-5 μM) lowers the concentration of phosphate or oxalate required to reduce slippage of the Ca2+ 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. Ca2+ 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 Ca2+ homeostasis. Higher curcumin concentrations (5-30 μM), however, inhibit overall ATPase activity and Ca2+ transport by interfering with phosphoenzyme formation with ATP or Pi.