Inhibition of Sarcoplasmic Reticulum Ca²⁺-ATPase by 2, 5-Di (Tert-Butyl)-1, 4-Benzohydroquinone

Abstract

We characterized the interaction of 2, 5-di (tert-butyl)-1, 4-benzohydroquinone (tBuBHQ) with the sarcoplasmic reticulum (SR) Ca²⁺-ATPase from rabbit fast-twitch skeletal and canine cardiac muscles by examining the effect of this agent on the ATPase reaction. tBuBHQ at less than 10μM inhibited ATP hydrolysis by both isoforms of Ca²⁺-ATPase by up to 80 and 90%, respectively. The half maximal inhibition of these enzymes was observed at about 1.5μM tBuBHQ. Thus, this agent potently inhibits the fast-twitch skeletal and slow-twitch skeletal/cardiac isoforms of SR Ca²⁺-ATPase. tBuBHQ at 5-10μM inhibited the rate of decomposition of the phosphoenzyme intermediate (EP), measured as a ratio between ATPase activity and the EP level in the steady state, by 35-40%. It also inhibited formation of EP by decreasing the rate of Ca²⁺ binding to the Ca²⁺-deficient, nonphosphorylated enzyme to about 1/8 of the control value. These results indicate that tBuBHQ has at least two sites of action in the reaction sequence for the SR Ca²⁺-ATPase.