Stimulatory and Inhibitory Effects of Dimethyl Sulfoxide and Ethylene Glycol on ATPase Activity and Calcium Transport of Sarcoplasmic Membranes

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Abstract
The effect of dimethyl sulfoxide (Me2SO) and ethylene glycol on 2 different [rabbit skeletal muscle] preparations of the sarcoplasmic reticulum, i.e., native membranes and membranes whose phospholipids were hydrolyzed by phospholipase A, were investigated using ATP and p-nitrophenylphosphate [NPP] as substrates. Me2SO and ethylene glycol inhibit both Ca-dependent ATP hydrolysis and ATP- supported Ca transport by native vesicles. Ca-dependent p-nitrophenylophosphatase [NPPase] activity and NPP-supported Ca transport were activated by both agents at concentrations lower than 30% (vol/vol). Me2SO strongly stimulated NPP activity of vesicles treated with phospholipase A, but had relatively little effect on activity of native vesicles. Up to a concentration of approximately 40% Me2SO (vol/vol) the inhibiting effect on the Ca-dependent ATPase was fully reversible, but only partially reversible on Ca transport. In the concentration range where Me2SO inhibited ATP hydrolysis and Ca transport, it did not affect ATP binding to the membranes nor Ca-dependent formation of phospho-protein. The rate of dephosphorylation and the rate of Pi exchange between ATP and ADP were markedly reduced by the presence of 30% Me2SO (vol/vol). While Me2SO inhibited passive Ca efflux, ethylene glycol produced a considerable activation. ADP-dependent Ca efflux and ATP synthesis were activated by 15% Me2SO (vol/vol). Ethylene glycol reduced both activities. The respective substrate-enzyme complexes were differently affected by the agents, resulting either in inhibition or stimulation.