The effect of deuterium ion concentration on the properties of sarcoplasmic reticulum

Summary 99.8% Deuterium oxide, as obtained commerically, has been shown to contain a contaminant which strongly inhibits calcium transport and binding by sarcoplasmic reticulum (S.R.) and the associated ATPase activity. The contaminant is removed by distillation of deuterium oxide. Calcium binding by S.R. is maximal at pH 6.5 whereas calcium transport (in the presence of oxalate) is maximal at a pH of 7.2 to 7.5. In the presence of deuterium oxide, these maxima are shifted to a pD of 7.2 and a pD of 7.5 to 8.0, respectively. The maximum binding and transport rates are not affected by the change from aqueous to deuterium oxide medium. The same phenomena are observed with the ATPase activity. In the presence of oxalate, calcium;magnesium ATPase is maximal at pH 7.2 and pD 8.0. The maximum rate is unchanged, however,At pH 7.2 or higher, the amount of calcium which may be bound by S.R. remains constant with time. At lower pH, calcium initially bound is slowly displaced from the membrane with time. It has been reported that deuterium oxide inhibits excitation-contraction coupling. The results presented here indicate that S.R. is probably not the site of deuterium oxide inhibition, and raise the possibility that the measured inhibition is due to an impurity in the deuterium oxide.