Inactivation of taurine transporter by calcium in purified human placental brush border membrane vesicles

Abstract

We investigated the effects of calcium on the activity of the taurine transporter in purified human placental brush border membrane vesicles. Treatment of the membrane vesicles with calcium markedly inhibited taurine uptake. The magnitude of inhibition was dependent on the calcium concentration and the treatment time. Free ionized Ca2+ was responsible for this effect because EGTA, a Ca2+ chelator, totally abolished the calcium-induced effect. Uptake of succinate, which occurs via a Na(+)-dependent process as does the uptake of taurine, was reduced only to a small extent by the calcium treatment. This result indicates that the effect of Ca2+ on taurine uptake was not due to an accelerated dissipation of the Na+ gradient as a result of an increased Na+ permeability of the membrane. Preloading the vesicles with phospholipase inhibitors such as neomycin and quinacrine significantly protected the taurine transporter from the Ca2+ effect, raising the possibility that Ca(2+)-activated phospholipases may mediate the Ca2+ effect. Kinetic analysis revealed that Ca2+ decreased the affinity of the transporter for taurine as well as the translocation rate of the taurine-loaded transporter complex.