Abstract
Inhibitors of SO4(2-) entry into and efflux from vesicles prepared from the brush border membrane of term human placenta have been studied, using 35SO4(2-) and an anion-exchange column assay. Divalent anions were found to be either potent or relatively feeble inhibitors of SO4(2-) uptake. Those which, like SO4(2-) itself, have a tetrahedral configuration, were strongly inhibitory and all of these anions were the metal oxides of elements of group VI (A and B) of the Periodic Table (chromate, molybdate, tungstate, selenate and thiosulphate). Divalent anions which were only weak inhibitors of SO4(2-) uptake were arsenate, phosphate and tetraborate. Chromate and to a lesser extent molybdate were effective inhibitors of SO4(2-) efflux from vesicles pre-loaded with SO4(2-). SO4(2-) efflux was insensitive to the electrical potential across the vesicle membrane. These results are discussed with respect to the mechanism for SO4(2-) transport across this membrane from mother to fetus and in the context of the transport to the feto-placental unit of the essential trace elements, selenium, chromium and molybdenum.