Vanadate inhibition of ATP-dependent H+ transport in membrane vesicles from turtle bladder epithelial cells

The ATP-dependent proton transport into vesicles of a mixed membrane fraction obtained from turtle bladder epithelial cells consists of at least two kinetically defined moieties: one, which is maximally inhibited by 25% with nanomolar levels of vanadate, but not inhibited at all with equimolar levels of N-ethylmaleimide, and another, which is maximally inhibited by 70% with micromolar levels of N-ethylmaleimide and by 25% with equimolar levels of vanadate. In contrast to the transport function, the associated enzymatic function (the ouabain-resistant ATPase activity) in these membranes, not inhibited by nanomolar levels of vanadate or N-ethylmaleimide, is maximally inhibited by 40% with micromolar levels of vanadate and by 13% with equimolar levels of N-ethylmaleimide. Independent of these kinetic differences between the enzyme and the transport functions, membranes containing the N-ethylmaleimide-sensitive proton transport function are electrophoretically separable from those containing the vanadate-sensitive transport function. For example, the kinetically defined, vanadate-sensitive proton transport function is recovered exclusively and kinetically identified in one of four electrophoretic membrane fractions, EF-II; while the N-ethylmaleimide-sensitive function is recovered in EF-III as well as in EF-II. Membranes of EF-IV, maximally enriched in ouabain-resistant ATPase activity, possess no proton transport function at all, even in the absence of N-ethylmaleimide or vanadate. Additional data under in vivo as well as under in vitro conditions are required to prove that the vanadate-sensitive proton transport in these vesicles is an in vitro manifestation of the mechanism responsible for generating the vanadate-sensitive luminal acidification process under in vivo conditions in the intact turtle bladder.