Biotin transport in basolateral membrane vesicles of human intestine.

The characteristics of the exit process of biotin from the enterocyte, i.e., transport across the basolateral membrane, was determined using an enriched basolateral membrane vesicle preparation of human intestine. Purity and suitability of basolateral membrane vesicles for transport studies was confirmed by enzymatic and functional criteria. Orientation of human basolateral membrane vesicles was determined by [3H]ouabain binding studies and was found to be 64% inside-out vesicles and the rest right-side-out vesicles and membrane sheets. Osmolarity studies indicated that the uptake of biotin by these vesicles represents transport into the intravesicular compartment, with little binding to membrane surfaces. The rate of biotin transport was linear for approximately 40 s but decreased thereafter. Transport of biotin was (a) Na+-independent, (b) saturable as a function of concentration, with an apparent KM of 1.1 microM and Vmax of 0.9 pmol/mg protein.15 s, (c) inhibited by structural analogues (desthiobiotin and biotin methyl ester) and related compounds (thioctic acid and thioctic amide), and (d) stimulated by inducing a positive intravesicular electrical potential. These studies are the first to demonstrate the existence of a carrier-mediated transport system for biotin in the basolateral membrane of human intestine.