Evidence for facilitated transport of biotin by hamster enterocytes.

The uptake of biotin by isolated hamster intestinal cells was investigated in the zone of physiological concentrations. Uptake of the vitamin was not a linear function of the external concentrations and kinetics could be saturated [Km = 1.12 microM, Vmax = 33.9 pmol/(mg protein X min)]. Metabolic inhibitors (antimycin A, 2,4-dinitrophenol) had no effect, so uptake is not energy dependent. Ouabain and N-ethylmaleimide, inhibitors of cation gradients, had no effect on biotin uptake, thus showing the absence of cotransport. The inhibition of uptake by analogs of biotin, that is, biocytin and DL-thioctic acid, indicates that the terminal carboxyl group and the thiophane ring play a role in the recognition of biotin. Counterflow experiments showed competitive inhibition of efflux when excess biotin was present in the cells. These findings are consistent with biotin uptake by isolated hamster enterocytes being a process of facilitated diffusion.