Thiamine transport in the brush border membrane vesicles of the guinea-pig jejunum.

Transport of [35S]thiamine was studied with membrane vesicles prepared from the brush border of guinea-pig jejunum, in which thiamine pyrophosphokinase (EC 2.7.6.2) was not detected. The presence of an Na+ gradient from outside to inside of the vesicles did not affect thiamine transport, whereas L-proline uptake into the vesicles of the same preparation was stimulated under identical conditions. The equilibrium level of thiamine uptake decreased with increasing osmolarity of the medium, which indicates that thiamine is transported into the membrane vesicles. The initial rate (30 s) of thiamine uptake increased linearly with increasing thiamine concentration throughout the range from 0.06 to 10 microM in the medium, in the presence and absence of an Na+ gradient. No effect of other monovalent cations, including K+, Li+ and choline+, was observed on thiamine transport. Pyrithiamine, an antimetabolite of thiamine, and unlabeled thiamine, both added in very excessive amounts, did not inhibit labeled thiamine transport into the membrane vesicles. These results confirm the assumption that thiamine passes through the brush border membrane of guinea-pig jejunum by simple diffusion.