S-12-3 Some Aspects of Thiamin Transport in Mammals

II. ENTEROCYTE ENTRY In brush-border membrane vesicles (BBMV) from rat small intestine the time course of T transport is not influenced by the presence or the absence of Nat(or Kt) in the incubation medium [1].At concentrations <1.25 ƒÊM, T is taken up mainly by a saturable mechanism (Km, 0.8, 0; Jmax, 0.35 pmol. mg-1 protein. 4 s-1), while at higher concentrations a passive diffusion prevails (Fig. 1A). T is not biotransformed during its transfer into the intravesicular space, and the transfer is inhibited competitively by T structural analogs. These findings suggest that the brush-border entry of T occurs by Nat-independent facilitated diffusion rather than by simple diffusion,as also supported by the results of experiments on T countertransport in rings of everted small intestine [2]. Since T is a weak organic cation, an investigation was carried out on the influence of pH on its transport by BBMV. Gradients of H+exert a pronounced effect on the time course of T uptake: an outwardly directed gradient produces a much faster rate of accumulation as compared to the absence of a gradient in the acidic pH range. In general, irrespectively of the gradient, the higher the Ht concentration inside the vesicles, the higher is the uptake. By contrast, an inwardly directed gradient of Ht or the absence of a gradient in the physiological pH range reduce T transport to about 34% of that observed with an outwardly directed gradient. These results seem to indicate that T entry is associated with an antiport of H+, a mechanism utilized by other organic cations for transport in jejunal BBMV [3]. Further research is in progress to better characterize this aspect. In summary, the entry of low concentrations of T into BBMV, and therefore into enterocytes, behaves as a carrier-mediated process, which is easily saturable, Nat-independent, and stimulated by an outwardly directed gradient of H+.