Intestinal thiamin transport in rats

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Abstract
Rat everted jejunal sacs were incubated at 37°C for 15–60 min in Krebs-Henseleit buffer, pH 7.4, with or without (control experiments) 0.2 μM [thiazole-2-14C]-thiamin. The determination of thiamin and its phosphoesters in the sac wall and in serosal fluid was carried out by an electrophoretic micromethod. Irrespective of the presence of14C-thiamin, the tissue content of endogenous thiamin pyro- and triphosphate decreased during the incubation, whilst that of thiaminmonophosphate remained relatively constant. The tissue content of free thiamin increased substantially only in control experiments. Endogenous free thiamin, together with a small amount of monophosphate, was found to enter the serosal fluid. The transfer of both compounds was greatly enhanced by the incubation with14C-thiamin, when an efficient thiamin phosphorylation could be demonstrated. During incubation with14C-thiamin, the concentration of14C-thiaminpyrophosphate and, to a lesser extent, that of free14C-thiamin increased progressively in the tissue, while14C-thiamin-monophosphate content remained almost unchanged. No14C-thiamin-triphosphate was detected. There was a rapid increase in the tissue specific radioactivity of free thiamin and thiamin-monophosphate, which preceded the rise in the specific radioactivity of thiamin-pyrophosphate. The specific radioactivities of the former compounds in the serosal fluid reflected those observed in the intestinal tissue. These results are interpreted as evidence suggesting that the active transport of thiamin is efficient only when intracellular thiamin phosphorylation is operating.

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