Intestinal thiamin transport in rats. Thiamin and thiamin phosphoester content in the tissue and serosal fluid of everted jejunal sacs.

Rat everted jejunal sacs were incubated at 37 degrees C for 15-60 min in Krebs-Henseleit buffer, pH 7.4, with or without (control experiments) 0.2 microM [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 of 14C-thiamin, the tissue content of endogenous thiamin pyro- and triphosphate decreased during the incubation, whilst that of thiamin-monophosphate 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 with 14C-thiamin, when an efficient thiamin phosphorylation could be demonstrated. During incubation with 14C-thiamin, the concentration of 14C-thiamin-pyrophosphate and, to a lesser extent, that of free 14C-thiamin increased progressively in the tissue, while 14C- thiamin-monophosphate content remained almost unchanged. No 14C-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.