STUDIES ON THE ABSORPTION OF S-BENZOYLTHIAMINE O-MONOPHOSPHATE : (II) PERMEABILITY TO RED CELL MEMBRANES

Abstract

S-Benzoylthiamine O-monophosphate (BTMP) as well as thiamine penetrated human red cell membrane very slowly, while S-benzoylthiamine (SBT) was found to penetrate very rapidly. Thus, SBT was suspected to be an actual form in the absorption of BTMP from intestine, after dephosphorylation of BTMP by alkaline phosphatase at the intestinal mucosa. As a possible model, it was demonstrated in the red cell suspension that BTMP showed a high permeability to the cell membrane under the presence of alkaline phosphatase in the medium. In the red cell suspension, it was found that SBT was accumulated in the cells and the cell to medium concentration ratio of the total thiamine reached over 2.8. From studies on i) transport of SBT into red cell ghosts as well as the intact cells, ii) decomposition of SBT in the red cell homogenate and in an aqueous medium, iii) effect of the concentration gradient on the uptake, and iv) effect of the extracellular pH on the uptake, it was concluded that the penetration of SBT into red cells proceeds through two steps ; the first, a passive diffusion of SBT through the cell membrane, and the second, a rapid decomposition of SBT to thiamine and the resultant accumulation in the cell. The decomposition of SBT to thiamine in the cell was shown to be mainly through a non-enzymatic reaction, mostly through exchange with SH group in glutathione. Possible mechanisms for the appearence of thiamine in the incubation medium accompaning the transport of SBT were also discussed.