| Abstract: | A kinetic study of 14C]thiamine uptake over a concentration range from 0.1 microM to 4 mM was performed in isolated rat hepatocytes. The results showed that two processes contribute to the entry in rat hepatocytes: a low affinity process with a Kt of 34.1 microM and Vmax of 20.8 pmol/10(5) cells per 30 s and a high affinity process with a Kt of 1.26 microM and Vmax of 1.21 pmol/10(5) cells per 30 s. The uptake of thiamine by the high affinity process was concentrative and reduced in a betaine medium or K+ medium. Both ouabain and 2,4-dinitrophenol decreased the thiamine uptake by the high affinity process. These findings indicate that the transport of thiamine via a high affinity process is dependent on Na+ and biological energy. The uptake of thiamine was strongly inhibited by thiamine analogs such as dimethialium and chloroethylthiamine. Among quarternary ammonium compounds other than thiamine derivatives, choline and acetylcholine significantly inhibited thiamine uptake by rat liver cells, whereas betaine and carnitine did not. A kinetic study of thiamine uptake by rat hepatocytes preloaded with pyrithiamine, a potent inhibitor of thiamine pyrophosphokinase, revealed that the biphasic property of thiamine uptake disappeared and a single carrier system for thiamine with a Kt of 40.5 microM, which was similar to the Kt value of the low affinity process, was retained. These results strongly suggest that thiamine transport system in rat liver cells is closely connected with thiamine pyrophosphokinase, which accelerates the uptake rat of thiamine by pyrophosphorylation at physiological concentrations of thiamine. |
