Free thiamine as the likely precursor of endocellular thiamine phosphates in everted rings of rat jejunum.

Labeled and unlabeled (endogenous) free and phosphorylated thiamine were measured in isolated everted rings of rat jejunum in vitro during short incubation periods (1-10min). Shortly after the addition of thiamine-14C to the incubation medium, the intracellular specific activity of the free form was higher than the specific activity of the phosphorylated fraction. In the course of time this difference diminished and finally the two specific activities became equal. The conclusion was reached that free thiamine is the likely precursor of intracellular phosphorylated thiamine. Moreover evidence is presented which indicates that free thiamine entered actively into intestinal epithelial cells. Since free thiamine was modified into phosphorylated form inside the cells, its movement against the endocellular concentration gradient was noticeably favoured.