Abstract
The intestinal transport of the naturally occurring folate coenzyme, 5-methyltetrahydrofolate, was studied using everted sacs of rat jejunum. The study provides evidence that intestinal transport of 5-methyltetrahydrofolate is composed of two systems: 1) an active, carrier-mediated system which is demonstrable at low concentrations; and 2) a diffusion system which is demonstrable at high concentrations. The active system is characterized by: 1) saturation kinetics with Km congruent to 0.3 microM; 2) accumulation against a concentration gradient with a serosal-to-mucosal ratio of 1.8; 3) inhibition by metabolic poisons; 4) inhibition by oxidized and reduced folate analogs; 5) temperature dependence; 6) sodium dependence; 7) glucose dependence; and 8) specificity for the jejunum. These features are strongly pH-dependent, and demonstration of active transport of 5-methyltetrahydrofolate requires a buffer pH of 6, glucose in the incubation medium and a substrate concentration of less than 10(-6) M. The diffusion process is characterized by: 1) linear increase in the mucosal-to-serosal transport of 5-methyltetrahydrofolate with increasing mucosal concentration to 10(-6) M and above; 2) energy independence; 3) pH independence; and 4) temperature independence. These studies clarify the mechanism of intestinal transport of 5-methyltetrahydrofolate, show the similarities to transport of other folate compounds and provide a unified concept of intestinal folate transport.
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|