Intestinal absorption of water-soluble vitamins in rainbow trout (Onchorhynchus mykiss).

The intestinal uptake of water-soluble vitamins, nicotinamide, riboflavin, biotin and folic acid, was studied in isolated everted intestinal sleeves of the cold-water teleost rainbow trout (Onchorhynchus mykiss). The presence of a carrier-mediated transport mechanism was determined by competitive inhibition and by Michaelis-Menten kinetics. The uptake of riboflavin, biotin or folic acid was not only subject to competitive inhibition but also a saturable function of increasing vitamin concentration in the incubation medium. The kinetic constants of the saturable mechanism were for riboflavin: K(m), 2.32 +/- 0.76 microM; Vmax, 0.26 +/- 0.04 pmol/mg min; for biotin: K(m), 9.70 +/- 3.76 microM; Vmax, 0.31 +/- 0.07 pmol/mg min; and for folic acid: K(m), 32.9 +/- 21.2 microM; Vmax, 3.63 +/- 0.99 pmol/mg min. In contrast, the uptake of nicotinamide was not subject to competitive inhibition and was a linear function of concentration (Kd, 0.140 +/- 0.012 pmol/mg min microM). Folic acid was absorbed more rapidly than and was not inhibited by its derivative, 5-methyl-tetrahydrofolate. Thus, the intestinal uptake of riboflavin, biotin and folic acid is carrier-mediated while that of nicotinamide occurs by simple diffusion. These mechanisms are similar to those found in the channel catfish for the same vitamins, except for folic acid, which is absorbed by diffusion in this warm-water omnivorous species.