INTESTINAL-ABSORPTION OF WATER-SOLUBLE VITAMINS IN RAINBOW-TROUT (ONCHORHYNCHUS MYKISS)

The intestinal uptake of water-soluble vitamins, nicotinamide, ribofla vin, 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 determin ed by competitive inhibition and by Michaelis-Menten kinetics. The upt ake of riboflavin, biotin or folic acid was not only subject to compet itive inhibition but also a saturable function of increasing vitamin c oncentration in the incubation medium. The kinetic constants of the sa turable mechanism were for riboflavin: K-m, 2.32 +/- 0.76 mu M; V-max, 0.26 +/- 0.04 pmol/mg min; for biotin: K-m, 9.70 +/- 3.76 mu M; V-max , 0.31 +/- 0.07 pmol/mg min; and for folic acid: K-m, 32.9 +/- 21.2 mu M; V-max, 3.63 +/- 0.99 pmol/mg min. In contrast, the uptake of nicot inamide was not subject to competitive inhibition and was a linear fun ction of concentration (K-d, 0.140 +/- 0.012 pmol/mg min mu M). Folic acid was absorbed more rapidly than and was not inhibited by its deriv ative, 5-methyl-tetrahydrofolate. Thus, the intestinal uptake of ribof lavin, biotin and folic acid is carrier-mediated while that of nicotin amide occurs by simple diffusion. These mechanisms are similar to thos e found in the channel catfish for the same vitamins, except for folic acid, which is absorbed by diffusion in this warm-water omnivorous sp ecies. Copyright (C) 1997 Elsevier Science Inc.