INTESTINAL-ABSORPTION OF WATER-SOLUBLE VITAMINS IN CHANNEL CATFISH (ICTALURUS-PUNCTATUS)

The absorption of selected water-soluble vitamins was studied in isola ted intestinal sleeves of channel catfish (Ictalurus punctatus). The u ptakes of riboflavin and biotin were each inhibited by their nonlabele d form, indicating the presence of a saturable mechanism [riboflavin: Michaelis constant (K-m), 4.84 +/- 1.23 mu M; maximal velocity (V-max) , 0.59 +/- 0.07 pmol . mg(-1). min(-1) biotin: K-m, 22.3 +/- 6.95 mu M ; V-max 0.67 +/- 0.10 pmol . mg(-1). min(-1)]. In contrast, uptakes of nicotinamide, folic acid, and its metabolic derivative 5-methyltetrah ydrofolate were not significantly inhibited by their respective nonlab eled forms. Their uptakes were a linear function of concentration [dis sociation constant, 0.173 +/- 0.006, 0.050 +/- 0.002, and 0.031 +/- 0. 004 pmol . mg(-1). min(-1).mu M(-1), respectively]. Folic acid was abs orbed more rapidly than 5-methyltetrahydrofolate, and neither vitamin inhibited the absorption of the other. Intestinal uptake of riboflavin and biotin is carrier mediated, whereas uptake of nicotinamide, folic acid, and 5-methyltetrahydrofolate occurs by simple diffusion. These mechanisms are similar to those found in mammals for the same vitamins , except for the hydrophobic folates, which are actively transported i n mammals but where diffusion may suffice to fulfill metabolic require ments of catfish.