AJP - Regulatory, Integrative and Comparative Physiology, Vol 269, Issue 3 490-R496, Copyright © 1995 by American Physiological Society
Intestinal absorption of water-soluble vitamins in channel catfish (Ictalurus punctatus)
D. M. Casirola, R. R. Vinnakota and R. P. Ferraris
Department of Physiology, New Jersey Medical School, Newark 07103-2714, USA.
The absorption of selected water-soluble vitamins was studied in isolated
intestinal sleeves of channel catfish (Ictalurus punctatus). The uptakes of
riboflavin and biotin were each inhibited by their nonlabeled form,
indicating the presence of a saturable mechanism [riboflavin: Michaelis
constant (Km), 4.84 +/- 1.23 microM; maximal velocity (Vmax), 0.59 +/- 0.07
pmol.mg-1.min-1 biotin: Km, 22.3 +/- 6.95 microM; Vmax, 0.67 +/- 0.10
pmol.mg-1.min-1]. In contrast, the uptakes of nicotinamide, folic acid, and
its metabolic derivative 5-methyltetrahydrofolate were not significantly
inhibited by their respective nonlabeled forms. Their uptakes were a linear
function of concentration [dissociation constant, 0.173 +/- 0.006, 0.050
+/- 0.002, and 0.031 +/- 0.004 pmol.mg-1.min-1.microM-1, respectively].
Folic acid was absorbed 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 in
mammals but where diffusion may suffice to fulfill metabolic requirements
of catfish.
