Intestinal absorption of water-soluble vitamins in channel catfish
( ictalurus punctatus).
Casirola, D. M., R. R. Vinnakota, and R. P. Ferraris.
Department of Physiology, New Jersey Medical School, Newark, NJ
07103-2714
APStracts 2:0083R, 1995.
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
nonlabelled form, indicating the presence of a saturable mechanism
(riboflavin: Km, 4.84 + 1.23 [mu]M; Vmax, 0.59 + 0.07 pmol/mg min;
biotin: Km, 22.3 + 6.95 [mu]M; Vmax, 0.67 + 0.10 pmol/mg min). In
contrast, the uptakes of nicotinamide, folic acid and its metabolic
derivative 5-methyl-tetrahydrofolate, were not significantly
inhibited by their respective nonlabelled forms. Their uptakes were a
linear function of concentration (Kd, 0.173 + 0.006, 0.050 + 0.002
and 0.031 + 0.004 pmol/mg min [mu]M, respectively). Folic acid was
absorbed more rapidly than 5-methyl-tetrahydrofolate, and neither
vitamin inhibited each other's absorption. Intestinal uptake of
riboflavin and biotin is carrier-mediated while that of nicotinamide,
folic acid and 5-methyl-tetrahydrofolate 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.
Received 21 July 1994; accepted in final form 23 March 1995.
APS Manuscript Number R401-4.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 April 1995.