HYDROLYSIS AND ABSORPTION OF 2 MONOPHOSPHATE DERIVATIVES OF ASCORBIC-ACID BY CHANNEL CATFISH ICTALURUS-PUNCTATUS INTESTINE

Brush-border membrane vesicles prepared from channel catfish intestine released phosphate from two monophosphate-stabilized forms of ascorbi c acid (AA) at comparable rates, with activity in proximal intestine e xceeding that in distal intestine. Apical absorption of AA by everted intestinal sleeves was greater in proximal intestine with maximal rate s of absorption for the phosphate-stabilized forms comparable, but abo ut 40% of those for native AA. Rates of passive influx were negligible for all forms of AA. Although the presence of phosphate groups delaye d absorption, presumably because the phosphate had to be removed befor e AA translocation, affinity of carriers was higher for the monophosph ate than native AA. This suggests less of the monophosphate AA is oxid ized prior to uptake allowing a greater percentage to be transported v ia the high affinity, Na+-dependent transporter. These findings imply bioavailability of the two monophosphate-stabilized forms of AA is dep endent on a two-stage process: (1) intestinal hydrolases remove the ph osphate group then (2) absorption of released AA via carrier-mediated processes that are comparable to those of mammals. Capacities of catfi sh intestine to release and absorb AA from both phosphate-stabilized f orms exceed estimates of daily requirements by at least three orders o f magnitude.