HYDROLYSIS, ABSORPTION AND BIOAVAILABILITY OF ASCORBIC-ACID ESTERS IN FISH

Accurate methods for quantitatively measuring ascorbic acid and its derivatives are indispensable for probing the known and for discovering new functions of this vitamin. The most commonly used assays are summarized and their limitations characterized as they relate to biological samples in fish nutrition. Because ascorbic acid (AA) is extremely sensitive to degradation during processing and storage in fish diets, AA derivatives with sulfate (AS), phosphate (AP) or glucose (AG) at the C-2 position in the lactone ring have been used to increase its resistance to oxidation. We report data on the vitamin C availability in some of the derivatives based on in vitro hydrolysis experiments with fish enzymes. Enzyme-substrate affinity for enzymatic hydrolysis of ascorbyl mono-, tri- and poly-phosphates was different based on K(m) (mM) values of 1.18, 4.12 and 3.72, respectively. The experiments on in vivo absorption of AA and its derivatives revealed differences relating to the type of diet used (casein-gelatin or fishmeal based). In addition, they showed that AS apparent absorption was not significantly different from that of AA and AP in a practical diet for rainbow trout. We found that AS supplementation resulted in significantly lower concentrations of ascorbate in liver, kidney and intestine of rainbow trout and common carp than when AA was given. In experiments with rainbow trout broodstock, we found that the total ascorbate concentration in eggs from females fed an AA-free diet was significantly lower (82 +/- 32 mug.g-1) than in females fed an AP diet (316 +/- 43 mug.g-1). We concluded that there was no important storage form (AS or AP) in rainbow trout eggs independent of the dietary ascorbate source. Although suggestions have been made regarding AS function in fish as storage form, many are questionable because of the assay method used or because of speculative assumptions that AP may be converted to AS in fish tissues. In the final set of experiments with rainbow trout fry fed diets supplemented with AP, we demonstrated that tissue ascorbate concentration was maintained at a higher level even during depletion than in fish fed AA diets. In conjunction. the former had a much better chance of survival when exposed to a scorbutic diet devoid of vitamin C as a result of processing and/or storage.