Degradation of Transferrin in the Presence of Ascorbic Acid and Oxygen

The liberation of ammonia in the reaction of transferrin and ascorbic acid in phosphate buffer at pH 7.4, 37°C was investigated. It was found that transferrin is degraded in the reaction into smaller molecular weight polypeptides. The ammonia evolution was inhibited by catalase and by hydroxyl free radical trapping agents such as benzene, benzoate, and iodide. Dehydroascorbic acid and hydrogen peroxide in the presence of traces of ferric or cupric ions were also studied. It is suggested that the ammonia evolution observed results from deamination of amino acids and polypeptide fragments as a result of the formation of reactive hydroxyl free radicals in the reaction mixture. It has been reported that ascorbic acid under physiological conditions accelerates the degradation of transferrin and the pentapeptide GlyArgAsnArgGly. It was suggested that this degradation resulted from deamidation of glutaminyl and asparaginyl residues {Robinson et al., 1973). These observations are interesting since Robinson and coworkers have hypothesized that non-enzymatic sequencecontrolled deamidation of amide residues in peptides and proteins is an important biological timing mechanism