Flavin-sensitized photooxidation of amino acids present in a parenteral nutrition infusate: Protection by ascorbic acid

Abstract

The visible light mediated photo-oxidation of amino acids present in a parenteral nutrition infusate was studied, using the sensitizing agents usually included in these solutions: riboflavin, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and the multivitamin mixture. Of the 14 amino acids studied (alanine, arginine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, and valine), only histidine, methionine, and tryptophan were photo-oxidized by the action of visible light in the presence of these sensitizers. When a mixture of the three photo-oxidizable amino acids was irradiated the photoconversion of tryptophan predominated. Riboflavin and FMN had about the same efficiency as sensitizers, whereas FAD was substantially less effective. The photo-oxidative efficiency of the multivitamin infusate on the amino acids, measured on the basis of observed molecular oxygen consumption, was greater significantly than that found in the presence of FMN. This difference is because of the antioxidative effect created by the vitamin C present in the multivitamin infusate, in relation to the pro-oxidative action of the flavin in its excited state. It was found that a solution of ascorbic acid and FMN, whose concentrations were equivalent to the one in a parenteral nutrition infusate, has the same rate of molecular oxygen consumption as a solution of the multivitamin infusate when irradiated with visible light. The generation of some oxidation products of the tryptophan, generated after irradiation of this amino acid in the presence of FMN, was monitored with emission spectroscopy. On completion of this same experiment, but adding vitamin C, it was observed that for an initial period of time no generation of tryptophan products occurred, after which, tryptophan was modified and had a rate of modification similar to that shown previously. Tryptophan is protected for the time necessary to consume all the vitamin C present in the solution.