Oxidation of Free and Peptide Bound Trytophan

Abstract

Food is often processed or stored under conditions suitable for the oxidative damage of proteins, in particular of the amino acids methionine, lysine, histidine and tryptophan (Trp). The forming products can affect sensory quality, nutritional value and food safety. Focusing on Trp as an essential amino acid, most authors have described the decrease of free or protein bound Trp during oxidative treatment such as the addition of hydrogen peroxide (H₂O₂) (de Week et al., 1987) or of oxidizing lipids (Matoba et al., 1984), or have determined the bioavailability of Trp in oxidized food proteins (Nielsen et al., 1985). Although many oxidation products of Trp are known (Savige, 1975), little is known about their formation in proteins. Krogull and Fennema (1987) determined N-formylkynurenine (NFK) and kynurenine (Kyn) after lipid oxidation; Holt et al. (1977) identified oxindolylalanine (Oia), aspartic acid (Asp) and Kyn after photo oxidation of Trp containing peptides. The aim of the present study was to identify and compare the degradation products of free, peptide and protein bound Trp formed in a simple model system with H₂O₂ as oxidative agent.