Oxidation of Free Tryptophan and Tryptophan Residues in Peptides and Proteins

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Abstract
The oxidation of free tryptophan (Trp) and Trp residues in peptides and proteins by hydrogen peroxide as oxidative agent has been used to evaluate Trp losses and the pattern of degradation products formed. Besides free Trp, four Trp-containing peptides and lysozyme were used as substrates in the aqueous model system. The oxidation rate of Trp and the formation of 16 possible degradation compounds were examined using RP-HPLC and UV, fluorescence, and photodiode array detection. The rate of Trp degradation increased from lysozyme to short-chain peptides to unbound Trp. Only ∼20% of the total Trp loss could be elucidated by the determined Trp degradation compounds. Oxindolylalanine (Oia), 3a-hydroxy-1,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole-2-carboxylic acid (PIC), N-formylkynurenine (NFK), dioxindolylalanine (DiOia), kynurenine (Kyn), and 5-hydroxytryptophan (5-OH-Trp) were identified in this order of quantity as degradation compounds, showing the Trp pyrrole moiety to be most susceptible to oxidation. As short peptides such as H-Ala-Trp-Ala-OH were completely hydrolyzed with immobilized Pronase E, Oia and NFK could be identified as main degradation compounds, as could minor amounts of Kyn, DiOia, PIC, and 5-OH-Trp. Acid (6 M HCl), alkaline (4.2 M NaOH), and enzymatic hydrolyses were compared for the determination of Trp degradation compounds in lysozyme. Kyn, Oia, and DiOia could be detected in the hydrogen peroxide treated protein. Keywords: l-Tryptophan; high-performance liquid chromatography (HPLC); oxidation; enzymatic hydrolysis