Reactions of protein with phenols and quinones evaluation of amino acid modification and protein digestibility Public Deposited

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  • Plant tissues contain a wide variety of phenolic compounds, frequently in high concentration. Both non-covalent association of intact and oxidized phenols and covalent linkage of oxidized phenols to protein occur. Such interactions can have important biochemical and nutritional significance. A model system approach was used to identify and quantitate amino acid residue modification in protein exposed to oxidized phenolic material and to identify the probable adduct compound. Digestibility studies were carried out on tanned bovine serum albumin to assess further protein-phenol association and to estimate the extent of covalent and non-covalent binding of oxidized phenolic material to protein. Preliminary studies indicated that non-enzymatic base-catalyzed air oxidation of pyrocatechol occurred rapidly above pH 7.5. Spectral analysis of oxidized pyrocatechol showed a continuous absorption throughout the visible region of the spectrum indicating the formation of a wide variety of products. The presence of bovine serum albumin in such a system changed the nature of oxidized products as evidenced by alterations in the visible absorption spectrum. Tanned bovine serum albumin showed a visible absorption spectrum similar to that of oxidized pyrocatechol which, in conjunction with the lack of visible absorption by bovine serum albumin, indicated association of pyrocatechol oxidation products with the protein. The strong nature of these associations was indicated by the failure of solvent stripping agents to completely remove bound material. Spectral analysis of the products of mushroom tyrosinase oxidation of pyrocatechol indicated that o-benzoquinone was the initial oxidation product and that product inhibition of tyrosinase occurred in the systems. Binding of enzymatic pyrocatechol oxidation products to glycine, bovine serum albumin, and poly-L-lysine was indicated. Amino acid analysis of tanned bovine serum albumin indicated that cystine, lysine, and histidine were the primary residues modified by covalent linkage to oxidized pyrocatechol. A similar pattern of amino acid modification was found in bovine serum albumin tanned by either base-catalyzed or tyrosinase oxidation of pryocatechol, or by incubation with p-benzoquinone. The extent of modification was greatest in the p.-benzoquinone system implying that the quinone was the reactive species. Observed modifications increased with increasing pyrocatechol or R-benzoquinone concentrations and with increasing pH. Gas-liquid chromatography of N-trifluoroacetyl-L-tryptophan methyl ester and N-trifluoroacetyl-DL-methionine methyl ester exposed to enzymatically oxidized pyrocatechol indicated that these amino acids were also modified, the extent of modification increasing with increasing pyrocatechol level. Trypsin digestibility of tanned BSA was markedly reduced while pepsin digestibility was less affected. Protein-phenol complex formation as evidenced by decreased digestibility, is a result of covalent and non-covalent binding of phenolic material. Digestibility decreases greater than the observed amino acid modifications resulting from covalent binding of oxidized phenolic material suggest, as one possibility, that non-covalent association of phenolic material is predominant over amino acid modifications. Modifications occurring to the nutritionally essential amino acids lysine, methionine, tryptophan, and histidine (infants only), and the decrease in digestibility of tanned protein may drastically reduce the nutritional value of a protein. These same modifications may alter the biochemical properties of protein. Procedures to isolate plant proteins must be designed to minimize these effects. Ultraviolet, infrared, and mass spectral analyses of a glycine-p-benzoquinone reaction mixture indicated that a phenolic secondary amine (Ar-NH-R) was one of the products. Results suggest a 1, 4-addition reaction as the probable means of amino acid modification in oxidized phenolic systems (literature on amino-acid quinone interaction supports this conclusion).
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