Chemical and physical modification of proteins by the hydroxide ion.

Proteins are exposed to alkaline conditions during solubilization and/or purification, during food storage and processing, in removal of toxic constituents, and for characterization. During alkali treatment, there are changes in solubility and aggregation, hydrolysis, elimination reactions involving the side chains of certain amino acids, racemization of amino acid residues, addition of compounds to proteins, fragmentation of the peptide chain, as well as modification or elimination of nonprotein constituents. The rates of these reactions are affected by pH, temperature, cations (in some cases), ionic strength (in some cases), protein concentration, and to some extent by the specific nature of the protein. The general mechanisms and stoichiometry of these reactions are described. Other constituents of high protein foods also undergo reactions in alkaline solutions and the products of these reactions may in turn react with proteins. We have described the effect of alkali on enediol formation and fragmentation of carbohydrates, the hydrolysis of lipids in alkaline solution and effect on rate of peroxidation of the polyunsaturated fatty acids, the oxidation of amino acid residues, especially methionine, the oxidation of phenols to benzoquinones, and the catalytic effect of metal ions in alkaline solutions. Alkali treatment is also used in the specific modification of proteins to distinguish between O-glycosyl and amide-linked glycosyl groups, to effect specific cleavage of peptide bonds via beta elimination, in the formation of anhydrotrypsin, anhydrochymotrypsin, anhydrosubtilisin and thiol-subtilisin, and in formation of intrachain crosslinking in proteins.