Behavior of O-glycosyl and O-phosphoryl proteins in alkaline solution

Aspects of the formation of lysinoalanine in milk and milk products

Treatment of food proteins with heat and/or alkali may convert some of the constituent amino acids into the unnatural amino acid lysinoalanine (LAL), which has been found to cause kidney damage (nephrocytomegaly) when fed to rats. LAL may be produced by β-elimination of a phosphoseryl, of a glycosylated seryl or of a cystyl residue followed by a nucleophilic addition to a lysyl residue. The formation of LAL in αs0-, αsl- and β-casein is discussed in relation to the location of phosphoseryl residues immediately adjacent to lysyl residues. The influence of pH, temperature and exposure time on the formation of LAL in whole casein is described in detail. The formation of LAL can be prevented by the addition of reducing agents, dephosphorylation or chemical modification of the є-amino group of lysine. The LAL content of foodstuffs can be measured accurately by ion-exchange chromatography and a survey is given of the LAL content of commercial milk and milk products.

The biological effects of free and protein-bound LAL are discussed. It seems that renal changes due to the presence of LAL in the diet are species-specific to the rat although recent findings have shown that no effect is observed in rats fed a mixed diet containing proteins with and without protein-bound LAL. When LAL is removed from the diet, nephrocytomegaly disappears. From these results it may be concluded that the health risks for man, connected with the eating of proteins which have been subjected to treatment with heat and/or alkali, are small.

The decrease in lysine content of milk and milk products due to formation of LAL is small (0–1 or 2%) in comparison with the decrease in available lysine possibly caused by the Maillard reaction (0–24%) and so the decrease in nutritive value of food products due to LAL formation may be neglected.

Lysinoalanine in food and in antimicrobial proteins

Heat and alkali treatment of food proteins widely used in food processing results in the formation of crosslinked amino acids such as lysinoalanine, ornithinoalanine, lanthionine, and methyl-lanthionine and concurrent racemization of L-amino acid isomers to D-analogues. The mechanism of lysinoalanine formation is a two-step process: first, hydroxide ion-catalyzed elimination of cysteine and serine residues to a dehydroalanine intermediate; second, reaction of the double bond of dehydroalanine with the epsilon-NH2 group of lysine to form a lysinoalanine crosslink. The corresponding elimination-addition reaction of threonine produces methyl-dehydroalanine, which then reacts with the NH2 and SH groups to form methyl-lysinoalanine and methyl-lanthionine, respectively. The crosslinked amino acids lanthionine and methyl-lanthionine are formed by analogous nucleophilic addition reactions of the SH group of cysteine to dehydroalanine and methyl-dehydroalanine, respectively. Processing conditions that favor these transformations include high pH, temperature and exposure time. Factors which minimize lysinoalanine formation include the presence of SH-containing amino acids such as cysteine, N-acetyl-cysteine, and glutathione, dephosphorylation of O-phosphoryl esters, and acylation of epsilon-NH2 groups of lysine side chains. The presence of lysinoalanine residues along a protein chain decreases digestibility and nutritional quality in rodents but enhances nutritional quality in ruminants. Protein-bound and free lysinoalanines are reported to induce enlargement of nuclei of rat kidney cells. All of the mentioned dehydro and crosslinked amino acids also occur naturally in certain peptide and protein antibiotics. These include duramycin, cinnamycin, epidermin, subtilin and the widely used food preservative nisin. Mechanistic rationalizations are offered for the observed antimicrobial activities of these compounds in relation to their structures. The cited findings and new research to better define the chemistry and dietary and antimicrobial roles of lysinoalanine and related compounds should lead to better and safer foods.

Beta-elimination of phosphate and subsequent addition of pyridoxamine as a method for identifying and sequencing peptides containing phosphoseryl residues

Peptides containing phosphoseryl residues can be modified by removal of the phosphate groups via beta-elimination followed by addition of pyridoxamine to the resulting dehydroalanyl residue. Peptides containing the modified residues can be detected at nanomole levels by monitoring absorbance at 328 nm or at picomole levels by monitoring fluorescence. Photolysis of the modified peptide converts the pyridoxamino adduct to a form which can be readily identified after Edman degradation.

Protein-alkali reactions: chemistry, toxicology, and nutritional consequences

Heat and alkali treatment of proteins catalyzes formation of crosslinked amino-acid side chains such as lysinoalanine, ornithino-alanine and lanthionine, and concurrent racemization of L-isomers of all amino acid residues to D-analogues. Factors that favor these transformations include high pH and temperature, long exposure, and certain inductive or steric properties of the various amino acid side chains. Factors that minimize crosslink formation include the presence of certain additives, such as cysteine or sulfite ions, and acylation of epsilon-NH2 groups of lysine side chains. Free and protein-bound lysinoalanine and D-serine induce nephrocytomegaly in rat kidney tissues. The presence of lysinoalanine and D-amino acid residues along a protein chain decreases its digestibility and nutritional quality. Understanding the factors that govern the formation of potentially harmful unnatural amino acid residues in food proteins and the toxic and nutritionally antagonistic action of these compounds in animals should lead to better and safer foods.

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.

Inhibition of lysinoalanine synthesis by protein acylation

Treating wheat gluten, soy protein, and lactalbumin under alkaline conditions at 65 degree C for various times destroys part of the threonine, cystine, lysine, tyrosine, and arginine residues in these proteins. The losses were accompanied by the appearance of lysinoalanine and unidentified ninhydrin-positive substances. Treating wool under somewhat milder alkaline conditions destroys part of the cystine and lysine residues, but not the other amino acids cited. In this case, loss of cystine and lysine was accompanied by the appearance of lanthionine and lysinoalanine. Amino acid analysis of alkali-treated acylated proteins revealed that acylation by acetic and succinic anhydrides prevents or minimizes destruction of lysine residues and the formation of lysinoalanine in wheat gluten and soy protein and, in wool, minimizes destruction of cystine and lysine residues and the formation of lanthionine lysinoalanine. Mechanisms are proposed to explain the observed inhibiting effects of protein acylation and oertain additives on lysinoalanine formation.