Metabolic transit of lysinoalanine (LAL) bound to protein and of free radioactive [14C]-lysinoalanine

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.

Excretion and tissue distribution of radioactive lysinoalanine, N epsilon-DL-(2-amino-2-carboxyethyl)-U-14C-L-lysine (LAL) in Sprague-Dawley rats

Rats were fed either a control soybean protein diet or a diet containing 3,000 ppm soybean protein-bound lysinoalanine (LAL) for 4 or 6 weeks, at which time all rats were dosed by stomach tube with 14C-LAL labeled in the lysine moiety. Urinary and fecal excretion and tissue distribution were followed in one experiment at 6, 12, 18, 24, 48 and 72 hours. Excretion in urine, feces and expired air was followed in the other metabolic experiment at 2-hour intervals for 48 hours, and at 24-hour intervals for the next 7 days. Tissue samples were counted and LAL determination was made by amino acid analysis in both experiments. The group of rats fed LAL excreted slightly more LAL than the group fed the control diet. Very little LAL remained in the rat tissues after either experiment, and the largest remaining quantity of radioactivity was found as lysine. Quantitation of 14C-lysine in the original material and in the material from rat organs showed that the rat has some capacity for converting LAL to lysine. Less than 0.5% of the original 14C remained in any organ examined 9 days after dosing in the either control or LAL-fed rats. Autoradiographs of the kidneys 24 hours after dosing showed that the radioactive material had accumulated in the proximal convoluted tubules of the corticomedullary junction.

Effect of protein-bound lysinoalanine, N epsilon-DL-(2-amino-2-carboxyethyl)-L-lysine on fetal and neonatal rats

Diets containing 5% to 30% of an alkali-treated isolated soybean protein (ATSP) containing 1% lysinoalanine (LAL) were fed to female Sprague-Dawley rats during gestation and lactation to provide 500, 1,000, 2,000, or 3,000 ppm LAL in the diet. An isolated soybean protein containing no LAL was used as control. No teratological effects were observed. No significant differences in birth weight, mortality, live births/litter, or number of pups/litter was found at any LAL level fed. However, significantly decreased weight gains were observed in pups from dams fed either 2,000 or 3,000 ppm LAL. No LAL was found in the milk, and protein content of all milks was similar. The decreased weight gains appeared to be due to reduced milk production in dams fed high levels of ATSP.