Aspects of the formation of lysinoalanine in milk and milk products

Abstract

SUMMARYTreatment 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.

Changes in the acid gelation of skim milk as affected by heat-treatment and alkaline pH conditions

“…This result thus ruled out alkaline hydrolysis as the main cause of the degradation of the milk proteins, unless this reaction was not extended enough to produce small peptides soluble in 12% trichloroacetic acid. Other possible causes are the formation of "unnatural" amino acids like lysino-alanine or lanthionine through the alkaline mediated elimination of the thiol of a cystein residue or of the organic phosphate moiety of a serine residue (both examples being particularly relevant to milk) and reaction of the dehydro-alanine protein with the ε-NH 2 group of lysine of another protein [11,15]. However, if the crosslinked proteins are being hydrolysed to yield the release of lysino-alanine then it is expected that NPN increases in the milk; also, both αs1and β-casein have been shown to produce lysinoalanine when heated at alkaline pH [28] where as α s1 -casein was barely affected in our conditions.…”

Contribution of mass spectrometry to assess quality of milk‐based products

“…When lysine residues are involved in such reaction, lysinoalanine (LAL) is formed. LAL is one of the most common modifications monitored in milk products, as marker of industrial treatments (De Koning & Van Rooijen, 1982; Fritsch, Hoffmann, & Klostermeyer, 1983). Indeed, this compound could also be formed upon heat treatment and has been detected in many heated food proteins, and in higher amounts in liquid formulas compared to powder ones (Finot, 1983).…”

Ultra-High-Temperature Processing of Milk and Milk Products

“…This formation is not a consequence of the Maillard reaction, but since it adds to the loss of availability of lysine which is a major ultimate result of theMaillardreaction.itis being considered here. Lysino-alanine (LAL) can be measured by means of an amino-acid analyser after hydrolysis to free LAL from the proteinbound form (Fritsch & Klostermeyer, 1981a,b;de Koning & van Rooijen, 1982). It has been detected in significant levels in incontainer and UHT-sterilized milks and creams.…”