Damage of Amino Acid Residues of Proteins after Reaction with Oxidizing Lipids: Estimation by Proteolytic Enzymes

A mixture of casein and methyl linoleate was stored at 50°C and 80% relative humidity for 0 - 14 days and damage to amino acid residues assessed. The damage was estimated by determining the amino acid composition of the hydrolysate by using proteolytic enzymes (pepsin-pancreatin digestion, followed by aminopeptidase-prolidase hydrolysis). The damage to amino acid residues was the most extensive in methionine, followed by tryptophan, histidine, and lysine. The degree of damage to these amino acids was also determined by chemical methods without using proteolytic enzymes. The efficiency of detecting damage by the present enzymatic method was close to that of the chemical methods.