Chemical and enzymic assays for available lysine

Lysine is of prime nutritional significance since it is the first limiting amino acid in many foods of plant origin and is easily rendered unavailable upon heat processing or upon unfavourable storage conditions. The term "available lysine" refers to forms of lysine which contain free €-amino groups within the peptide chain. Once the Є-amino group is blocked, lysine becomes unavailable since it cannot be hydrolyzed by proteolytic enzymes. The availability of lysines in casein, lysozyme, β-lactoglobulin, acid solubilized gluten and whole egg was determined by the pepsin pancreatin digestion test, the dinitrobenzene sulfonic acid (DNBS) and the trinitro-benzene sulfonic acid (TNBS) methods. The results were compared to the fluorodinitrobenzene (FDNB) method. Good agreement was obtained between the FDNB official method and the DNBS technique, with a correlation coefficient of 0.989. When the TNBS method was compared to the FDNB difference technique, a correlation coefficient of 0.988 was found. The pesin pancreatin digestion test indicated the relative amount of lysine released by the enzymes under the conditions specified by the test. A correlation coefficient of 0.995 was found between the FDNB official method and the enzymatic test. The specificity of DNBS for the Є-amino group of lysine was determined using α- and Є-formyl-lysines, L-lysine, L-lysyllysine, L-lysylalanine and ribonuclease-S-peptide. DNBS was found to react mainly with Є-amino group but with a slight reactivity with α-amino group. However, in the case of proteins with several Є-amino groups and N-terminal lysine, the contribution of the Є-amino group to the results becomes negligible. The DNBS method was found to be the simplest and most reliable method for determination of available lysine, for the following reasons: a) it does not require acid hydrolysis of the proteins; b) a large number of samples can be analyzed simultaneously in a few hours, and c) it does not require expensive and lengthy chromatographic amino acid analysis.Land and Food Systems, Faculty ofGraduat