Abstract
It was found that lysine reacts with an acidic ninhydrin-copper reagent to give a characteristic orange color. The reagent and the color are very stable. Moreover, the reaction is very specific for lysine. Other amino acids with the exception of proline and glycine, salts, glucose, urea, ammonia do not give similar color and do not interfere the reaction. Applying this reaction, the authors developed a simple and rapid method for the detection and colorimetric determination of lysine in lysine-enriched foods.
A sample is extracted with hot water. Carbohydrate in the extract is hydrolysed with amylase under suitable conditions to obtain clear solution. After centrifugation, if necessary, the clear super-natant solution is concentrated to 15 to 50mg/dl. Two ml of this solution is mixed with 4ml of the reagent solution (5.0g of ninhydrin and 6.7g of cupric chloride dihydrate are dissolved in 125ml of 0.1M citrate buffer (pH 1.30) and 375ml of methylcellosolve, then diluted with 500ml of water). The mixture is heated in a boiling water bath for 20 minutes and cooled. Then 15ml of water is added to the mixture. The absorbance at 475mμ in a 10-mm cell is read against a reagent blank. Lysine in lysine-enriched bread and other foods were determined by using this procedure. The results were in close agreement with those obtained by a microbioassay or by a conventional colorimetric method with Folin's reagent.
