AMINO-ACIDS INTERFERE WITH THE NINHYDRIN ASSAY FOR ASPARAGINE

A colorimetric ninhydrin assay, reportedly specific for asparagine, ha s been published (Sheng et al. (1993). Anal. Biochem., 211, 242-9). Th e suitability of this assay for determining asparagine in food samples was tested by using it to measure the asparagine levels in tips of ha rvested asparagus spears. Compared with our standard chromatographic p rocedure (x), the colorimetric assay (y) severely overestimated aspara gine in asparagus (y = 129 . 6 + 1 . 12x, S-y/x = 32 . 2) even though the results from the two methods were highly correlated (r = 0 . 997, P = 0 . 003). By testing other amino acids in the assay, we showed tha t they also react with ninhydrin under the conditions of Sheng et al. In particular, proline gave a response two-fold greater than asparagin e and the response with phenylalanine was half that of asparagine. It is believed that the overestimation of asparagine by the ninhydrin met hod is due to the additive response of the other amino acids present i n asparagus. It is concluded that the assay is unsuitable for the rout ine determination of asparagine in asparagus, or in complex mixtures o f amino acids in other foods.