Amino acids interfere with the ninhydrin assay for asparagine

A colorimetric ninhydrin assay, reportedly specific for asparagine, has been published (Sheng et al. (1993). Anal. Biochem., 211, 242–924). The suitability of this assay for determining asparagine in food samples was tested by using it to measure the asparagine levels in tips of harvested asparagus spears. Compared with our standard chromatographic procedure (x), the colorimetric assay (y) severely overestimated asparagine in asparagus (y = 129·6 + 1·12x, Syx = 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 that they also react with ninhydrin under the conditions of Sheng et al. In particular, proline gave a response two-fold greater than asparagine and the response with phenylalanine was half that of asparagine. It is believed that the overestimation of asparagine by the ninhydrin method is due to the additive response of the other amino acids present in asparagus. It is concluded that the assay is unsuitable for the routine determination of asparagine in asparagus, or in complex mixtures of amino acids in other foods.