Tryptophan Contents of Pea and Broad Bean Seeds as a Function of Nitrogen Content

Abstract

The tryptophan contents of pea and broad bean seeds were determined for samples originating from commercial and experimental cultivars, using a procedure that ensured the true quantity of this amino acid to be evaluated. For both species a relationship was found indicating a strict proportionality between the levels of tryptophan (Trp) and nitrogen (N) of the seed. The values available in the literature for tryptophan (159 for pea and 53 for broad bean) were compared with those predicted from the nitrogen of samples through the relationship between Trp and N. They ranged from 68 to 153% with a weighted mean of 100% for pea and from 98 to 114% with a mean of 101% for broad bean when they were expressed as percentage of predicted value. The lowest percentages were related to underestimation owing to degradation of tryptophan by alkali or its incomplete release by enzyme during hydrolysis preparatory to analysis, and the highest to overestimation attributed to lysinoalanine generated during alkaline hydrolysis and coeluted with tryptophan when ion exchange chromatography is used for analysis. Some linear relationships previously reported appeared to be close from those determined in the present study, suggesting the absence of tryptophan degradation with some hydrolytic conditions and limited influence of genetic factors upon tryptophan content of pea and broad bean seeds. Keywords: Tryptophan; nitrogen; pea seed; broad bean seed