A colorimetric enzyme assay using Escherichia coli to determine nutritionally available lysine in biological materials

A novel bacterial method is described for determining nutritionally available lysine in protein foods with a lysine auxotroph of Escherichia coli (strain M2626). Lysine‐dependent synthesis of the induced enzyme bT‐galactosidase is determined by a colorimetric method. With this approach sensitivity is increased ca 100‐fold and assay time decreased to ca 2 h. The improved procedure was applied to the assay of lysine present as the free amino acid or in small peptides, and after enzymic pre‐digestion in vitro with a mixture of pronase and intestinal peptidases, to pure proteins and a variety of feed meals and rice cultivars. In addition, heat treatment of complex samples was shown to lower their content of available lysine, as judged by the decreased nutritional response of the E.coli strain.