Photometric estimation of proline and ornithine.

Some years ago Grassmann and von Arnim (1, 2) described in detail a red reaction product of proline and ninhydrin formed at pH 7; it was suggested that this reaction product might be of use as a measure of the concentration of proline in biological materials. Hydroxyproline gave a similar though not identical reaction product. More recently, Van Slyke et al. (3), in studies of the ninhydrin method for the gasometric estimation of free amino acids, reported that, at a pH of approximately 1.0, a red water-insoluble reaction product is formed by proline with ninhydrin; lysine forms a black product and ornithine a red one. No significant amounts of color are formed with most other amino acids at a pH near 1.0. Under the conditions specified below, it has been found that the amount of colored product formed by the reaction of certain amino acids with ninhydrin at approximately pH 1.0 may be estimated spectrophotometritally and used as a measure of the amount of these amino acids present. The procedure is applicable to proline, ornithine, lysine, and hydroxylysine in pure solution. Most other amino acids and particularly hydroxyproline do not interfere significantly.