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A technique is described by which both the numbers of tryptophan residues and their approximate locations in the peptide chain of a protein can be determined by cleavage with N-bromosuccinimide followed by polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate. The number of new peptide bands appearing in the gel is a function of the number of tryptophan residues, and the relative migration of the bands permits calculation of peptide molecular weights and an estimation of the positions of the tryptophan residues in the peptide chain. The technique uses a sample of about 0.5 mg and is suitable for any protein that contains a small number of tryptophan residues. These are the very specimens that are difficult to assay accurately for tryptophan by spectrophotometric or colorimetric methods. Tryptophan residues which are within about 20 residues of the ends of the peptide chain or of each other would not be detected. The specificity of the cleavage with N-bromosuccinimide was ascertained by utilizing human serum albumin, which is known to have a single tryptophan residue at position 214. The technique was then applied to a comparative study of the numbers and locations of tryptophans in the serum albumins of 16 species, namely 11 mammals, three birds and two amphibians. The number of tryptophan residues were confirmed by an independent colorimetric method. All of the mammalian albumins contained a tryptophan residue near position 213. The three avian albumins examined have no tryptophan. Frog and toad albumins contained two tryptophan residues, which appear to be situated at different positions from those in mammalian albumins.

Determination of the number and relative position of tryptophan residues in various albumins