Tryptophan content of the serum albumin of normal and of cadmium-poisoned monkeys.

2 residues of trytophan per molecule of the protein, whereas all albumins in the poisoned monkeys, whether of normal size or low-molecular weight, contained less tryptophan, this amino acid being absent entirely in the minialbumins of both serum and urine. Serum albumin of the usual molecular weight (66,ooo) in the cadmium-poisoned monkeys contained approximately 30% less tryptophan than its normal counterpart in untreated animals. Taken in conjunction with previous observations on the behaviour of minialbumins, which aggregate readily in low-salt media including isotonic saline, it is concluded that approximately 30% of the circulating serum albumin in the poisoned monkeys arose by aggregation of minialbumin molecules. On the basis of the close similarity in amino acid composition, in nearly all other respects, between the various albumins, it is suggested that minialbumin comprises a mixture of peptides derived by fission of the normal albumin molecule along its whole chain length, and that subsequently a peptide containing both tryptophan residues is either deleted metabolically or excluded in the preparation procedures.