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1. A fragment designated D was isolated from human serum albumin degraded by CNBr. Its properties show that it is made up of the B and C fragments isolated by McMenamy et al. (1971) (J. Biol. Chem., 246, 4744-4750). 2. Reduction of fragment D gives rise to two chains, one of which consists of the second subfragment of reduced fragment B linked to fragment C by an amino acid different from methionine. It thus demonstrates the existence of albumin molecules from which the second methionine residue located between fragments B and C is missing.

1. ‘Inhibitor fragment’ isolated from human serum albumin degraded by rabbit cathepsin D is composed of one peptide chain with two intrachain disulphide bonds. There are two kinds of inhibitor molecules having different N-terminal amino acids: one is threonine and the other glutamine. 2. Fragment F1, isolated from inhibitor degraded by trypsin, is composed of two chains linked by a disulphide bond. There are three kinds of fragment F1. All have one alpha chain in common, which has an intrachain disulphide bond. They differ by the nature of the chain, which is linked to the alpha chain by a disulphide bond. The epsilon chain is present in trace amounts. The two other chains, beta and gamma, differ by their C-terminal amino acid, which is respectively arginine and lysine. 3. Inhibitor is composed of the last 92 or 89 residues of the human albumin molecule and fragment F1 is composed of two parts of this C-terminal portion of the albumin molecule.

When human albumin was treated with CNBr, a fragment designated D was obtained and attributed to the absence from some of the albumin molecules of methionine at position 123 [Lapresle & Doyen (1975) Biochem. J. 151, 637-643]. The present study shows that methionine-123 is converted into homoserine without cleavage of the subsequent methionine-cystine bond. With bovine alpha-lactalbumin, a further example of non-cleavage of a methionine-cystine bond with conversion of methionine into homoserine is reported.