Isolation and characterization of fragments of human serum albumin containing some of the antigenic sites of the whole molecule

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Abstract
Human serum albumin was digested with chymotrypsin in a dialysis sac and from the ultrafiltrate 4 components having some of the antigenic sites of the original molecule were isolated. The largest of these antigenic components will precipitate antibody from rabbit anti-human serum, and is apparently a mixture of 2 antigenic components. Two further antigenic components of molecular weight 23400 and 19000 will precipitate 10-20% of antibody and the former gives total inhibition and the latter partial inhibition of precipitation of antibody with human albumin. The smallest antigenic component, molecular weight 7100, does not precipitate antibody either in solution or in agar gel but gives partial inhibition of precipitation. The antigenic site or sites of this molecule are also part of the antigenic structure of the two larger fragments. The ratio of inhibitor to albumin for half the maximum inhibition varies from 0.3 to 1.5 for the three inhibitors, and the bearing this has on estimates made of the size of an antigenic site is discussed. The amino acid composition of the inhibitor of molecular weight 7100 has been determined and found to contrast very sharply with the composition of the whole molecule.