The first 107 residues of Fragment C of human serum albumin have been sequenced and two positions at which affinity labels block the indole site determined. Histidine 23 is the position of blockage by bromoacetyl-L-tryptophan and lysine 67 is the position of blockage by 5-dimethylaminonaphthalene-1-sulfonyl chloride and probably pyridoxal-5'-phosphate. The presence of an indole ligand at the binding site markedly reduces incorporation of the label into the above lysyl residue, and in the case

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... 5-dimethylaminonaphthalene-1-sulfonyl chloride, increases incorporation into three other positions, lysine residues 13, 39, and 84. It is concluded that binding of the indole ligand on the site brings about conformational changes in the albumin structure exposing new reactive positions for 5-dimethylaminonaphthalene-1-sulfonyl chloride. There is a large accumulation of basic and hydrophobic residues and no glycine, serine, threonine, valine, aspartate, or cysteine residues in the sequence 10 to 43. Lysine 71 has been identified by amino acid analyses and sequence studies as the position acetylated by acetylsalicylic acid (Hawkins, D. R., Pinckard, N., Crawford, C. P., and Farr, R. S. J. Clin. Invest. (1969) 48, 536), establishing the structural relationships of two major ligand binding sites on albumin. The lone tryptophan is at position 86. Evidence indicates that within residues 1 to 86 of Fragment C and within residues of the A-Phe fragment (Mr equals approximately 10,000), the latter known to be adjacent to Fragment C in the whole albumin structure, exists the major binding sites of all ligands for human serum albumin.