Steric accessibility of tyrosine residues in human serum albumin

Human serum albumin was nitrated by an excess of tetranitromethane at pH 8.0. As shown by amino acid analysis, of the 18 tyrosine residues present in albumin about 7-7.5 residues remain unaltered, 9 residues are converted into 3-nitrotyrosine, and 1.2 residue into 3,5-dinitrotyrosine. The nitrated albumin was digested with cyanogen bromide to three fragments which comprise the whole original molecule. The individual fragments were converted into their S-sulfo derivatives and the latter digested with chymotrypsin or stepwise with trypsin and thermolysin. The yellow, nitrotyrosine-containing peptides were isolated from the digest and the positions of nitrated tyrosine residues in albumin thus located. Residues No 30, 148, 150, 161, 334, 341, 401, and 411 were identified as strongly nitrated and residues No 84, 138, 452, and 497 as medium nitrated. Residues No 140, 263, 319, 332, 353, and 367 either react weakly or were not found in nitrated form. Residue No 411 and partly also 161 were converted into 3,5-dinitrotyrosine. The accessibility of the individual tyrosine residues to the nitrating agent is discussed with respect to their positions in disulfide loops and hypothetic parts of the secondary structure of albumin.