[Modification of tryptophan residues in immunoglobulin M by 2-hydroxy-5-nitrobenzyl bromide].

The modification of tryptophan residues in monoclonal immunoglobulin M (IgM) by 2-hydroxy-5-nitrobenzyl bromide (RK) was studied at pH 2.0-2.85 and 7.0 and a RK to tryptophan molar ratio (K) from 1 to 40. At pH 2.85, the number of RK residues bound to IgM (N) in account to one HL-fragments does not exceed 10 (the HL-fragment of IgM contains 14 tryptophan residues); the plot of N vs K reaches a plateau at K greater than 20. When the pH is lowered to approximately 2, N rises to approximately 15, but the plateau is not reached. At pH 7.0, the modified IgM with N greater than 1 gives a sediment, while the product with N approximately equal to 1 remains in solution. Evidently, the latter contains the most accessible tryptophan residue (calculated per one HL-fragment). This residue was found to be one of the three residues localized in the C mu 2-domain and the adjoining N-terminal part. The possibility of multiple modification of tryptophan residues during the RK interaction with IgM in acid medium at high values of K is discussed.