[Accessibility of tryptophan residues in immunoglobulin M molecule as an indicator of its conformational variability].

The accessibility of tryptophan residues in immunoglobulin M to modification with the Koshland reagent (2-hydroxy-5-nitrobenzyl bromide) was used as an indicator of its conformational variability. Of 14 tryptophan residues (per HL-fragment) in the native IgM, only one (presumably Trp312 in the mu-chain) was the most accessible. Irreversible acid- or temperature-induced conformational changes of IgM increased almost 2-fold the number of accessible tryptophan residues. After partial enzymatic deglycosylation of IgM (especially by an intense splitting of mannose), all tryptophan residues became inaccessible. Modification of the most accessible tryptophan residue increased 2- to 3-fold the number of tyrosine residues accessible to nitration with tetranitromethane. Using the spin label method, it was demonstrated that modification of four tryptophan residues in IgM considerably decreased the mobility of the Cmu 3 domain together with an essential drop in. the solubility of the modified IgM.