Kinetics of interaction of heavy and light chains in immunoglobulins. Use of a sulfhydryl-specific light chain fluorescent probe.

The fluorescent probe N-(iodoacetylaminoethyl)-8-naphthylamine-1-sulfonic acid (1,8-I-AEDANS) reacts stoichiometrically with the COOH-terminal cysteine residue of a human immunoglobulin light (L) chain. The absorption and fluorescence spectral properties of the L-AEDANS product and the environmental sensitivity of the fluorescence suggest the utility of L-AEDANS in the study of the subtle conformational changes accompanying interactions between light chain and other proteins or ligands. Its applicability to the problem of immunoglobulin assembly is illustrated by the association of L-AEDANS with heavy chain dimers, which results in an enhancement of fluorescence. The data indicate that L-AEDANS binds to kinetically equivalent, noninteracting sites with an apparent second order rate constant of 6 X 10(6) liters mol(-1) s(-1) at 20 degrees, pH 7.5.