Interaction of the fluorescence-labeled secretory component with human polymeric immunoglobulins

The secretory component (SC) isolated from human milk was labeled with 2 mol of the fluorescent thiol reagent N-[7-(dimethylamino)-4-methylcoumarinyl]maleimide (DACM) per mol of SC through the reactive disulfide bond of SC. The binding of the labeled SC to polymeric immunoglobulins was examined by gel filtration by measuring the fluorescence of DACM at 478 nm. The labeled SC was bound to immunoglobulin M (IgM) and its (Fc)5 mu fragment and to dimeric immunoglobulin A (IgA). When the labeled SC was bound to IgM or the (Fc)5 mu fragment, the fluorescence of DACM increased about 30%. By use of this fluorescence change, quantitative studies were made on the equilibrium and kinetics of the reversible interactions of the labeled SC with two IgM proteins and their (Fc)5 mu fragments at pH 7.0 and 25 degrees C. All the IgM proteins and their (Fc)5 mu fragments had one binding site per mole of polymers. The affinity constant (6 X 10(8) M-1), the association rate constant (7 X 10(7) M-1 min-1), and the dissociation rate constant (0.1 min-1) of one IgM were different from those of the other IgM (1.7 X 10(9) M-1, 1.0 X 10(8) M-1 min-1, and 0.06 min-1, respectively). However, the values for the (Fc)5 mu fragments of the two proteins were the same (1.9 X 10(9) M-1, 1.1 X 10(8) M-1 min-1, and 0.06 min-1, respectively) and were very similar to those of the IgM with the higher affinity constant.(ABSTRACT TRUNCATED AT 250 WORDS)