Relative rates of the non-covalent and covalent binding of secretory component to an IgA dimer.

The rate of binding of free secretory component to an IgA dimer in vitro was studied by high-voltage gel electrophoresis at different times after mixing. The rate of the formation of the covalent component of the interaction (i.e. the disulphide interchange reaction) was monitored separately by denaturing the proteins in 6 M guanidine hydrochloride at different times after mixing and subsequently estimating the amount of covalently bound secretory componenet by gel chromatography in the denaturing solvent. The rates of the two reactions could not be distinguished in experiments at 37 degrees or 20 degrees C. At 4 degrees C, however, secretory component bound to the IgA dimer almost as rapidly as at higher temperatures, while the rate of the disulphide interchange was considerably lower. This indicates that the noncovalent interactions are the primary type of bonds formed between secretory component and IgA, and that the formation of these bonds initiate the disulphide interchange reaction, the rate of which is highly dependent on temperature.