Effect of interchain disulfide bond on hapten binding properties of light chain dimer of protein 315.

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Abstract
Hapten binding characteristics of the covalent L chain dimer, derived from the murine Ig[immunoglobulin]A secreted by plasmacytoma MOPC-315, to 2 nitroaromatic compounds, .epsilon.-N-(2,4-dinitrophenyl)-L-lysine and 4-(.alpha.-N-alanine)-7-nitrobenz-2-oxa-1,3-diazole, were investigated by differential spectroscopic titrations. Binding curves for both haptens displayed sigmoidity similar to that reported earlier for the reduced and alkylated dimer held together by noncovalent bonds only. Presence of the interchain disulfide bond in the covalent dimer caused marked changes in its binding properties. The data, like those obtained for the noncovalent dimer, fit the allosteric model of Monod, Wyman and Changeux in which binding of the 1st hapten to the dimer causes a conversion of both sites of the protein molecule from a lower to a higher affinity conformation. The binding parameters showed that affinity and positive cooperativity in the interaction between haptens and the covalent dimer were significantly enhanced. Differences in parameters of the binding and of the allosteric transition caused by the presence of the interchain disulfide bond demonstrated the existence of longitudinal interactions in Ig derivatives. These properties of the L chain dimer make it a potential model for receptors present on thymus-derived lymphocytes.