Thermodynamic and conformational studies on an immunoglobulin light chain which reversibly precipitates at low temperatures

Abstract

A lambda light chain, isolated from an immunoglobulin G molecule, was found to reversibly precipitate at low temperatures. This cryoprecipitation was a function of pH, ionic strength, protein concentration, and time as well as temperature. The lambda chain underwent a cooperative conformational change as the temperature was lowered from 26 to 0 degrees C as judged by ultraviolet difference spectroscopy and circular dichroism. Normal lambda chains showed no conformational change. By difference spectroscopy it was possible to calculate the equilibrium constant governing the conformational change. The change was strongly exothermic (delta H approximately -80 kcal mol-1) and accompanied by a large decrease in entropy (delta S approximately -280 eu). The midpoint of the transition was dependent on the initial protein concentration, suggesting that only the noncovalent dimer of the lambda chain exhibited the conformational change. The existence of a monomer-dimer eqiulibrium (KA approximately 4 X 10(5) M-1) was confirmed by sedimentation velocity. No conformational change was observed by circular dichroism at concentrations where greater than 95% of lambda chain was in the form of a monomer. Although high ionic strength inhibited cryoprecipitation, it had no effect on the conformational change. Stabilization of the dimer by forming an interchain disulfide bond between two monomers abolished both the conformational change and cryoprecipitation. A fragment corresponding to the constant region was isolated from both peptic and tryptic digests of the lambda chain. This fragment neither cryoprecipitated nor showed temperature dependence conformational changes. It proved impossible to isolate a fragment corresponding to the variable region. Both qualitative and quantitative models are presented to account for the behavior of the lambda chain at low temperatures.