The effect of pH and temperature on the self-association of recombinant human interleukin-2 as studied by equilibrium sedimentation

The self-association of recombinant human interleukin-2 (rhIL-2) in solution was investigated as a function of pH and temperature using equilibrium sedimentation. Studies were performed at pH 3.6, 6.5 and 8.2, at 1 degree C and 20 degrees C. A model assuming an ideal single molecular species describes the data observed at pH 6.5 at both temperatures. At pH 8.2, the data from both temperatures can be better described by a weak monomer-dimer association equilibrium. The values of the association constants obtained indicate the presence of less than 10% dimer at a concentration of 1 mg/ml at both temperatures. At pH 3.6, aggregates with a Z average molecular weight of over 35 times that of monomeric rhIL-2 were formed. The smallest associating species present under these conditions corresponds to the monomer, which produces aggregates with a wide range of molecular weights. The monomer appears to be in equilibrium with the smallest aggregates, in that a model describing an indefinite association fits the data obtained at the highest centrifugal speed. No model was found to successfully describe the association of the monomer into the much larger aggregates observed at lower speeds. This may be the result of the lack of rapid thermodynamic reversibility of the larger aggregates. Temperature was found to have no significant effect on the largest aggregates that were formed at pH 3.6.