Salt-Induced Aggregation of Lysozyme Studied by Cross-Linking with Glutaraldehyde: Implications for Crystal Growth

Glutaraldehyde cross-linking followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis has been used to detect aggregates of iysozyme in solutions which lead to crystals. In solutions of varying NaCl content, the number of aggregates was found to be related to the ionic strength of the solution. Solutions of 1% NaCl, pH 4.0 were monomeric while those containing 7-15% NaCl, pH 4.0 were shown to be as much as 36% aggregated and 64% monomeric. The aggregates detected at the highest salt and protein concentration studied were composed of dimers, trimers and tetramers. The aggregates increased by addition of single units suggesting the aggregation pathway to be that of monomer addition. The kinetics of the cross-linking reaction were slow preventing a study of either the time dependence of aggregation or the effect of temperature on aggregate distributions. Comparison of the total aggregate concentrations for NaCl and Na₂SO₄ showed that the concentration of aggregates was related to the ionic strength of the solution suggesting that in both crystallization and precipitation, electrostatic shielding of like-charged protein molecules is necessary in order for aggregation to occur.