Electrofocusing-induced alteration in the net charge of human macrophage-stimulating protein

Macrophage-stimulating protein is a mammalian serum protein, detected by its capacity to render mouse resident peritoneal macrophages responsive to chemoattractants. The purified protein was shown by column electrofocusing over a period of 48 h to have an isoelectric point of 7.0. In this study we determined the isoelectric point of the native molecule by electrofocusing human serum on a flat bed of prefocused ampholytes. Values in the range of 5.5–6.2 were obtained. A shift in the isoelectric point to 7.0–7.6, without significant alteration in the biological activity of the molecule, occurred when electrofocusing was prolonged to 48 h or was carried out in 6 m urea.