Molecular weight, isoelectric point, and stability of a murine lymphokine that induces macrophage tumoricidal activity

A factor in antigen- or mitogen-stimulated murine spleen cell culture supernatants that induces macrophage-mediated tumor cytotoxicity was characterized physicochemically. Although activity was eluted from Sephadex G-100 columns in a region corresponding to a MW of 55,000, rechromatography of the first and second halves of the eluted peak resulted in two separate peaks, corresponding in location to the first and second halves of the original peak. This shows that the original peak comprises two activation factors of slightly different MW. In contrast, electrofocusing experiments designed to assure that the isoelectric position had been reached by the end of the run showed a single isoelectric point of pH 5.8, with no sign of charge heterogeneity. Denaturation studies showed rapid loss of activity at 60 degrees C, stability over a pH range of 5 to 10, loss of activity at pH 4.0, and loss of two-thirds of the activity in 6 M urea. The number of criteria by which this lymphokine has now been characterized should suffice to distinguish it from other lymphokines evaluated by similar methods.