HETEROGENEITY OF S-100 PROTEINS OF BRAIN: SUBUNIT COMPOSITIONS

Abstract— Two populations of S-100 proteins (III-IVa-1) and (III-IVb-1) were isolated from bovine brain by a simple three-step procedure involving precipitation with ammonium sulfate, ion exchange chromatography and gel electrophoresis. These two fractions reacted with anti-S-100 serum and showed all the properties of S-100 protein, but differed in the distribution of protein among the four subunits obtained in SDS-gels. These subunits had apparent molecular weights of 7800, 6800, 6100, and 5200. Separations based on charge differences revealed only three subunits. Similar fractions isolated from rat brain contained only two subunits with apparent molecular weights of 7800 and 6500. The two S-100 protein fractions also differed in the specific extinction at 280 nm and in the protein distribution among the separate bands which formed either in a 14% acrylamide-agarose gel-discontinuous buffer system or in 7.5% gels in the presence of calcium. The yield of S-100 protein isolated in the presence of aids (EDTA, EGTA, mercaptoethanol, protease inhibitors) was considerably higher than in their absence (80 mg vs 60 mg/kg tissue), the predominant loss occurring in fraction III-IVa-1. It is concluded that 'S-100 protein'contains a number of molecular species which have different subunit compositions and differences in stability.