Specific soluble malarial antigens were isolated from the serum and plasma of chickens acutely infected with Plasmodium gallinaceum and were characterized by biophysical and biochemical procedures. Since the physical, chemical, and immunoserologic properties of these antigens differed from those of cellular parasitic antigens, the former are here termed explasmodial. Explasmodial antigens were purified by differential ultracentrifugation, agar, sephadex and biogel column-chromatography techniques. Biochemical and biophysical studies revealed that the antigens were proteinaceous in nature and associated with albumin and globulin serum fractions of low molecular weight; their molecular weight was expressed in Svedburg units of S-3 and S-5. Their amino-acid content, which was quantitated in µmoles, revealed differences in concentration from those in plasmodial antigens and in normal chicken serum.
Trypsin, pepsin, and papain destroyed the serologic activity of the explasmodial antigens in gel-precipitation and tube latex-agglutination tests. The explasmodial antigens were stable at 56°C, but were partially inactivated at 65°C; temperatures above 65°C completely destroyed their serologic activity. The antigens can be preserved at 3 to 5°C for 4 or 5 months and for more than 1 year by deep-freezing at -65°C as well as by lyophilization for an indefinite period. Phenol (1:100) destroyed their serologic activity. The activity was also destroyed by dialysis against distilled water, NaCl concentrations in excess of 4%, and by a pH above 9.0. These data suggest that further work on the serologic potential of explasmodial antigens should be carried out.