Metabolic labelling and characterisation of S-antigens, the heat-stable, strain-specific antigens of Plasmodium falciparum.

The C-10 clone of Plasmodium falciparum was metabolically labelled with [3H]glycine. Analysis by sodium dodecylsulphate/polyacrylamide gel electrophoresis (SDS/PAGE) revealed that S-antigens were the only significantly labelled products released into culture supernatants by the end of the asexual cycle. This result indicates conclusively that heat-stable, strain-specific antigens (the 'S-antigens') emanate from the parasite and not the host. The S-antigen of clone C-10 was resolved on SDS/PAGE as two labelled products with molecular weights of 156000 +/- 5000 and 130000 +/- 2000. The two components were differentially susceptible to proteolysis with trypsin. In other experiments, boiled plasma from Aotus monkeys infected with a different isolate of P. falciparum was partially purified by isoelectric focusing and the fraction containing S-antigen was iodinated by the Iodogen procedure. Analysis by SDS/PAGE of immunoprecipitated material revealed an iodinated component corresponding to the lower molecular weight band of the metabolically labelled S-antigen.