Evidence for the involvement of sulfhydryl groups in the expression of antitumor activity of streptococcal acid glycoprotein (SAGP) purified from crude extract of Streptococcus pyogenes.

An acidic glycoprotein (SAGP), purified from crude extract (CE) of Streptococcus pyogenes, inhibits the growth of murine fibrosarcoma (Meth A) cells in vitro and prolongs the life span of mice inoculated with the same cells. This study revealed that the growth inhibitory activity of SAGP on Meth A cells in vitro was lowered in the presence of sulfhydryl-oxidizing agents such as cystamine and 5, 5'-dithiobis (2-nitrobenzoic acid) in a dose dependent manner. The activity of SAGP was also diminished by pretreatment of SAGP with cystamine. Inactivation of SAGP by cystamine was reversed by dithiothreitol. Furthermore, the effect of CE on prolonging the life span of Meth A-bearing mice was reduced by the administration of cystamine. These findings suggest that SH-groups are involved in the expression of anti-tumor activity of SAGP or CE in vitro and in vivo.