Purification and characterization of a novel cytotoxic substance from cell-free extract of Streptococcus pyogenes

A cytotoxic substance designated as streptococcal cytotoxic protein (SCP) was isolated from a cell-free extract of the Su strain of Streptococcus pyogenes possessing cytotoxic and antitumor activity. SCP was purified with a series of column chromatography and preparative PAGE to give a homogeneous single band as revealed by PAGE analysis. The purified SCP has a molecular mass of 165 kDa, composed of four 43 kDa subunits, and its pI is 4.3. SCP was sensitive to proteinases and was labile to heat and at acidic or alkaline pH. SCP showed inhibitory effects on the [3H]thymidine, [3H]uridine and [3H]leucine uptakes and on the growth of cells, and released 51Cr from cells when the protein was added to the cultures of Ehrlich ascites carcinoma (EAC), mouse mammary tumor (MM-2), leukemia (L-1210) and NIH-3T3 mammalian cells in vitro. SCP also showed an antitumor effect on EAC or MM-2 tumor-bearing mice but not on L-1210 tumor-bearing mice in vivo.