Active Oxygen‐mediated Cytotoxic and Antitumor Actions of Streptococcal Cytotoxic Protein

Streptococcal cytotoxic protein (SCP), obtained from the cell‐free extract of Streptococcus pyogenes, inhibited the uptake of [methyl‐3H]thymidine by Ehrlich ascites carcinoma cells depending on the concentration of fetal calf serum (FCS) added to the culture medium. The same results were found using sera from different animals. The inhibitory activity of SCP was completely suppressed by catalase but not by hydroxyl radical scavengers or superoxide dismutase. The antitumor activity of SCP in tumor‐bearing mice was inhibited by administration of catalase together with SCP. SCP enzymatically produced hydrogen peroxide in the presence of FCS as detected by the 2,2′‐azino‐di‐[3‐ethyl‐benzothiazoline‐(6)‐sulfonic acid]/horseradish peroxidase method. The kinetic parameters, Km and Vmax, for hydrogen peroxide production of SCP were 2.13% FCS and 0.53 nmol/min/μg SCP, respectively. These results indicate that SCP is an enzyme which produces hydrogen peroxide and exerts potent cytotoxic and antitumor effects using an active oxygen species, hydrogen peroxide, produced by the enzymatic reaction of SCP with an unknown substrate contained in FCS or sera from various animals and molecular oxygen.