Nitric oxide synthases catalyze the activation of redox cycling and bioreductive anticancer agents

Nitric oxide synthases (NOSs) play a crucial role in the control of blood f low, memory formation, and the immune response. These proteins can be struc turally divided into oxygenase and reductase domains, The reductase domain shares a high degree of sequence homology with P450 reductase, which is tho ught to be the major enzyme responsible for the one-electron reduction of f oreign compounds, including bioreductive antitumor agents currently undergo ing clinical trials. In view of the structural similarities between NOS and P450 reductase, we investigated the capacity of NOS to reduce the hypoxic cytotoxin tirapazamine, the antitumor agent doxorubicin, and also the redox cycling compound menadione, All three isoforms exhibited high levels of ac tivity toward these compounds. In the case of doxorubicin and menadione, th e activity of NOS II was 5-10-fold higher than the other enzymes, whereas w ith tirapazamine, the activities were broadly similar. NOS-mediated metabol ism of tirapazamine resulted in a large increase in plasmid DNA strand brea ks, demonstrating that the reduction was a bioactivation process. In additi on, tirapazamine inhibited NOS activity. Because nitric oxide is implicated in maintaining tumor vascular homeostasis, it is conceivable that tirapaza mine could potentiate its own toxicity by increasing the degree of hypoxia, This study suggests that the NOSs could play a key role in the therapeutic effects of tirapazamine, particularly because NOS activity is markedly incr eased in several human tumors. In addition, the presence of NOS in the hear t indicates that these enzymes may contribute to the cardiotoxicity of redo x cycling drugs, such as doxorubicin.