Fig. 17.

Primary and secondary singlet oxygen triggered by the singlet oxygen generator photofrin in contact with tumor cells. A. Illuminated photofrin generates primary singlet oxygen (#1) that causes local inactivation of catalase (#2). This allows for the generation of secondary singlet oxygen through reactions # 3 - #8, followed by autoamplification of secondary singlet oxygen and catalase inactivation. B. The action and signature of primary singlet oxygen can be specifically studied when the generation of secondary singlet oxygen is inhibited by AEBSF. Please note that only histidine can inhibit the signature of primary singlet oxygen from photofrin, whereas the signature of primary singlet oxygen generated through interaction between H₂O₂ and nitrite (Fig. 16) can be inhibited by histidine, FeTPPS, EUK-134 and mannitol.