title: Human red blood cell membrane oxidase and horseradish peroxidase cleavage of folic acid: evidence for formation of singlet oxygen.
abstract: 1. Human red blood cell membrane oxidase catalyzes the transformation of folic acid to pterin-6-aldehyde and p-aminobenzoyl glutamic acid provided hydrogen peroxide, a xanthine oxidase inhibitor, is present. Horseradish peroxidase produces the same product in the absence of hydrogen peroxide. 2. The oxidation of folic acid by horseradish peroxidase is accompanied by photon emission. Several lines of evidence suggest that singlet oxygen is the emitting species and is generated directly: a) the effects of singlet oxygen traps such as bilirubin, and of singlet oxygen enhancers such as 1,4-diazobicyclo 2.2.2 octane (DABCO) and eosin; b) the emission spectrum maximum of the unsensitized reaction was greater than 560 nm; c) enhancement of photon emission when the reaction was carried out in D2O, and d) no enhancement of the emission was observed when anthracenic energy acceptors were present. 3. Singlet oxygen production and the inactivation of xanthine oxidase may be important when considering folic acid metabolism by cancer cells, in view of the fact that the level of this enzyme is low in these cells.
mesh_terms: D000818:Animals; D055598:Chemical Phenomena; D002621:Chemistry; D004910:Erythrocyte Membrane / Q000201:enzymology*; D004912:Erythrocytes / Q000201:enzymology*; D005260:Female; D005492:Folic Acid / Q000378:metabolism*; D006735:Horseradish Peroxidase / Q000378:metabolism*; D006801:Humans; D009369:Neoplasms / Q000201:enzymology; D010101:Oxygen Consumption; D010544:Peroxidases / Q000378:metabolism*; D051381:Rats; D014969:Xanthine Oxidase / Q000378:metabolism
chemical_list: D005492:Folic Acid; C036913:folic acid oxidase; D006735:Horseradish Peroxidase; D010544:Peroxidases; D014969:Xanthine Oxidase