Dihydroorotase catalyzes the ring opening of the hydrolysis intermediates of the cardioprotective drug dexrazoxane (ICRF-187).

The enzyme kinetics of the hydrolysis of the one-ring open metabolites of the antioxidant cardioprotective agent dexrazoxane [ICRF-187; (+)-1,2-bis(3,5-dioxopiperazin-1-yl)propane] to its active metal ion binding form ADR-925 [N,N'-[(1S)-1-methyl-1,2-ethanediyl]bis[N-(2-amino-2-oxoethyl)glycine] by dihydroorotase (DHOase) has been investigated by high-performance liquid chromatography (HPLC). A spectrophotometric detection HPLC assay for dihydroorotate was also developed. Dexrazoxane is clinically used to reduce the iron-based oxygen free radical-mediated cardiotoxicity of the anticancer drug doxorubicin. DHOase was found to catalyze the ring opening of the metabolites with an apparent V(max) that was 11- and 27-fold greater than its natural substrate dihydroorotate. However, the apparent K(m) for the metabolites was 240- and 550-fold larger than for dihydroorotate. This report is the first that DHOase might be involved in the metabolism of a drug. Furosemide inhibited DHOase, but the neutral 4-chlorobenzenesulfonamide did not. Because dihydroorotate, the one-ring open metabolites, and furosemide all have a carboxylate group, it was concluded that a negative charge on the substrate strengthened binding to the positively charged active site. The presence of DHOase in the heart may explain the cardioprotective effect of dexrazoxane. Thus, dihydropyrimidinase and DHOase acting in succession on dexrazoxane and its metabolites to form ADR-925 provide a mechanism by which dexrazoxane is activated to exert its cardioprotective effects. The ADR-925 thus formed may either remove iron from the iron-doxorubicin complex, or bind free iron, thus preventing oxygen radical formation.