1. Isoquinoline, cinnoline, quinoxaline, quinazoline and phthalazine were incubated with preparations of rabbit liver aldehyde oxidase. 2. The oxidation products, 1-hydroxyisoquinoline, 4-hydroxycinnoline, 2-hydroxy- and 2,3-dihydroxy-quinoxaline, 4-hydroxy- and 2,4-dihydroxy-quinazoline, and 1-hydroxyphthalazine were identified by comparison of their spectral and chromatographic characteristics with those of authentic compounds. 3. Michaelis-Menten constants are reported for the action of the parent heterocycles with aldehyde oxidase. The compounds reported in this study are among the most efficient substrates yet described for rabbit liver aldehyde oxidase. 4. The compounds in 1 above were incubated with bovine milk xanthine oxidase: only quinazoline and phthalazine yielded significant amounts of metabolites. Km values were calculated for these compounds. 5. Incubation of the heterocycles with rat liver preparations gave qualitatively the same results as those obtained using rabbit liver, but smaller amounts of the oxidation products were detected from rat liver incubations.

Molybdenum(V) e.p.r. spectra from reduced forms of aldehyde oxidase were obtained and compared with those from xanthine oxidase. Inhibited and Desulpho Inhibited signals from aldehyde oxidase were fully characterized, and parameters were obtained with the help of computer simulations. These differ slightly but significantly from the corresponding parameters for the xanthine oxidase signals. Rapid type 1 and type 2 and Slow signals were obtained from aldehyde oxidase, but were not fully characterized. From the general similarities of the signals from the two enzymes, it is concluded that the ligands of molybdenum must be identical and that the overall co-ordination geometries must be closely similar in the enzymes. The striking differences in substrate specificity must relate primarily to structural differences in a part of the active centre concerned with substrate binding and not involving the catalytically important molybdenum site.

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