Interaction of phthalazines with molybdenum hydroxylases : phthalazine and its 1-substituted derivatives as substrates, inhibitors and inducers of aldehyde oxidase and xanthine oxidase, both in vitro and in vivo
The interaction of the 2,3-diazanaphthalene, phthalazine and its 1-substituted derivatives with the molybdenum hydroxylases, aldehyde oxidase and xanthine oxidase, has been investigated both in vivo and in vitro. Metabolic studies, carried out by treating rabbits with both cold and ¹⁴C-labelled phthalazine, have shown that this compound is extensively metabolised in vivo, the major metabolite being a glucuronide conjugate. Very little unchanged phthalazine or its molybdenum hydroxylase mediated oxidation product 1-hydroxyphthalazine were excreted in the urine. Pretreatment of rabbits with phthalazine or 1-hydroxyphthalazine had no effect upon the activity of the microsomal monooxygenases but caused a significant increase in the specific activities of both aldehyde oxidase and xanthine oxidase. Determination of the molybdenum content of purified aldehyde oxidase fractions using electrothermal atomic absorption spectroscopy has confirmed that an increase in the molybdenum content of the enzyme fraction accompanies the increase in activity. A qualitative assessment of purified aldehyde oxidase fractions using iso-electric focusing has indicated that this enzyme may be composed of 2 or 3 active variants and following pretreatment with either phthalazine or 1-hydroxyphthalazine a further band of enzyme activity is apparent on the electropherogram. The Km value for phthalazine is significantly reduced with enzyme prepared from phthalazine treated rabbits, indicating that a form of the enzyme with a high affinity for phthalazine may have been induced. 1-Hydrazinophthalazine (Hydralazine) and two other hydrazine substituted N-heterocycles, endralazine and 1-hydrazinoisoquinoline have been shown to exert a potent progressive inhibition of aldehyde oxidase in vitro, effective only in the presence of substrate, but are inactive towards xanthine oxidase. In addition, administration of hydralazine to rabbits results in a significant reduction in liver aldehyde oxidase activity. Investigations into the interaction of some of the metabolites of hydralazine with aldehyde oxidase in vitro suggest that hydralazine is also the inhibiting species in vivo.