Monensin (MON) is an ionophore antibiotic widely used in veterinary practice as a coccidiostatic or a growth promoter. The aims of this study were to characterize the P-450 isoenzyme(s) involved in the biotransformation of the ionophore and to investigate how this process may be affected by tiamulin and other chemotherapeutic agents known to produce toxic interactions with MON when administered concurrently in vivo. In liver microsomes from untreated rats (UT) or from rats pretreated, respectively, with ethanol (ETOH), b-naphthoflavone (bNAF), phenobarbital (PB), pregnenolone 16acarbonitrile (PCN), or dexamethasone (DEX), the rate of MON Odemethylation was the following: DEX > PCN > PB .. UT 5 ETOH > bNAF; similar results were obtained by measuring total MON metabolism. In addition, the extent of triacetyloleandomycinmediated P-450 complexes was greatly reduced by the prior addition of 100 mM MON. In DEX-treated microsomes, MON O-demethylation was found to fit monophasic Michaelis-Menten kinetics (KM 5 67.6 6 0.01 mM; Vmax 5 4.75 6 0.76 nmol/min/mg protein). Tiamulin markedly inhibited this activity in an apparent competitive manner, with a calculated Ki (Dixon plot) of 8.2 mM and an IC50 of about 25 mM. At the latter concentration, only ketoconazole or metyrapone, which can bind P-450 3A, inhibited MON O-demethylase to a greater extent than tiamulin, whereas a-naphthoflavone, chloramphenicol, or sulphametasine was less effective. These results suggest that P-450 3A plays an important role in the oxidative metabolism of MON and that compounds capable of binding or inhibiting this isoenzyme could be expected to give rise to toxic interactions with the ionophore.
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