This study aimed to assess the overall glucuronidation capacity of cats, using prototypic substrates identified for human UDP-glucuronosyltransferases (UGTs). To this end, Michaelis-Menten kinetics were established for the substrates using feline hepatic microsomal fractions, and results were compared with similar experiments carried out with dog liver microsomes. Cats are known for their low capacity of glucuronide formation, and UGT1A6 was found to be a pseudogene. However, functional studies with typical substrates were not performed and knowledge of the enzymology and genetics of other glucuronidation enzymes in felidae is lacking. The results of this study showed extremely low formation of naphthol-1-glucuronide (1.7 ± 0.4 nmol/mg protein/min), estradiol-17-glucuronide (<0.7 nmol/mg protein/min), and morphine-3-glucuronide (0.2 ± 0.03 nmol/mg protein/min), suggesting a lack of functional UGT1A6 and UGT2B7 homologues in the cat's liver. Dog liver microsomes were producing these glucuronides in much higher amounts. Glucuronide capacity was present for the substrates 17β-estradiol (estradiol-3-glucuronide, 2.9 ± 0.2 nmol/mg protein/min) and 4-methylumbelliferone (31.3 ± 3.3 nmol/mg protein/min), assuming that cats have functional homologue enzymes to at least the human UGT1A1 and probably other UGT1A isozymes. This implies that for new drugs, glucuronidation capacity has to be investigated on a substance-to-substance base. Knowledge of the glucuronidation rate of a drug provides the basis for pharmacokinetic modeling and as a result proper dosage regimens can be established to avoid undesirable drug toxicity in cats
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