Functional and immunochemical comparison of hepatic UDP-glucuronosyltransferases in a piscine and a mammalian species

1. 1. The aglycone specificity of hepatic microsomal glucuronidation was compared under uniform conditions in a fish, Pleuronectes platessa and a mammal, Rattus norvegicys, representative of the most primitive and advanced vertebrate classes. 2. 2. Both species exhibited comparable UDP-glucuronosyltransferase (UDPGT) activity towards planar phenolic substrates (1-napthol, 4-nitrophenol); however, plaice activity towards bulky non-planar substrates such as (-)-morphine was either 200-fold lower, or for arylacetic acid (RS-2-phenylpropionic acid) and an aryloxyacetic acid (clofibric acid) non-detectable. 3. 3. Conjugation of the endogenous substrates, bilirubin and steroids were 4- to 40-fold lower in the plaice than in the rat. Whilst both species formed diglucuronides of the asymmetrical bilirubin IXα, they displayed a reciprocal preference for the initial esterification, conjugation of the C-8 side chain predominating in the rat and of C-12 in the fish. 4. 4. Immunoblot analysis using two polyclonal antisera preparations raised against rat UDPGTs demonstrated the presence of multiple weakly cross-reacting polypeptides in fish microsomes indicative of multiple isoforms and conservation of common structural motifs over more than 350 million years since evolutionary divergence of the mammals.