Multiplicity and Specificity of UDP-Glucuronyltransferase

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Abstract
The activity of UDP-glucuronyltransferase (EC 2.4.1.17) was assayed in whole liver homogenates of different species and in microsome preparations with 3 different substrates: bilirubin, 4-methylumbelliferone and p-nitrophenol. Mg++ and Ca++ added to the incubation medium have a similar effect on the enzyme system estimated with all 3 substrates : an activity maximum followed by a decrease with higher Mg++ and Ca++ concentrations. These results are in contradiction with previous observations reported by other authors who found an activation of the bilirubin system by Mg++ and an inhibition of the PNP system by Mg++ and Ca++. EDTA inhibits all 3 systems especially if Mg++ is present in the incubation medium. Mn++ inhibits the bilirubin conjugation but can replace Mg++ for the glucuronid synthesis of 4MU and PNP; Zn++ inhibits all 3 systems studied. The presented results are discussed.

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