title: The effect of steroids and nucleotidesoon solubilized bilirubin uridine diphosphate-glucuronyltransferase.
abstract: 1. It was confirmed that bilirubin glucuronyltransferase can be obtained in solubilized form from rat liver microsomes. 2. Michaelis-Menten kinetics were not followed by the enzyme with bilirubin as substrate when the bilirubin/albumin ratio was varied. High concentrations of bilirubin were inhibitory. 3. The K(m) for UDP-glucuronic acid at the optimum bilirubin concentration was 0.46mm. 4. Low concentrations of Ca(2+) were inhibitory in the absence of Mg(2+) but stimulatory in its presence; the converse applied for EDTA. 5. UDP-N-acetylglucosamine and UDP-glucose enhanced conjugation by untreated, but not by solubilized microsomes. 6. The apparent 9.5-fold increase in activity after solubilization was probably due to the absence of UDP-glucuronic acid pyrophosphatase activity in the solubilized preparation. 7. The activation of solubilized enzyme activity by ATP was considered to be a result of chelation of inhibitory metal ions. 8. The solubilized enzyme activity was inhibited by UMP and UDP. The effect of UMP was not competitive with respect to UDP-glucuronic acid. 9. A number of steroids inhibited the solubilized enzyme activity. The competitive effects of stilboestrol, oestrone sulphate and 3beta-hydroxyandrost-5-en-17-one, with respect to UDP-glucuronic acid, may be explained on an allosteric basis.
mesh_terms: D000227:Adenine Nucleotides; D000255:Adenosine Triphosphate; D000818:Animals; D001663:Bilirubin; D002118:Calcium; D003687:Dehydroepiandrosterone; D004054:Diethylstilbestrol; D004492:Edetic Acid; D004958:Estradiol; D004970:Estrone; D005964:Glucosyltransferases; D006728:Hormones; D007700:Kinetics; D008274:Magnesium; D008862:Microsomes, Liver / Q000201:enzymology; D009711:Nucleotides; D011276:Pregnanediol; D011755:Pyrophosphatases; D051381:Rats; D012995:Solubility; D014500:Uracil Nucleotides
chemical_list: D000227:Adenine Nucleotides; D006728:Hormones; D009711:Nucleotides; D014500:Uracil Nucleotides; D004970:Estrone; D003687:Dehydroepiandrosterone; D004958:Estradiol; D004054:Diethylstilbestrol; D000255:Adenosine Triphosphate; D004492:Edetic Acid; D005964:Glucosyltransferases; D011755:Pyrophosphatases; D008274:Magnesium; D011276:Pregnanediol; D001663:Bilirubin; D002118:Calcium