Reinigung und charakterisierung einer löslichen uridin diphosphat-glucuronat: 17β-hydroxysteroid-glucuronyl-transferase beim menschen

The ground plasma (150 000 × g supernatant) of the human intestine contains a uridinediphosphate glucuronate glucuronyltransferase (EC 2.4.1.17) which catalyses exclusively the formation of 17β-glucuronides of oestriol, 17β-oestradiol and testosterone. This enzyme was found in the fractions obtained between 60 and 80% saturation of the ground plasma with ammonium sulphate; it could be separated from two other uridine diphosphate glucuronate glucuronyltransferases which, with oestriol as substrate, were capable of forming the 3-glucuronide and the 16α-glucuronide. The two latter enzymes were present in the fractions obtained at 0–30% and 30–60% saturation of the ground plasma with ammonium sulphate. The low specific activity of the enzyme catalysing the formation of the 3-glucuronide suggests the probability of its origin by microsomal leakage.

The kinetics of the uridinediphosphate glucuronate: 17β-hydroxysteroid glucuronyltransferase are described. The formation of the 17β-glucuronide of oestriol shows a maximum at pH 6.8. The Michaelis-Menten constant was found to be 3.5 · 10⁻⁴M, whereas the activation energy of the enzyme amounted to 12.2 kcal/mole.