An enzyme that conjugates the 16alpha-hydroxyl group of oestriol with glucuronic acid was found in the cytosol fraction of human liver. The enzymic activity could not be sedimented when the cytosol fraction was centrifuged at 158000g(av.) for 120min. The oestriol 16alpha-glucuronyltransferase was purified 100-fold by 0-30% saturation of the cytosol fraction with ammonium sulphate followed by filtration of the precipitate through Sephadex G-200. The activity was eluted at the void volume. The product of the reaction, oestriol 16alpha-monoglucuronide, was identified by paper chromatography and by crystallization of radioactive product to constant specific radioactivity. The optimum temperature was 37 degrees C, and the activation energy was calculated to be 11.1kcal/mol. The apparent Michaelis-Menten constants for oestriol and UDP-glucuronic acid were 13.3 and 100mum respectively. Cu(2+), Zn(2+) and Hg(2+) inhibited, whereas Mg(2+), Mn(2+) and Fe(2+) stimulated the enzyme. Substrate-specificity studies indicated that the amount of oestradiol-17beta, oestradiol-17alpha and oestrone conjugated was not more than about 5% of that found for oestriol. Oestriol 16alpha-monoglucuronide, a product of the reaction, did not inhibit the 16alpha-oestriol glucuronyltransferase; in contrast, UDP, another product of the reaction, inhibited the enzyme competitively with respect to UDP-glucuronic acid as the substrate, and non-competitively with respect to oestriol as the substrate. ATP and UDP-N-acetylglucosamine did not affect the oestriol 16alpha-glucuronyltransferase. 17-Epioestriol acted as a competitive inhibitor and 16-epioestriol as a non-competitive inhibitor of the glucuronidation of oestriol. 5alpha-Pregnane-3alpha,20alpha-diol also inhibited the enzyme non-competitively. It is most likely that the oestriol 16alpha-glucuronyltransferase described here is bound to the membranes of the endoplasmic reticulum.

Adenosine Triphosphate↗

PubMed1970

Studies on a testosterone glucuronyltransferase from the cytosol fraction of human liver.

An enzyme that conjugates the 17beta-hydroxyl group of testosterone was found in the cytosol fraction of human liver. The same enzyme preparation also conjugates the 16alpha-hydroxyl group of oestriol. The enzymic activity could not be sedimented by centrifuging the cytosol fraction at 158000g(av.) for 120min. The testosterone-conjugating as well as the oestriol-conjugating activities were found in the precipitate obtained after 30% saturation of the cytosol fraction with ammonium sulphate. Filtration of the precipitate through Sephadex G-200 enriched the testosterone-conjugating enzyme 50-fold and the oestriol-conjugating enzyme 100-fold. No separation of the two activities was achieved. With labelled testosterone the product of the reaction, testosterone 17beta-glucuronide, was identified by paper chromatography and by crystallization to constant specific radioactivity. Testosterone 17beta-glucuronyltransferase was active between pH7.0 and 8.6 in tris-HCl and tris-maleate buffers. The apparent K(m) values for testosterone and UDP-glucuronic acid were 6.4 and 25mum respectively. The enzyme was active between 37 and 45 degrees C; the activation energy was calculated to be 5kcal/mol. Oestriol did not influence the glucuronidation of testosterone. Controlled heating as well as alternate freezing and thawing of the purified enzyme preparation led to an inactivation of both testosterone-conjugating and oestriol-conjugating activities at similar rates. Testosterone and oestriol, when incubated together, gave a reaction rate that was approximately equal to the sum of the rates when the two substrates were incubated separately. The present findings suggest that testosterone and oestriol are conjugated by two separate enzymes.

Adolescent↗

PubMed1970