The biosynthesis of tyramine glucuronide by liver microsomal fractions

Labelled tyramine glucuronide was synthesized in vitro from UDP-[14C]glucuronic acid, [14C]tyramine or [3H]tyramine. The glucuronidation was carried out at pH9.2 in the presence of a monoamine oxidase inhibitor, trans-2-phenylcyclopropylamine. The Km values for tyramine were 69 and 125 micrometer and those for UDP-glucuronic acid were 260 and 290 micrometer respectively for guinea-pig and rat liver microsomal preparatons. The specific activities of microsomal glucuronyltransferase measured in fresh hepatic preparations of guinea pig, mouse and rat were respectively 601, 251 and 235 pmol of [14C]tyramine glucuronide/min per mg of protein. Increase in activity ranged from 2- to 6-fold in preparations which were frozen and thawed once and 5.4- to 10-fold when the freezing and thawing was repeated. Rabbit liver has very low activity, and monkey liver and intestine were completely devoid of this conjugating capacity.