Sensitive Radio-Assays for the Determination of Microsomal UDP-Glucuronyltransferase in Human Liver Biopsy Specimens

Abstract

UDP-glucuronyltransferase (GT) can accurately and conveniently be determined in 0,2 – 1 mg of liver biopsies using ¹⁴C-1-naphthol as substrate. After incubation (37°C, pH 7.4, 0.5 mM 1-naphthol, 3 mM UDP-glucuronic acid, 5 mM MgCl₂) > 98% of free 1-naphthol is removed from the incubation mixture by one extraction with CHCl₃ and the radioactivity of the glucuronide is determined in the aqueous phase. Two k_M-values were found for 1-naphthol. This may suggest that the substrate is glucuronidated by more than one GT. In biopsies from patients with no histological liver damage and no exposure to enzyme inducers native and Brij 58-activated GT was 0.11 ± 0.04 (n=7) and 0.87 ± 0.23 (n=18) μmol/min/g liver, respectively. GT in biopsies from patients with cholelithiasis and histological pericholangitis was significantly decreased whereas GT from patients with alcoholic liver damage was unchanged or slightly increased. GT (¹⁴C-morphine as substrate) which in the rat can be purified and separated from 1-naphthol-GT (Bock et al., Biochem. Pharmacol. 26, 1097 (1977)) can be assayed in 10 mg liver biopsies. Activated morphine-GT was 0.07 ± 0.02 μmol/min/g liver (n=18). GT activities with morphine and 1-naphthol as substrates were correlated in biopsies from 18 patients (r=0.58, p< 0.01). A lack of alternate substrate inhibition was found for the two GTs. The assays may help to elucidate the multiplicity of GT as well as the effects of inducers and pathological factors on the activity of GT in human tissues.