UDP-glucuronosyltransferase activity toward digitoxigenin monodigitoxoside in human liver microsomes

The properties of UDP-glucuronosyltransferase (UDPGT) toward digitoxigenin-monodigitoxoside (DT1) have been studied in human liver microsomes. The enzyme activity determined in nonactivated microsomes was very low (20 pmol/min/mg protein) compared with previously published values in rats (104 pmol/min/mg protein) or mice (379 pmol/min/mg protein) DT1-UDPGT activity was increased (180 to 220% of control activity) by Lubrol PX, Triton X-100, or (3-[3-cholamidopropyl]dimethylammonio)-1-propane sulfonic acid. The rate of DT1 glucuronidation determined for 29 different human liver microsomes was variable (18 to 87 pmol/min/mg protein). The KM found was approximately 4.5 microM. DT1-UDPGT activity was tentatively correlated with other known UDPGT activities. No significant correlations were found between DT1 and p-nitrophenol or 4-hydroxybiphenyl UDPGT activities. On the other hand, a strong correlation (r = 0.64, p < 0.05) was observed between DT1-UDPGT activity and digoxigenin-monodigitoxoside (DG1, another cardiac glycoside) glucuronidation rate.