RT Journal Article
SR Electronic
T1 Bovine Serum Albumin Decreases <em>K</em><sub>m</sub> Values of Human UDP-Glucuronosyltransferases 1A9 and 2B7 and Increases <em>V</em><sub>max</sub> Values of UGT1A9
JF Drug Metabolism and Disposition
JO Drug Metab Dispos
FD American Society for Pharmacology and Experimental Therapeutics
SP 2117
OP 2129
DO 10.1124/dmd.111.041418
VO 39
IS 11
A1 Nenad Manevski
A1 Paolo Svaluto Moreolo
A1 Jari Yli-Kauhaluoma
A1 Moshe Finel
YR 2011
UL http://dmd.aspetjournals.org/content/39/11/2117.abstract
AB The human UDP-glucuronosyltransferase (UGT) enzymes UGT1A9 and UGT2B7 play important roles in the hepatic glucuronidation of many drugs. The presence of bovine serum albumin (BSA) during in vitro assays was recently reported to lower the Km values of both these UGTs for their aglycone substrates without affecting the corresponding Vmax values. Nonetheless, using the specific substrates entacapone and zidovudine (AZT) for UGT1A9 and UGT2B7, respectively, and using an improved ultrafiltration method for measuring drug binding to BSA and to biological membranes, we found that the presence of BSA during the glucuronidation reaction leads to a large increase in the Vmax value of UGT1A9, in addition to lowering its Km value. On the other hand, in the case of UGT2B7, our results agree with the previously described effect of BSA, namely lowering the Km value without a large effect on the enzyme's Vmax value. The unexpected BSA effect on UGT1A9 was independent of the expression system because it was found in a recombinant enzyme that was expressed in baculovirus-infected insect cells as well as in the native enzyme in human liver microsomes. Moreover, the effect of BSA on the kinetics of 4-methylumbelliferone glucuronidation by recombinant UGT1A9 was similar to its effect on entacapone glucuronidation. Contrary to the aglycone substrates, the effect of BSA on the apparent Km of UGT1A9 for the cosubstrate UDP-α-d-glucuronic acid was nonsignificant. Our findings call for further investigations of the BSA effects on different UGTs and the inhibitors that it may remove.