PT - JOURNAL ARTICLE AU - Tang, Cuyue AU - Ma, Bennett TI - GLYCOSIDATION OF AN ENDOTHELIN ET<sub>A</sub> RECEPTOR ANTAGONIST AND DICLOFENAC IN HUMAN LIVER MICROSOMES: AGLYCONE-DEPENDENT UDP-SUGAR SELECTIVITY AID - 10.1124/dmd.105.005801 DP - 2005 Dec 01 TA - Drug Metabolism and Disposition PG - 1796--1802 VI - 33 IP - 12 4099 - http://dmd.aspetjournals.org/content/33/12/1796.short 4100 - http://dmd.aspetjournals.org/content/33/12/1796.full SO - Drug Metab Dispos2005 Dec 01; 33 AB - Following the finding that UGT2B7 catalyzes the transfer of the glycosyl group from both UDP-glucuronic acid (UDP-GlcA) and UDP-glucose (UDP-Glc) to an endothelin ETA receptor antagonist, Compound A [(+)-(5S,6R,7R)-2-isopropylamino-7-[4-methoxy-2-((2R)-3-methoxy-2-methylpropyl)-5-(3,4-methylenedioxyphenyl)cyclopenteno[1,2-b] pyridine 6-carboxylic acid], to form an acyl glucuronide and a glucoside (Tang et al., 2003), two additional nucleotide sugars [UDP-galactose (UDP-gal) and UDP-N-acetyl glucosamine (UDP-GlcNAc)] were examined as cosubstrates in human liver microsomes. It was found that UDP-gal, but not UDP-GlcNAc, also served as a sugar donor primarily through catalysis by UGT2B7, although at a significantly reduced catalytic rate. These three UDP-sugars showed pH-dependent kinetics and appeared to compete with each other, with IC50 values parallel to their respective apparent Km values. In contrast, only UDP-GlcA served as the sugar donor for the conjugation of diclofenac, a known UGT2B7 substrate, with an apparent Km for UDP-GlcA of 96 ± 17 μM. UDP-Glc and UDP-gal, two futile sugar donors for diclofenac, were found to competitively inhibit the glucuronidation of this aglycone. Different from the case with Compound A, UDP-Glc and UDP-gal displayed Ki values of 2054 ± 108 μM and 1277 ± 149 μM, &gt;10-fold greater than the Km for UDP-GlcA, indicating that these two nucleotide sugars were also capable of binding to the enzyme but with a lower affinity. The findings of this study indicate that the selectivity of UGT2B7 toward UDP-sugars is aglycone-dependent. With Compound A as the acceptor substrate, human UGT2B7 becomes more accommodative in the transfer of the glycosyl group from UDP-sugars beyond UDP-GlcA. The mechanism may involve enzyme conformational changes associated with Compound A binding to the enzyme. The American Society for Pharmacology and Experimental Therapeutics