Photoaffinity Labeling of the Aglycon Binding Site of the Recombinant Human Liver UDP-Glucuronosyltransferase UGT1A6 with 7-Azido-4-methylcoumarin

7-Azido-4-methylcoumarin (AzMC) is a fluorescent photoactive compound structurally related to 4-methylumbelliferone (4-MU), a marker substrate of the human liver recombinant UDP-glucuronosyltransferase (UGT) 1A6. AzMC was synthesized and utilized to label the substrate binding site of UGT1A6. AzMC exhibits a fluorescence spectrum with maximum excitation and emission wavelengths of 380 and 442 nm, respectively. Upon irradiation, the probe irreversibly inhibited glucuronidation activity measured with para-nitrophenol (pNP) as substrate and interacted with UGT1A6 according to a saturable process indicative of reversible binding before covalent incorporation of the photoaffinity label. This inhibition was both time and concentration dependent and led to the calculation of an inhibition constant, k2 = 0.113 mM min−1, and dissociation constant, Kd = 2.89 mM, for the reaction. Partial photoinactivation of UGT1A6 with AzMC revealed that the probe decreased the apparent Vmax of the pNP glucuronidation reaction, but not the Km. Moreover, inhibition was partially prevented by 1-naphthol, a surrogate substrate for the enzyme, or by preincubation with an active-site directed inhibitor, 5′-O-[[(2-decanoylamino-3-phenyl-propyloxycarbonyl)amino]-sulfonyl]-2′,3′-O-isopropylideneuridine. In contrast, UDP-glucuronic acid (UDP-GlcUA) did not have any protective effect against photoinactivation and AzMC did not affect the photoaffinity labeling of UGT1A6 by 5-[β-32P]N3UDP-GlcUA, a photoaffinity analog of UDP-GlcUA. Additionally, in the absence of irradiation, AzMC was found to be a competitive inhibitor of 4MU glucuronidation. Collectively, these results strongly indicate that AzMC specifically binds to the UGT1A6 aglycon binding site. Amino acid alignment of phenol-binding proteins revealed a conserved motif, YXXXKXXPXP. It is possible that this motif is involved in phenol binding to UGT1A6 and other phenol-accepting proteins.