Structural Modifications at the C-4 Position Strongly Affect the Glucuronidation of 6,7-Dihydroxycoumarins

Abstract

Esculetin (6,7-dihydroxycoumarin) and its C-4 derivatives have multiple pharmacologic activities, but the poor metabolic stability of these catechols has severely restricted their application in the clinic. Glucuronidation plays important roles in catechols elimination, although thus far the effects of structural modifications on the metabolic selectivity and the catalytic efficacy of the human UDPglucuronosyltransferase (UGT) enzymes remain unclear. This study was aimed at exploring the structure-glucuronidation relationship of esculetin and its C-4 derivatives, including 4-methyl esculetin, 4-phenyl esculetin, and 4-hydroxymethyl esculetin as well as 4-acetic acid esculetin. It was achieved by identifying the main human UGTs responsible for the different reactions and by careful characterization of the reactions kinetics. These catechols, with the exception of 4-acetic acid esculetin, are selectively metabolized to the corresponding 7-O-glucuronides. UGT1A6 and UGT1A9 are the two major UGTs involved in the 7-O-glucuronidation of 4-methyl esculetin and esculetin. UGT1A6 was the major contributor for 7-O-glucuronidation of 4-hydroxymethyl esculetin, and UGT1A9 played a significant role in the 7-O-glucuronidation of 4-phenyl esculetin. The results of the kinetic analyses revealed that the K m values of the compounds, in both UGT1A9 and human liver microsomes, decreased with increasing hydrophobicity of the C-4 substitutions. The outcome of this was that C-4 hydrophobic and hydrophilic groups on 6,7-dihydroxycoumarin exhibited contrasting effects on UGT affinity. All of these findings provide helpful guidance for further structural modification of 6,7-dihydroxycoumarins with improved metabolic stability.

Antidiabetic activity screening and nmr profile of vegetable and spices commonly consumed in Indonesia

“…Typical NMR signals of esculetin was identified in P. indica spectra after comparing the spectra with NMR data from Xia et al (2015). This compound was assigned based on the existence of doublet at δ 6.22 (d, J = 9.6 Hz, 1H, H-3), singlets at δ 6.68 (s, 1H, H-8) and δ 6.80 (s, 1H, H-5) and doublet at δ 7.59 (d, J = 9.9 Hz, 1H, H-4) ( Figure 3A).…”

Recent progress and challenges in screening and characterization of UGT1A1 inhibitors

“…The human UGT1A subfamily members share high amino acid sequence homology (>65%), and their substrate specificity frequently overlap, even if at variable kinetic constants. In fact, many UGT1A1 substrates could be glucuronidated by multiple UGT enzymes, a good example for which is the C-7 phenolic coumarin derivatives, such as 4-methyl-umbelliferone (4-MU) 54 . It is also noteworthy that no high-resolution crystal structure of the substrate binding domain of UGT1A1, or any other mammalian UGT, has been reported yet.…”

Assessment of the inhibition potential of Licochalcone A against human UDP-glucuronosyltransferases

“…Most UGTs are expressed in the liver, but some isoforms, such as UGT1A7, UGT1A8, and UGT1A10, are expressed predominantly in the gastrointestinal tract (Gregory et al, 2004). Human intestinal UGTs are considered as important determinants in intestinal metabolism, especially for those oral phenolic compounds, such as flavonoids and courmarins Wu et al, 2011;Xia et al, 2015). Therefore, the inhibition against human UGTs in the gastrointestinal tract may make a great impact on the oral bioavailability of many phenolic drugs or natural products.…”