UDP-Glucuronosyltransferases (UGTs) 2B7 and UGT2B17 Display Converse Specificity in Testosterone and Epitestosterone Glucuronidation, whereas UGT2A1 Conjugates Both Androgens Similarly

Abstract

ABSTRACT:Testosterone and epitestosterone are endogenous steroids that differ in the configuration of the hydroxyl-bearing carbon at C-17. Testosterone is the predominant male sex hormone, whereas the role of epitestosterone is largely unclear. In humans, both androgens are excreted mainly as glucuronide conjugates and the urinary ratio of testosterone to epitestosterone (T/E), used to expose illicit testosterone abuse by male athletes, indicates the relative concentrations of the respective glucuronides. Some male athletes have T/E values greater than the accepted threshold value (4.0), even without testosterone abuse. We have analyzed athletes' urine samples and found that the main reason for such false-positive results in doping tests was a low epitestosterone glucuronide concentration not a high level of testosterone glucuronide. Sulfate conjugates of both testosterone and epitestosterone were also detected in the different urine samples. Glucuronidation assays with the 19 human UDP-glucuronosyltransferases (UGTs) of subfamilies UGT1A, UGT2A, and UGT2B revealed that UGT2B17 is the most active enzyme in testosterone glucuronidation. UGT2B17 does not glucuronidate epitestosterone, but inhibition studies revealed that it binds epitestosterone with affinity similar to that of testosterone. Epitestosterone glucuronidation is catalyzed mainly by UGT2B7, and the K m of this reaction is significantly lower than the K m of UGT2B17 for testosterone. Although UGT2B7 and UGT2B17 exhibited high, although converse, stereoselectivity in testosterone and epitestosterone glucuronidation, UGT2A1, an extrahepatic enzyme that is expressed mainly in the nasal epithelium, catalyzed the glucuronidation of both steroids at considerable rates and similar kinetics. The results shed new light on the substrate specificity and stereoselectivity of human UGTs.

Characterization of the ZebrafishUgtRepertoire Reveals a New Class of Drug-Metabolizing UDP Glucuronosyltransferases

“…Steroid hormones are mainly metabolized via glucuronidation (James, 2011). In human, steroid hormones are predominantly glucuronidated by members of the UGT2 family (Sten et al, 2009). In zebrafish, however, they are mainly conjugated by members of the UGT5 family (Tables 2 and 3).…”

Effect of Dose and 5α‐Reductase Inhibition on the Circulating Testosterone Metabolite Profile of Men Administered Oral Testosterone

“…Other than UGTs, 17β‐HSD2, 5‐AR, CYP3A4, and sulfotransferases can also metabolize testosterone, however, the quantitative contribution of these enzymes in testosterone first‐pass metabolism is unknown 28, 29, 30. Dutasteride is a selective inhibitor of the type 1 and type 2 isoforms of the 5‐AR enzyme ( Figure 1).…”

Effects of Cell Differentiation and Assay Conditions on the UDP-Glucuronosyltransferase Activity in Caco-2 Cells

“…1) was designed to reveal information on the activity of many individual UGTs, and care was taken to ensure that the majority of the compounds in this collection are not substrates for UGT1A6. For example, epitestosterone and AZT are markers for UGT2B7 (Court et al, 2003;Sten et al, 2009), ethinylestradiol is mainly for UGT1A1 (Soars et al, 2003), oxazepam is for UGT2B15 (S-oxazepam glucuronide) (Court et al, 2002), propofol is for UGT1A9 (and UGT1A10 to some extent) (Court, 2005; M. Kurkela and M. Finel, unpublished observation from our laboratory), testosterone is mainly for UGT2B17, and TFP is for UGT1A4 (Uchaipichat et al, 2006). The marker substrate for UGT1A6 was 1-naphthol, a compound that is also glucuronidated by many other UGTs, but at much lower rates than by UGT1A6.…”