Studies related to the metabolism of anabolic steroids in the horse: The phase I and phase II biotransformation of 19-nortestosterone in the equine castrate

Abstract

1. The metabolism of 19-nor[4-¹⁴C]testosterone has been studied in the equine castrate.

2. Following XAD-2 extraction of aliquots of the 0—24 h urine samples, the glucuronic acid and sulphate conjugates were separated by Sephadex LH-20 column chromatography. After hydrolysis of the conjugates, the neutral phase I metabolites of 19-nortestosterone were extracted, purified and identified by g.l.c.-mass spectrometry.

3. In phase I metabolism stereospecificity was observed in the reduction of the A-ring with the formation of the 5α,β-isomers of estranediol. Epimerization at C-17 and hydroxylation at C-16 were the other major pathways.

4. In phase II metabolism the C-17α steroid epimers were predominantly conjugated with glucuronic acid and the C-17β epimers with sulphuric acid.

5. One animal showed a slight variation in metabolism with a tendency towards the formation of polar metabolites.