DIFFERENT INVITRO METABOLISM OF 7 ALPHA-METHYL-19-NORTESTOSTERONE BY HUMAN AND EQUINE AROMATASES

The ability of human and equine placental microsomes to aromatize 7alpha-methyl-19-nortestosterone (MNT) was studied. Kinetic analysis indicates that MNT shares the androgen-binding site of human and equine placental microsomal aromastases. Human placental microsomal estrogen synthetase had about a 2.5-fold higher relative affinity for MNT than the equine placental enzyme (K(iMNT)/K(m) androstenedione of 32 versus 87). However, MNT was not metabolized by human placental microsomes, whereas it was very actively metabolized by equine placental microsomes. Further studies using purified equine cytochrome P450arom indicated that the presence of a 7alpha-methyl group and the absence of a C19 methyl group did not impair its conversion by the purified enzyme. The product of this reaction was separated and identified as 7alpha-methylestradiol by gas chromatography coupled to mass spectrometry.