Two isozymes of porcine aromatase, the placental and the blastocyst forms,
were expressed in CHO cells using the mammalian cell transfection method. U
sing an 'in-cell' assay (a H-3-water release method), catalytic parameters
of the porcine placental aromatase were found to be very similar to those o
f the human enzyme; however, the activity of the blastocyst isozyme was fou
nd to be one-thirtieth that of the placental isozyme. Product isolation ass
ay (using testosterone as the substrate) revealed that the major steroid pr
oducts were 17 beta-estradiol and 19-nortestosterone. The product ratio of
estradiol/19-nortestosterone was found to be 94 : 6 for the porcine placent
al form, 6 : 94 for the porcine blastocyst form, and 92 : 8 for the human w
ild-type aromatase. Therefore, the porcine blastocyst aromatase isozyme cat
alyzes mainly androgen 19-desmethylation rather than aromatization. In addi
tion, inhibition profile analyses on the placental and blastocyst isozymes
were performed using three steroidal inhibitors [4-hydroxyandro-stenedione
(4-OHA), 7 alpha-(4'-amino)phenylthio-1,4-androstandiene-3,17-dione (7 alph
a-APTADD), and bridge (2,19-methyleneoxy) androstene-3,17-dione (MDL 101,00
3)], and four nonsteroidal inhibitors [aminoglutethimide (AG), CGS 20267, I
CI D1033, and vorozole (R83842)]. While the two isozymes of porcine aromata
se share 93% amino-acid sequence identity, our results indicate that the tw
o porcine aromatase isozymes have distinct responses to various aromatase i
nhibitors.