THE EFFECTS OF PROGESTERONE, 4,16-ANDROSTADIEN-3-ONE AND MK-434(TM) ON THE KINETICS OF PIG TESTIS MICROSOMAL TESTOSTERONE-4-ENE-5-ALPHA-REDUCTASE ACTIVITY

The enzyme 3-oxo-steroid: NADPC(+) 4-oxidoreductase (EC 1.3.1.22; 5 al pha-reductase) was assayed in testicular microsomes of pigs of 3, 20 a nd 24 weeks of age. The activity was very low in 3-week-old animals an d approximately 10-fold higher in 5- and 6-month-old pigs. The pH opti mum was 6.3 in 6-month-old animals, 5.7 in 5-month-old animals, but co uld not be reliably determined in 3-week-old animals. The kinetic para meters for 5 alpha-reductase in testis microsomes from g-month-old ani mals were; K-m(app), 8.0 mu mol/l, V-max(app), 6.7 nmoles/90 min/mg pr otein. Progesterone was a competitive inhibitor of testosterone 5 alph a-reduction with an apparent K-i(app) of 0.86 mu mol/l. However, 4,16- androstadien-3-one (dienone), which undergoes 5 alpha-reduction in the biosynthesis of the pheromonally active 16-androstenes, was a compara tively poor inhibitor with a K-i(app) of 4.9 mu mol/l. Similarly, MK43 4, which is a selective inhibitor of the human type 2 5 alpha-reductas e, but which inhibits both types 1 and 2 in the rat, was also a poor c ompetitive inhibitor of testosterone 5 alpha-reductase in the pig test is (K-i(app), 3.1 mu mol/l. It would appear from these studies that th e pig testis microsomal 5 alpha-reductase corresponds to a type 1 isoz yme that is not capable of reducing dienone other than under condition s where the dienone concentration would be in considerable excess of t estosterone. It is, therefore, probable that substrate-specific 5 alph a-reductases exist in the pig testis for the 5 alpha-reduction of test osterone and dienone. (C) 1997 Elsevier Science Ltd.