Kinetic studies on androstenedione production in ovarian microsomes from immature rats.
Androstenedione formation from progesterone by P-450(17 alpha,lyase) was investigated in ovarian microsomes of immature rats treated with pregnant mare serum gonadotropin. Successive monooxygenase reactions in the formation of androstenedione without the intermediate leaving P-450(17 alpha,lyase) were demonstrated by a double-substrate double-label experiment using [14C]progesterone and 17 alpha-[3H]hydroxyprogesterone as substrates and also by specific reduction in the concentration of intermediate 17 alpha-hydroxyprogesterone in the reaction medium by reaction of liposomal P-450C21. A detailed kinetic study on the reactions of P-450(17 alpha,lyase) in microsomes was conducted in the steady state. Kinetic parameters indicated the C17,C20-lyase reaction for 17 alpha-hydroxyprogesterone (Km = 80 nM) to be strongly inhibited by progesterone (Ki = 8 nM). In the presence of a high concentration of progesterone, as in the case of in vivo rat ovary, most androstenedione is concluded to be formed directly from progesterone by successive monooxygenase reactions catalyzed by P-450(17 alpha,lyase). 20 alpha-Dihydroprogesterone competitively inhibited the C17,C20-lyase reaction for 17 alpha-hydroxyprogesterone with Ki = 23 nM, but had only slight effect on progesterone metabolism to androstenedione. 20 alpha-Dihydroprogesterone, thus, cannot be a regulator for androstenedione formation in rat ovary.
Yamazaki
T
T
Faculty of Integrated Arts and Sciences, Hiroshima University, Japan.
Marumoto
T
T
Kominami
S
S
Ishimura
K
K
Yamamoto
A
A
Takemori
S
S
D000735:Androstenedione / Q000096:biosynthesis*; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D005260:Female; D006064:Gonadotropins, Equine / Q000494:pharmacology; D006908:Hydroxyprogesterones / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism*; D010053:Ovary / Q000187:drug effects / Q000378:metabolism*; D011374:Progesterone / Q000378:metabolism*; D051381:Rats; D013254:Steroid 17-alpha-Hydroxylase / Q000378:metabolism; D013255:Steroid 21-Hydroxylase / Q000378:metabolism
D006064:Gonadotropins, Equine; D006908:Hydroxyprogesterones; D000735:Androstenedione; D011374:Progesterone; D003577:Cytochrome P-450 Enzyme System; D013255:Steroid 21-Hydroxylase; D013254:Steroid 17-alpha-Hydroxylase; C022433:pregnenolone 17-alpha-hydroxylase