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J. Biol. Chem., Vol. 278, Issue 11, 9552-9559, March 14, 2003
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From the Department of Internal Medicine, Division of Endocrinology
and Metabolism, Department of Pharmacology, University of Texas
Southwestern Medical Center, Dallas, Texas 75390-8857
Progesterone has long been considered the primary
mediator of Xenopus oocyte maturation. We have recently
shown, however, that androgens, which are equal or more potent
promoters of maturation and are present at higher levels in ovulating
frogs, may also be playing an important physiologic role in mediating
maturation. Here, we examined the role of CYP17, a key enzyme mediating
sex steroid synthesis, in Xenopus ovarian androgen
production. We found that the 17,20-lyase activities of
Xenopus CYP17 exceeded the 17
Xenopus laevis Ovarian CYP17 Is a Highly
Potent Enzyme Expressed Exclusively in Oocytes
EVIDENCE THAT OOCYTES PLAY A CRITICAL ROLE IN XENOPUS
OVARIAN ANDROGEN PRODUCTION*
-hydroxylase activities in
both the 
4 and 5 pathways; thus, Xenopus CYP17
rapidly converted pregnenolone and progesterone to
dehydroepiandrosterone (DHEA) and androstenedione, respectively. This remarkably robust activity exceeds that of CYP17 from most higher vertebrates, and likely explains why virtually no progesterone is detected in ovulating frogs. Additionally, ovarian CYP17 activity was present exclusively in oocytes, although all other enzymes involved
in sex steroid production were expressed almost entirely in surrounding
follicular cells. This compartmentalization suggests a "two-cell"
model whereby Xenopus ovarian androgen production requires
both follicular cells and oocytes themselves. The requirement of
oocytes for ovarian androgen production further introduces the unusual
paradigm whereby germ cells may be responsible for producing important
steroids used to mediate their own maturation.
*
This work was supported in part by National Institutes of
Health Grant DK59913 and Welch Foundation Grant I-1506.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
W. W. Caruth, Jr. Endowed Scholar in Biomedical Research. To
whom correspondence should be addressed. Tel.: 214-648-4793; Fax:
214-648-7934; E-mail: stephen.hammes@utsouthwestern.edu.
Copyright © 2003 by The American Society for Biochemistry and Molecular Biology, Inc.
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