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CASE FILES: BIOCHEMISTRY
A nsw ers
[48.1]
A. Hyperandrogenism is primarily ovarian in origin in PCOS women,
although a minor contribution from the adrenal gland may occur.
Hyperinsulinemia is the primary stimulus. Fat cells convert androgens
to estrogens. This may explain the late menopause in this case. In nor-
mal postmenopausal women, ovarian production of testosterone con-
tinues and this provides the main source of circulating estrogen.
Typically an LH/FSH ratio of at least 2.5 is associated with PCOS.
[48.2]
B. Increased levels of FSH and LH result from decreased estrogen
levels and release of feedback inhibition. Androgen secretion contin-
ues although diminished. Osteoblast activity decreases, eventually
leading to risk of osteoporosis. Progesterone levels decrease, and GnRH
levels increase.
[48.3]
C. Progesterone is secreted by the corpus luteum under the influence
of LH. Together with estrogen, it promotes the thickening and main-
tenance of the endometrium. Progesterone inhibits GnRH release,
uterine contraction, and follicle development.
B IO C H E M IS T R Y PE A R L S
Puberty is initiated by the pulsatile release of gonadotropin-
releasing hormone (GnRH) from the hypothalamus.
GnRH binds to plasma membrane receptors in its target cells in the
anterior pituitary and triggers activation of the phosphatidylinos-
itol signaling pathway, stimulating the release of both follicle-
stimulating hormone (FSH) and luteinizing hormone (LH) from
the same cell.
FSH binds its plasma membrane receptor in the ovarian follicle to
stimulate, via adenylate cyclase activation, cyclic AMP produc-
tion, and protein kinase, increasing the ovarian synthesis and
secretion of 17-P-estradiol, the female sex hormone.
Follicular atresia is
the cause of hypoestrogenemia in the
menopause, which is associated with elevated gonadotropin lev-
els (FSH and LH).
REFERENCES
Litwack G, Schmidt TJ. Biochemistry of hormones I: polypeptide hormones. In:
Devlin TM, ed. Textbook of Biochemistry with Clinical Correlations, 5th ed.
New York: Wiley-Liss, 2002.
Pathophysiology of the endocrine system. An online textbook from Colorado State
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