Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

I. Introduction
637
e . Placental Gonadotropin
A gonadotropin, renamed equine chorionic gonadotropin
(eCG) because of its close relationship with human chorionic
gonadotropin (hCG) ( Farmer and Papkoff, 1979 ),
was formerly called pregnant mares ’ serum gonadotropin
(PMSG) and is produced by mares during early pregnancy
(day 40 to 140) by fetal trophoblastic cells of the chorionic
girdle, which attach to, invade, and phagocytose the maternal
epithelium and become embedded within the uterus as
specialized endometrial cups. This process begins on day
36 of pregnancy. No specific placental gonadotropins have
been demonstrated in other domestic species.
2 . Steroid Hormones
Steroid hormones are derived from a common precursor
molecule, cholesterol, via the metabolic pathway schematically
outlined in Figure 21-1 . More than 1500 biologically
active steroids have been isolated from biological material
or have been produced synthetically. The molecular weight
of steroid hormones is low, usually below 500 ( Table 21-1 ).
Examples of steroids that play an important role in reproductive
processes are estrogens, androgens, and progestagens,
with the main source being the gonads. The structure
of the most important sex steroids is presented in Figure
21-2 . The most common steroid hormones are usually designated
by a trivial name (e.g., estradiol, testosterone, or
progesterone). The International Union of Pure and Applied
Chemistry (IUPAC; www.iupac.org ) has recommended systemic
names for steroid hormones. These systemic names
describe the chemical and stereoisomeric characteristics of
the particular steroid hormone ( Table 21-1 ).
3 . Prostaglandins
Prostaglandins constitute a group of 20-carbon unsaturated
fatty acids with molecular weights usually between 300
and 400 ( Table 21-1 ). Prostaglandins are not hormones
TABLE 21-1 Nomenclature and Molecular Weights
of Some Biologically Important Steroids and
Prostaglandins
Trivial Name Systematic Name Molecular
Weight
Androstenedione
4-Androstene-3,
17-dione
17 β -Estradiol 1,3,5(10)-Estratriene-3,
17 β -diol
Estrone
17 α -
Hydroxyprogesterone
Pregnenolone
3-Hydroxy-1,3,5(10)-
estatrien-17-one
17 α -Hydroxy-4-
pregnene-3,20-dione
3 β -Hydroxy-5-pregnen-
20-one
286
272
270
331
317
Progesterone 4-Pregnene-3,20-dione 315
Testosterone
17 β -Hydroxy-4-
androsten-3-one
PGF 2 α 9 α ,11 α ,15-
Trihydroxyprosta-5,
13-dienoic acid
15-Keto-13, 14-dihydro- 9 α ,11 α ,Dihydroxy-15-
PGF 2 α
ketoprost-5-enoic acid
288
354
354
Acetate
Cholesterol
FIGURE 21-1 Pathway for the synthesis of
biologically active steroids from acetate. The
steroids secreted from the gonads and the adrenals
are formed from acetate and cholesterol.
5-Pregnenolone
Progesterone
17-Hydroxypregnenolone
17-Hydroxy-
Progesterone
Corticosteroids
Dehydroepiandrosterone
Androstenedione
Estrone
5-Androstenediol Testosterone Estradiol-17