[Steroid-protein interactions in human, amphibian and Cyclostoma sera. Analytical ion exchange chromatography]

Diversity and biological significance of sex hormone-binding globulin in fish, an evolutionary perspective

In fish, two different genes, shbga and shbgb, exist that encode for very different proteins. Shbga is the ortholog of mammalian Shbg and was found in all investigated teleosts. In contrast, Shbgb is highly divergent and appears to be a salmonid-specific protein. Here, we review existing data on fish Shbga and Shbgb that have been obtained in chondrichthyes and osteichtyes. Even though other significant expression sites exist, existing data indicate that Shbga is mainly expressed in liver and subsequently secreted into the blood as a homodimer. In contrast, Shbgb is mainly expressed in the ovary, probably secreted as a monomer, and could contribute to the regulation of local steroid action. Binding studies indicate a specialization of circulating Shbg during evolution towards the preferential binding of estradiol and testosterone in teleosts. In contrast, specific fish steroids such as 11-oxo-androgens and oocyte maturation-inducing steroids that are crucial for reproduction are poorly bound by either form of Shbg.

Plasma sex steroid binding proteins (SSBP) in the male lizard, Podarcis s. sicula, during the reproductive cycle

In male Podarcis s. sicula plasma, a sex steroid binding protein [SSBP(s)] binds testosterone (T) and estradiol-17 beta (E2) with moderate affinity (Kd = 0.23 +/- 0.08 x 10(-8) for 3H-E2, and 0.24 +/- 0.07 x 10(-8) for 3H-T) and high capacity. The SSBP binding affinity is unchanged throughout the sexual cycle, although its capacity is higher in nonreproductive males (winter and postreproductive period). This change may be related to changes in plasma T and E2 levels, and is likely to be involved in mechanisms whereby free steroid is delivered to target organs. SSBP, under isoelectrofocusing, is distributed between pH 5.5-6.5 and pH 7.1-7.5. The concentration of these two forms varies during the annual cycle.

Identification of an estrogen receptor in the testis of the sea lamprey, Petromyzon marinus

Employing a hydroxylapatite batch assay, estrogen-binding activities (EBAs) were demonstrated in the cytosol and nuclear extract of the testis of the anadromous sea lamprey, Petromyzon marinus. The lamprey testicular EBAs are sensitive to trypsin digestion and bind [3H]estradiol-17 beta with high affinities (cytosolic Kd = 0.52 nM; nuclear Kd = 0.39 nM) and limited capacities (cytosolic: 56.2 fmol/g tissue; nuclear: 68.2 fmol/g tissue). Androgens, progesterone, and corticosterone displayed little affinities for lamprey EBAs. Thus, lamprey testicular EBA possessed many definitive properties of an estrogen receptor as described in amphibian, reptilian, and mammalian studies. No specific binding to androgens was detected in either testicular subcellular fraction. The presence of a putative estrogen receptor in lamprey testis suggests a functional role of estrogen in testicular regulation in this ancient vertebrate.

Plasma sex-steroid binding protein in a seasonally breeding reptile, Alligator mississippiensis

The properties of a sex-steroid binding protein (SSBP) in the plasma of the American alligator, Alligator mississippiensis, were partially characterized. Alligator SSBP has a sedimentation coefficient of 4S in a 5-20% sucrose gradient. It binds to estradiol-17 beta (E2) and testosterone (T) with limited capacities and moderate affinities (association constant for [3H]E2 is 4.70 +/- 0.09 X 10(8) M-1 and for [3H]T is 1.05 +/- 0.07 X 10(8) M-1, mean +/- SEM of six determinations). Plasma SSBP level, as measured by plasma [3H]E2 binding capacity, varies from 30 to 140 nmol per liter plasma (nM) and was found to be dependent on the gender, sexual maturity, and reproductive state of the animal. Distinct annual fluctuations in plasma SSBP level were observed in female alligators. In adult females, plasma SSBP levels were high (122 +/- 6 nM) in the fall during the nonbreeding season and low (30-60 nM) in spring and early summer during the breeding season. A minimum (33 +/- 6 nM) was reached in mid-June coinciding with the time of oviposition and rapid decline in circulating estrogen levels. This decline in adult female plasma SSBP levels during the breeding season was not observed in immature females. On the contrary, plasma SSBP levels in immature females increased from 81 +/- 14 nM in April to 134 +/- 9 nM in June. Plasma SSBP levels in male alligators showed little changes throughout the entire breeding season; they remained within the range of 80-100 nM from March to June. We believe that seasonal fluctuations in plasma SSBP levels constitute part of the mechanism involved in the regulation of free steroid delivered to target organs in female alligators and that such a mechanism does not exist in male animals.

Corticosteroid binding in plasma of Xenopus laevis. Modifications during metamorphosis and growth

The binding of corticosteroids has been studied by equilibrium dialysis at 4 degrees C and electrophoresis on polyacrylamide gel. Aldosterone was not bound specifically. Large amounts of corticosterone (B) were bound. A high affinity (Ka = 2.8 X 10(8) M-1) low capacity (70.1 nM) component was found in tadpoles. Its affinity and its electrophoretic mobility were not significantly modified during metamorphosis and growth until adult. It differs from mammalian CBG with respect to affinity, electrophoretic mobility and specificity. During growth, the capacity of the high affinity component increased significantly (173 nM in adults) and a low affinity (Ka = 2.0 X 10(5) M-1) high capacity (18.26 microM) component was detected. This last component has the same electrophoretic mobility as bovine serum albumin. These modifications can be related to the large increase in the level of plasma proteins (especially albumins). The concentrations of bound and free B deduced from our data indicate that in tadpoles the increase in the total concentration in the climax is not a good reflection of modifications of these two fractions since the change of free B is larger than that of bound B. This information is an important consideration in interpreting the physiological role of interrenal secretion during metamorphosis.

Androgen transport proteins: physical properties, hormonal regulation, and possible mechanism of TeBG and ABP action

The physical properties and hormonal regulation of testosterone estradiol binding globulin (TeBG) and androgen binding protein (ABP) are reviewed. Brief mention is made of prostatic binding protein. The discussion focuses on the mechanism of action for these proteins in androgen secretion, transport, and absorption and androgen binding models. Speculations are presented for the functional significance of these proteins.

Sex steroid binding proteins in non-mammalian vertebrates

High affinity cytosolic and nuclear steroid binding macromolecules have been identified in steroid target tissues of representative amphibians, reptiles and birds. In addition to these traditional "receptors" evidence exists which indicates the presence of other steroid binding proteins, varying in affinity and specificity, in the cytosol of steroid targets (liver, oviduct) of these same species. The possible relationships between cytosolic binding proteins of differing affinity and specificity and their relevance to hormone action in non-mammalian species is discussed.