Drug-binding properties of rat alpha-foetoprotein. Specificities of the phenylbutazone-binding and warfarin-binding sites

Rat α-Fetoprotein binding affinities of a large set of structurally diverse chemicals elucidated the relationships between structures and binding affinities

Endocrine disrupting chemicals interfere with the endocrine system in animals, including humans, to exert adverse effects. One of the mechanisms of endocrine disruption is through the binding of receptors such as the estrogen receptor (ER) in target cells. The concentration of any chemical in serum is important for its entry into the target cells to bind the receptors. α-Fetoprotein (AFP) is a major transport protein in rodent serum that can bind with estrogens and thus change a chemical's availability for entrance into the target cell. Sequestration of an estrogen in the serum can alter the chemical's potential for disrupting estrogen receptor-mediated responses. To better understand endocrine disruption, we developed a competitive binding assay using rat amniotic fluid, which contains very high levels of AFP, and measured the binding to the rat AFP for 125 structurally diverse chemicals, most of which are known to bind ER. Fifty-three chemicals were able to bind the rat AFP in the assay, while 72 chemicals were determined to be nonbinders. Observations from closely examining the relationship between the binding data and structures of the tested chemicals are rationally explained in a manner consistent with proposed binding regions of rat AFP in the literature. The data reported here represent the largest data set of structurally diverse chemicals tested for rat AFP binding. The data assist in elucidating binding interactions and mechanisms between chemicals and rat AFP and, in turn, assist in the evaluation of the endocrine disrupting potential of chemicals.

Biological roles of alpha-fetoprotein during pregnancy and perinatal development

The use of alpha-fetoprotein (AFP) as a serum marker in cancer actually predates its employment in the detection of congenital defects; however, the latter use of AFP as a fetal defect marker has propelled its clinical utilization. Although the serum-marker capacity of AFP has long been exploited, less is known of the biological activities of this oncofetal protein during fetal and perinatal development. In the present review, the biological activities of AFP are discussed in light of this glycoprotein's presence in various biological fluid compartments: embryonic and fetal tissues, serum, urine, and reproductive fluids. After a review of the histochemical detection of AFP in various cells and tissues during development, AFP concentrations within various biological fluids were discussed in the context of gestational age and anatomic location. Discussion follows concerning the relationships and roles of AFP in developmental events such as erthyropoiesis, histogenesis/organogenesis, and ligand binding and in developmental disorders such as hypothyroidism, folate deficiencies, and acquired immunodeficiency disorder (AIDS). Based on its association with so many types of birth defects, malformations, and congenital anomalies, AFP can be viewed as a molecular "troubleshooter" until signal transduction pathways are established during pregnancy and prenatal development. The review concludes with a discussion of the place of AFP in the rapidly expanding field of proteomics.

Fetal tissues are exposed to biologically relevant free thyroxine concentrations during early phases of development

Maternal hypothyroxinemia in early pregnancy is often associated with irreversible effects on neuropsychomotor development. To evaluate fetal tissue exposure to maternal thyroid hormones up to midgestation, we measured total T(4) and free T(4) (FT(4)), T(3), rT(3), TSH, and possible binding proteins in first trimester coelomic and amniotic fluids and in amniotic fluid and fetal serum up to 17 wk. Samples were obtained before interruption of maternal-fetal connections. The concentrations in fetal compartments of T(4) and T(3) are more than 100-fold lower than those in maternal serum, and their biological relevance for fetal development might be questioned. We found, however, that in all fetal fluids the concentrations of T(4) available to developing tissues, namely FT(4), reach values that are at least one third of those biologically active in their euthyroid mothers. FT(4) levels in fetal fluids are determined by both their T(4)-binding protein composition and the T(4) or FT(4) in maternal serum. The binding capacity is determined ontogenically, is independent of maternal thyroid status, and is far in excess of the T(4) in fetal fluids. Thus, the availability of FT(4) for embryonic and fetal tissues would decrease in hypothyroxinemic women, even if they were euthyroid. A decrease in the availability of FT(4), a major precursor of intracellular nuclear receptor-bound T(3), may result in adverse effects on the timely sequence of developmental events in the human fetus. These findings ought to influence our present approach to maternal hypothyroxinemia in early pregnancy regardless of whether TSH is increased or whether overt or subclinical hypothyroidism is detected.

Drug binding in plasma. A summary of recent trends in the study of drug and hormone binding

The ligands are generally bound in plasma to a significant extent by several transport proteins (both high and low affinity), irrespective of their endogenous or exogenous origin. The protein binding of endogenous compounds (such as hormones) exhibits higher affinity and specificity than those of exogenous compounds (such as drugs). For plasma proteins that bind the same ligand(s), structural similarities or a common genetic origin may be found, although this is not a general rule. Alterations in ligand binding may be due to modifications of either the structure or the level of the binding protein. These modifications may result from genetic make up, physiology or pathology. In some situations, plasma binding may impair the distribution of drugs to tissues, with drug distribution then mainly restricted to the distribution compartment of the drug-binding protein. In other instances, the plasma drug-binding is permissive, and does not limit drug distribution to tissues. A given drug-transport protein system may have either a permissive or a restrictive effect on the drug distribution, depending on the tissue. The physiological significance of the high-affinity transport proteins is not completely understood. These proteins may increase the plasma concentration of poorly hydrosoluble ligands, ensure a more uniform tissue distribution and increase the life of the ligands. The life of the protein may also be increased by ligand binding. High-affinity transport proteins are also involved in some specific carrier mediated transfer mechanisms. It is possible to demonstrate structure-binding relationships or binding selectivity for the plasma transport proteins, but these are quite independent of relationships observed at the receptor level.

Localization of the estrogen-binding site of alpha-fetoprotein in the chimeric human-rat proteins

Rat alpha-fetoprotein (AFP) has been demonstrated to bind estrogen, whereas human AFP lacks the activity. We constructed four chimeric molecules from cDNAs encoding these AFPs with the use of two restriction sites common to them and expressed them as well as rat and human AFP cDNA in yeast. The recombinant molecules were purified, characterized, and found to have the predicted structures. Analyses of estrogen binding indicated that a rat AFP sequence composed of residues 423-506 that contains 31 rat-specific amino acids is essential for the activity.

Chemistry and biology of alpha-fetoprotein

Alpha-Fetoprotein (AFP) is a product of specific fetal tissues and of neoplastic cells of hepatocyte or germ cell origin in adults. This protein belongs to a gene family that is phylogenetically most closely related to serum albumin. Its primary, secondary, and tertiary structural aspects appear similar to the three-domain concept proposed for the latter protein. The primary sequence of AFP departs most widely from serum albumin in the first 135 amino acid residues, with about 42% of the remaining 590 residues of the human proteins being identical. Some evidence exists that there are limited sequence differences in the AFP of a given animal species. AFP shows considerable charge heterogeneity that appears to relate mostly to its glycoid moiety. The proteins of some species such as the rat show more pronounced heterogeneities than that of humans. The variations in extent and type of glycosylations are evidenced by differences in the binding to various lectins. These interactions are being extensively explored in attempts to differentiate the sources of the protein produced by various normal and neoplastic cells and may provide valuable diagnostic methods. AFP, like serum albumin, shows relatively strong binding affinities for a variety of ligands. The most notable difference is the strong preferential binding of polyunsaturated fatty acids by AFP. This protein may play a role in transporting these substances to developing and to malignant cells. Various agents affect the synthesis of this protein both by specific fetal tissues and by neoplastic cells. Marked differences in the responses of cells, particularly those of neoplastic types, are indicative of variations in the genetic factors responsible for control of its synthesis. The subject of the genomic repression of the synthesis of AFP seen in fetal life upon maturation of the liver and the reoccurrence of synthesis upon malignant conversion of hepatocytes and of certain germ cells are of particular interest. The regulation of the closely related AFP and albumin genes is providing a powerful and attractive model to examine molecular events in the activation and inactivation of specific genes during development and in oncogenic processes. Extensive measurements of AFP during pregnancy and in the course of neoplasias, notably hepatoma, are being made to aid in following changes in such developments. Various specific physiological roles for this protein are also being proposed. One of these is its possible action in the regulation of immune processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Estrogen-binding properties of rat serum alpha 1-fetoprotein and its isoforms. Investigation of the apparent non-integrality of sites on the unfractionated protein

Rat fetal serum alpha 1-fetoprotein (AFP), a heterogeneous glycoprotein, binds estrogens with high affinity but at a fractional number of sites even after treatment with charcoal (n = 0.6), which may mean 60% of the protein has 1 site and the remainder none. To investigate the origin of this fractional number of sites the "native" protein (purified by negative affinity chromatography) was further purified (step 1) and fractionated (step 2) into its two main charge variants (electrophoretically "slow" and "fast") by a two-step fast-protein liquid chromatography method. The binding parameters for estrone and estradiol-17 beta of the "native" and "repurified" proteins and of each charge variant were determined by equilibrium microdialysis. The molar extinction coefficient at 278 nm of each sample was also determined. (1) The "repurified" AFP and each charge variant had a number of binding sites for estrogens close to unity. This increase in the number of sites could neither be explained by the loss of a non-binding isoform (corresponding to 40% of the protein) during chromatography, nor by the existence of complex negative modulatory interactions between isoforms. (2) The affinities for estrogens of the "repurified" protein and the two charge variants were slightly decreased compared to that of "native" AFP, except that the "fast" form had the "native" protein's high affinity for estrone--but not for estradiol-17 beta. (3) The molar extinction coefficients at 278 nm of the "repurified" AFP and the isoforms were much lower than that of the "native" protein. These results suggest that the presence of (an) inhibitor(s) of estrogen binding on the "native" protein which is/are removed by the ion-exchange fast protein liquid chromatography (FPLC) column. A ligand absorbing at 278 nm, which may or may not be the inhibitor, is also removed. The isoform heterogeneity with respect to estrone binding is discussed.

Characterization of the aflatoxin B1-binding site of rat albumin

A fluorescence-enhancement method was used to investigate the non-covalent interaction between aflatoxin B1 and rat albumin. Solvent-induced shifts in the emission spectrum of aflatoxin B1 provided evidence that the aflatoxin B1-binding site of rat albumin is a highly nonpolar environment. A dissociation constant of 20 microM was determined at 20 degrees C. The possibility that aflatoxin B1 binds one of the three major drug sites of albumin was investigated by ligand-displacement experiments. Mechanisms whereby marker ligands displace aflatoxin B1 were further investigated by comparing the experimental binding parameters with those derived theoretically, assuming competitive binding. The results indicate that: aflatoxin B1 and phenylbutazone compete for a common high-affinity site on rat albumin; high-affinity binding of aflatoxin B1 and site-II marker ligands takes place independently; aflatoxin B1 does not compete with either cholate or warfarin for the same high-affinity site, but the simultaneous binding of warfarin or cholate negatively modulates the binding of aflatoxin B1 to albumin. Fluorescence energy-transfer studies show that the lone tryptophan residue, Trp-214, is not associated with the aflatoxin B1-binding site.

Participation of the lone tryptophan residue of rat alpha-foetoprotein in its drug-binding sites. Comparison with rat serum albumin

The participation in drug binding of the lone tryptophan residue of rat alpha-foetoprotein (alpha-FP) and serum albumin, the two main transport proteins of foetal serum, has been studied by two different techniques. Firstly, the effect on phenylbutazone and warfarin binding of the chemical derivatization of the lone tryptophan residue of both proteins by 2-nitrophenylsulphonyl chloride (NPS) was studied. Secondly, the effect of phenylbutazone binding on the intrinsic fluorescence of the tryptophan residue of rat alpha-FP and albumin was investigated. The specific modification of the proteins by NPS did not affect the binding of warfarin by rat alpha-FP and albumin, but greatly decreased the affinity of the high-affinity sites of rat alpha-FP for phenylbutazone, though the numbers of these sites were not significantly changed. However, for albumin a similar decrease in the affinity constant appeared to be due to the reaction conditions. The spectrofluorimetric studies showed that the lone tryptophan residue of alpha-FP and albumin was quenched by phenylbutazone binding, and the quenching paralleled the fractional saturation of the high-affinity site only in the case of albumin. The effect of phenylbutazone binding on the intrinsic fluorescence of rat alpha-FP indicated that the lone tryptophan residue of this foetal protein is not in the same molecular environment as that of albumin, not participating directly in the high-affinity site for phenylbutazone, and the effect may be via some induced conformational change in rat alpha-FP. These results also confirm our previous suggestion that the high-affinity sites for phenylbutazone and warfarin are different on the rat alpha-FP molecule. The results seem to indicate that this is also the case for albumin, but confirmation is necessary.