title: Isolation of two forms of rat alpha-fetoprotein and comparison of their binding parameters with estradiol-17beta.
abstract: In polyacrylamide gels, highly purified rat alpha1-fetoprotein shows a molecular heterogeneity, i.e. a "slow" and a "fast" moving fraction. We have isolated by electrophoretic fractionation and subsequent elution these two forms of alpha1-fetoprotein, and we have studied comparatively the binding parameters for estradiol-17beta of whole alpha1-fetoprotein preparations and of the isolated forms. We have shown that the number of binding sites per molecule of whole alpha1-fetoprotein is always, in our experimental conditions, a fractional number, inferior to unity (0.3). Furthermore, the analysis of the binding parameters of the "two forms" of alpha1-fetoprotein allows discrimination between different classes of binding sites. For the "slow" fraction, the number of predominant binding sites per molecule of protein is close to unity (0.7-0.9), whereas for the "fast" fraction, a very low fractional value is found (0.1). The corresponding association constants are reproducibly different for the two fractions: Ka = 0.1.10(8) M-1 for the "slow" alpha1-fetoprotein, and Ka = 0.7.10(8) M-1 for the "fast" alpha1-fetoprotein. Traces of a very high affinity (10(9) M-1) minor class of binding sites are demonstrated in the "slow" fraction. These results point to the existence of a molecular population of alpha1-fetoprotein, some forms of which have a strong or very strong affinity, and some a negligible affinity, for estrogens.
mesh_terms: D000818:Animals; D001665:Binding Sites; D006528:Carcinoma, Hepatocellular / Q000097:blood; D004958:Estradiol; D005260:Female; D005326:Fetal Proteins; D008113:Liver Neoplasms / Q000097:blood; D008297:Male; D011485:Protein Binding; D011487:Protein Conformation; D051381:Rats; D000509:alpha-Fetoproteins / Q000276:immunology / Q000302:isolation & purification
chemical_list: D005326:Fetal Proteins; D000509:alpha-Fetoproteins; D004958:Estradiol