Binding of aflatoxins B1 and G1 to human serum proteins

Aflatoxin B1 transport in rat blood plasma. Binding to albumin in vivo and in vitro and spectrofluorimetric studies into the nature of the interaction

Binding of [3H]aflatoxin B1 to rat plasma was investigated in vivo and in vitro. Column chromatographic and polyacrylamide gel electrophoretic analyses clearly demonstrated that aflatoxin B1 bound primarily plasma albumin. Very little binding activity was shown by other plasma proteins. Spectrofluorimetric studies were undertaken to gain some insight into the nature of the aflatoxin-albumin interaction. Quenching of the lone tryptophan fluorescence intensity upon aflatoxin binding was due, at least in part, to a ligand-induced conformational change in the albumin molecule. Aflatoxin B1 binds an apolar site with an association constant of 30 mM-1 at pH 7.4 and 20 degrees C. Neither charcoal treatment of rat albumin nor the presence of 0.15 M NaCl had any significant effect on the interaction. The association constant was pH-dependent, increasing about 1.7-fold as the pH increased from 6.1 to 8.4. This pH dependence is ascribed to a pH-induced conformational change in the albumin molecule. Thermodynamic studies indicated that the aflatoxin-albumin interaction was exothermic (delta H = -29.3 kJ X mol-1), with a delta S value of -13.8 J X mol-1 X K-1.

Immunospecific binding of aflatoxins to human immunoglobulins in vitro

The aflatoxins are a family of complex coumarins produced by species of Aspergillus that are toxic and carcinogenic in animals. Four of the naturally occurring aflatoxins (B1, B2, G and G2) were incubated in vitro with whole human serum and subsequently tested for effects on antigenicities and electrophoretic mobilities of the proteins. Immunoelectrophoresis showed deviations of some of the precipitin arcs from normal patterns. Double diffusion experiments with affinity-isolated antibodies that are specific for the heavy chains of immunoglobulins IgA, IgG and IgM showed that the antigen-antibody binding sites of the heavy chains of IgA and IgM were blocked after incubation, but not those of IgG. These results are of interest regarding the specificity of immunochemical reactions in vitro that might be linked to specific immunological responses in diseases mediated by these toxins.

Aflatoxin B1 dihydrodiol antibody: production and specificity

A specific antibody for 2,3-dihydro-2,3-dihydroxyaflatoxin B1 (AFB1-diol) was prepared, and its reactivity was characterized for the major aflatoxin (AF) B1 (AFB1) metabolites. Reductive alkylation was used to conjugate AFB1-diol to ethylenediamine-modified bovine serum albumin (EDA-BSA) and horseradish peroxidase for use as an immunogen and an enzyme-linked immunosorbent assay (ELISA) marker, respectively. High reactant ratios, 1:5 and 1:10, for AFB1-diol-EDA-BSA (wt/wt) resulted in precipitated conjugates which were poorly immunogenic. However, a soluble conjugate obtained by using a 1:25 ratio of AFB1-diol to EDA-BSA could be used for obtaining high-titer AFB1-diol rabbit antibody within 10 weeks. Competitive ELISAs revealed that the AFB1-diol antibody detected as little as 1 pmol of AFB1-diol per assay. Cross-reactivity of AFB1-diol antibody in the competitive ELISA with AF analogs was as follows: AFB1-diol, 100%; AFB1, 200%; AFM1, 130%; AFB2a, 100%; AFG1, 6%; AFG2, 4%; aflatoxicol, 20%; AFQ1, 2%; AFB1-modified DNA, 32%; and 2,3-dihydro-2-(N7-guanyl)-3-hydroxy AFB1, 0.6%. These data indicated that the cyclopentanone and methoxy moieties of the AF molecule were the primary epitopes for the AFB1-diol antibody. The AFB1-diol competitive ELISA was subject to substantial interference by human, rat, and mouse serum albumins but not by BSA, Tris, human immunoglobulin G, or lysozyme. By using a noncompetitive, indirect ELISA with an AFB1-modified DNA solid phase, a modification level of one AFB1 residue for 200,000 nucleotides could be determined.

Fluorescence studies of the interactions of serum albumin and rat alpha1-fetoprotein with aflatoxin B1. Specificity and binding parameters

The interaction of bovine serum albumin and rat alpha1-fetoprotein with aflatoxin B1 has been followed by the fluorescence quenching of the protein in presence of the ligand. The binding parameters (n, number of sites and Kd, dissociation constant) have been determined for the bovine serum albumin-alflatoxin B1 system: n = 3.5 and Kd = 3.1 +/- 0.5 . 10(-5) M and for the alpha-fetoprotein-aflatoxin system: n = 4 and Kd = 3.7 +/-0.5 . 10(-5) M. The competition of anilino-naphthalene-sulfonate and aflatoxin B1 for the same hydrophobic sites on bovine serum albumin has been demonstrated. The fluorescence quenching of various proteins (lysozymes, egg-albumin, gamma-globulin) by aflatoxin B1 have shown that there is not a strict specificity of aflatoxin towards proteins.