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

Injectable trace minerals (selenium, copper, zinc, and manganese) alleviate inflammation and oxidative stress during an aflatoxin challenge in lactating multiparous Holstein cows

Trace minerals are vital in the antioxidant response during oxidative stress; however, limited research is available on the effects of trace mineral supplementation during an aflatoxin (AF) challenge. The objective of the study was to determine the effects of 2 subcutaneous injections of 15 mg/mL of Cu, 5 mg/mL of Se, 60 mg/mL of Zn, and 10 mg/mL of Mn (Multimin 90, Multimin North America, Fort Collins, CO) given at 1 mL/90.7 kg of average body weight in response to an AF challenge. Fifty-eight Holstein cows [body weight (mean ± SD) = 734 ± 6 0kg; days in milk = 191 ± 93] were assigned to 1 of 3 treatments in a randomized complete block design. The experimental period (63 d) was divided into an adaptation phase (d 1-56) and a measurement phase (d 57-63). From d 57 to 59, cows received an AF challenge that consisted of 100 μg of aflatoxin B₁/kg of dietary dry matter intake (DMI) administered orally via balling gun. Treatments were saline injection and no AF challenge (NEG), saline injection and AF challenge (POS), and trace mineral injection and AF challenge (MM). Injections were performed subcutaneously on d 1 and 29. Milk was sampled 3 times daily from d 56 to 63, blood was sampled on d 0, 56, 60, and 63, and liver samples were taken on d 0 and 60. Two treatment orthogonal contrasts [CONT1 (NEG vs. POS) and CONT2 (POS vs. MM)] were made. Cows in NEG had lower AF excretion in milk and greater 3.5% fat-corrected milk (32.1 ± 1.37 kg/d) compared with cows in POS (28.6 ± 1.43 kg/d). Feed efficiencies (3.5% fat-corrected milk/DMI, energy-corrected milk/DMI, and milk/DMI) were greater for cows in NEG (1.42 ± 0.07, 1.46 ± 0.07, and 1.45 ± 0.07, respectively) than cows in POS (1.16 ± 0.08, 1.18 ± 0.08, and 1.22 ± 0.07, respectively). Cows in POS had greater milk urea nitrogen and blood urea nitrogen than cows in MM. Liver concentrations of Se and Fe were greater for cows in MM compared with cows in POS. Cows in MM tended to have greater plasma glutathione peroxidase activity compared with cows in POS. An upregulation of liver GPX1 was observed for cows in POS compared with cows in MM. In conclusion, subcutaneous injection of trace minerals maintained an adequate antioxidant response when an AF challenge was present.

Reaction of aflatoxin B(1) oxidation products with lysine

Aflatoxin (AF) B(1) exo-8,9-epoxide hydrolysis yields AFB(1) dihydrodiol, which undergoes base-catalyzed rearrangement to, and is in equilibrium with, AFB(1) dialdehyde. We investigated the reaction of AFB(1) dialdehyde with albumin to generate a Lys adduct, previously characterized by others [Sabbioni, G., Skipper, P. L., Büchi, G., and Tannenbaum, S. R. (1987) Carcinogenesis8, 819-824; Sabbioni, G. (1990) Chem.-Biol. Interact. 75, 1-15]. Pronase digestion of bovine albumin serum treated with AFB(1) dialdehyde and HPLC yielded the adduct, identified by its characteristic UV and mass spectra. The structure of the Lys-AFB(1) dialdehyde adduct is concluded to be (S)-alpha-amino-2,3-dihydro-2-oxo-4-(1,2,3,4-tetrahydro-7-hydroxy-9-methoxy-3,4-dioxocyclopenta[c][1]benzopyran-6-yl)-1H-pyrrole-1-hexanoic acid, structure B of the former paper and 8 of the latter, based on work with the methylamine adduct described in the following paper in this issue [Guengerich, F. P., Voehler, M., Williams, K. M., Deng, Z., and Harris, T. M. (2002) Chem. Res. Toxicol. 15, 793-798]. The time course of product formation at varying concentrations of AFB(1) dialdehyde could be described by complexation with albumin with a K(d) of 1.5 mM and a first-order reaction rate with the N6-amino group of Lys of 0.033 min(-)(1). The reaction of AFB(1) dialdehyde with N(2)-acetylLys was monitored by UV spectroscopy and yielded a final spectrum similar to that of the described Lys adduct. Kinetic analysis of the changes at pH 7.2 was best described with a scheme involving equilibrium of the dialdehyde with dihydrodiol and a rate-limiting reaction of AFB(1) dialdehyde with the N6 atom of N(2)-acetylLys, with an apparent second-order rate constant of 2.6 x 10(3) M(-)(1) min(-)(1), followed by putative carbinolamine formation and rearrangement, collectively described by a first-order rate constant of 7.6 min(-)(1). Competition experiments with the hydrolysis of AFB(1) exo-8,9-epoxide indicate that N2-acetylLys also reacts with the epoxide at pH 7.2 (k = 350 M(-)(1) min(-)(1)) and 9.5 (k = 1.8 x 10(3) M(-)(1) min(-)(1)). This reaction might contribute to the formation of protein Lys adducts, depending upon the local concentration of free or protein Lys. Mass spectral analysis of trypsin digests of bovine serum albumin modified with AFB(1) dialdehyde indicated selective modification of Lys455 and Lys548. Collectively, these results provide more insight into the mechanism of formation of AFB(1) dialdehyde-protein adducts and indicate that the formation of Lys adducts is a moderately efficient process. The binding of AFB(1) dialdehyde to albumin or the protonation of the N6-amino group retards the reaction with Lys residues.

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.

Drug-binding properties of rat alpha 1-foetoprotein. Binding of warfarin, phenylbutazone, azapropazone, diazepam, digitoxin and cholic acid

As part of an investigation into whether alpha 1-foetoprotein (alpha 1-FP) plays the same transport role in foetal serum as albumin does in the adult, the binding properties of both proteins were compared with respect to the binding of a series of compounds known to be bound by albumin's specific drug-binding sites. The binding of warfarin, phenylbutazone, azapropazone, diazepam, digitoxin and cholic acid by rat alpha 1-FP and serum albumin was studied by equilibrium dialysis at 4 degrees C. Rat alpha 1-FP was shown to have neither albumin's high-affinity site II (diazepam as marker) nor its site III (digitoxin and cholic acid as markers). High-affinity binding by alpha 1-FP was found for the specific markers (warfarin, phenylbutazone, azapropazone) of albumin's drug-binding site I. However, instead of albumin's one high-affinity site/molecule, a mean value of 0.5 site/molecule was obtained with rat alpha 1-FP. Charcoal treatment at neutral pH of rat serum albumin did not affect its measured binding properties, but treatment of the alpha 1-FP led to an increased affinity for warfarin, phenylbutazone and azapropazone without a change in the measured number of sites, indicating competition for binding at this site by (an) endogenous ligand(s). These results are discussed in terms of the structures of the two proteins and with respect to the physiological implications of the differences found.

Conformational transitions of human alpha-1 fetoprotein and serum albumin at acid and alkaline pH

Conformational transitions of HAFP in the pH-range 2-12 were studied by fluorescence spectroscopy, fluorescence polarization measurements, circular dichroism and hydrophobic chromatography in order to compare molecular architecture of HAFP and that of human serum albumin. It was found that HAFP has a remarkably hydrophilic exposed molecular surface at neutral pH and possesses extensive hydrophobic binding sites located in crevices. Conformational changes occur in HAFP in the acid and alkaline pH regions; extensive hydrophobic areas in HAFP are exposed by both acid and alkaline transitions. The alpha-helix contents of HAFP were determined as 67% at pH 7.6, 47% at pH 2.11.

Association of aflatoxin B1 with plasma components in vitro

The in vitro association of [3H]aflatoxin B1 with plasma from man, pig and rat was compared. The uptake of aflatoxin B1 into plasma was rapid and did not differ between species. Its association with lipoprotein fractions was minimal when incubated with whole plasma; column chromatography of plasma from all 3 species resulted in the elution of the aflatoxin B1 with the low Mr plasma proteins. It is concluded that although aflatoxin B1 is a hydrophobic compound, it does not partition into the plasma lipoproteins and that its in vitro uptake into plasma does not differ among the 3 species studied. Plasma transport is probably not one of the factors determining species specificity for carcinogenesis by aflatoxin B1.

Differences in the binding of thyroid hormones and indoles by rat alpha 1-fetoprotein and serum albumin

The transport of small molecules in the blood, normally assured by serum albumin in the adult, is not well known in the fetus since the albumin concentration is low in fetal serum and inversely related to the alpha 1-fetoprotein concentration. In order to investigate whether rat alpha 1-fetoprotein might be a fetal counterpart to albumin, the binding properties of these two proteins have been compared with respect to a series of molecules of biological importance, especially during fetal development: thyroid hormones and indole analogues. Though high-affinity binding of thyroxine was found with both rat alpha 1-fetoprotein and albumin, a significant difference in the number of binding sites for this hormone was found with the two proteins. Further, while rat serum albumin strongly bound L-tryptophan and indolyl-3-acetic acid (Ka approximately equal to 10(5) M-1), rat alpha 1-fetoprotein did not bind any of the indoles tested. These results are discussed with respect to the physiological and pharmacological significance of the transport role of these proteins.