The conditions for the formation of a non-covalent complex between 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the human transport fetal protein, K K-fetoprotein (AFP), have been studied. TCDD has been shown to form a stable complex with AFP in a 2:1 (TCDD :AFP) ratio. The apparent solubility of TCDD in water increases 10 5 -fold after complex formation. The toxicity of the TCDD:AFP complex injected into mice by the intravenous route is comparable with that of free TCDD administered in oil solution per os. The complex manifests very much higher toxicity (200^1400 times) against human tumor cells (CEM, MCF-7, HepG2) in vitro and surpasses TCDD in selectivity. AFP may facilitate TCDD transport in embryonic tissues and enhance its embryotoxic and teratogenic effects.z 1999 Federation of European Biochemical Societies.

The steady growth of inflammatory diseases of the udder in dairy cattle forces us to look for the causes of this phenomenon in the context of growing chemical pollution of the environment and feeds. Within the framework of this concept, an analysis was made of the polarity level of the three toxic impurity groups, which are commonly present in dairy cattle feeds. These impurities are presented by mycotoxins, polyaromatic hydrocarbons (PAH) and persistent organic pollutants (POP). It has been determined that 46% of studied mycotoxins (n = 1500) and 100% of studied polyaromatic hydrocarbons (n = 45) and persistent organic pollutants (n = 55) are lipophilic compounds, prone to bioaccumulation. A comparative evaluation of the sorption capacity of four adsorbents of a different nature and polarity with respect to the simplest PAH, naphthalene and lipophilic estrogenic mycotoxin, zearalenone in vitro has been carried out. The highest efficiency in these experiments was demonstrated by the reversed-phase polyoctylated polysilicate hydrogel (POPSH). The use of POPSH in a herd of lactating cows significantly reduced the transfer of aldrin, dieldrin and heptachlor, typical POPs from the “dirty dozen”, to the milk. The relevance of protecting the main functional systems of animals from the damaging effects of lipophilic toxins from feeds using non-polar adsorbents, and the concept of evaluating the effectiveness of various feed adsorbents for dairy cattle by their influence on the somatic cell count in the collected milk are discussed.

Membrane-bound sites of cytochrome P-450 2B4 (LM2) were determined by means of two different methods, photoactivated binding of membrane phospholipids to the protein and epitope mapping by antibodies. Phospholipids bearing photoreactive labels at different distances from the their polar 'head' were used in the former case. Phosphatidylcholine labelled at the apolar end of the fatty acid chain bound only to the N-terminal region of the hemoprotein. Other phospholipids labelled nearer to the head group bound not only to the N-terminus but also to the segments 273-314 and 427-491. Epitope mapping of the domain next to the N-terminus (residues 21-119) of the isolated hemoprotein was performed with the help of a peptide-scanning method, a programmable peptide synthesis on pins followed by ELISA testing with the polyclonal antiserum against cytochrome P-450 2B4. This domain was shown to possess a considerable density of sites with high antigenic activity. No membrane-penetrating part of this domain was found except for the fragment 1-21. A model of structure of P-450 2B4 was computed by comparison with the structure of cytochrome P-450,, on the basis of an alignment of 47 cytochromes P-450 with the former hemoprotein. Major parts of the protein sequences photoreacting with the phospholipid probes, but not the antibody-reactive epitopes of the region 21 -119, are located at the membrane-facing side in this model Though cytochromes P-450 are the most thoroughly studied membrane enzymes, there is no agreement on their topology in the lipid bilayer. At present there are several models of the membrane orientation of microsomal cytochrome P-450 [l-61 constructed on the basis of various methods of polypeptide chain-structure prediction or some experimental data [7-111. The aim of our investigation was to examine these models by analysis of the interaction of photoreactive phospholipids bearing the photolabel at different distances from their polar 'heads' with cytochrome P-450 2B4 in proteoliposomes and by surface epitope mapping of the 119 N-terminal amino acid residues in the solubilized protein with the help of the peptide-scanning technique (PEPSCAN) [ 121. MATERIALS AND METHODS MaterialsCytochrome P-450 2B4 was obtained from phenobarbital-induced rat liver microsomes [ 131. Proteoliposomes were prepared as described earlier [14]. Photoreactive lipid was added to the solution of egg phosphatidylcholine, a phosphoCorrespondence to V. Y. Uvarov, Institute Biomedical Chemis-

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We studied the effects of a hydrophobized reversed-phase feed adsorbent in the form of a polyoctylated polysilicate hydrogel (POPSH) on productivity indicators, metabolic adaptation, and on the level of polychlorinated biphenyls (PCBs) in the blood of growing Holsteinized black-and-white heifers during the transition period. Two groups of two-month-old heifers of 20 head each were used. The experimental group received POPSH in addition to the main diet. The use of the adsorbent led to an increase in daily weight gain by 19.9% and to a decrease in the total concentration of PCB congeners found in whole blood by 40%. The greatest decrease in concentration (35–52%) was observed for tetra-, penta- and hexachlorobiphenyls. These results demonstrate the possibilities of effective protection of calves from lipophilic toxins of feed and their active decontamination.