No abstract

Ochratoxin A (OA) has been reported to affect immune function both at the level of antibody synthesis and natural killer (NK) cell activity. In the present study we demonstrate that exposure of purified human lymphocyte populations and subpopulations to the toxin will abrogate the cells' ability to respond to activating stimuli in vitro. Thus, both IL-2 production and IL-2 receptor expression of activated T lymphocytes are severely impaired. When the cells are preincubated with the analogue ochratoxin B (OB) prior to OA exposure, the inhibitory effect of OA is reversed. Furthermore, the inhibitory effect of OA on antibody production is not only due to blocking of T helper cell function. Highly purified B lymphocytes will not respond to polyclonal activators in vitro after a brief pulse with OA. The results strongly suggest that the toxin causes its immunosuppression through interference with essential processes in cell metabolism irrespective of lymphocyte population or subpopulation.

Three metabolites were formed from ochratoxin A in the presence of rabbit liver microsomal fractions and NADPH. They were isolated by extraction, thinlayer chromatography, and high-pressure liquid chromatography. Two of them were identified as (4R)-and (4S)-4-hydroxyochratoxin A. It is suggested on the basis of mass and nuclear magnetic resonance spectroscopy that the third metabolite is 10-hydroxyochratoxin A. The formation of the metabolites was inhibited by carbon monoxide and metyrapone and was stimulated when microsomes from phenobarbital-treated animals were used. The results suggest that cytochrome P-450 catalyzes the formation of these metabolites.

Two metabolic products were formed from ochratoxin A by human, pig, and rat liver microsomal fractions in the presence of reduced nicotinamide adenine dinucleotide phosphate. They were isolated from the incubation mixture in the presence of pig liver microsomes by extraction, thin-layer chromatography, and high-pressure liquid chromatography. Their structures are suggested to be (4R)and (4S)-4-hydroxyochratoxin A on the basis of mass and nuclear magnetic resonance spectroscopy. Km and the maximum velocity for the formation of the two metabolites by human, pig, andrat microsomes were determined. Their formation was inhibited by carbon monoxide and metyrapone. The results indicate that the microsomal hydroxylation system is a cytochrome P-450 and that different species are involved in the formation of the two epimeric forms of 4hydroxyochratoxin A.

No abstract

No abstract

The diffusion and sedimentation constants of the pH 6 acetolactate‐forming enzyme, determined by analytical ultracentrifugation, were used to calculate a molecular weight of 200 000 for the enzyme. The amino acid composition of the enzyme has been worked out, and 3 molecules of cocarboxylase, and 6 atoms of phosphorus were found to be tightly bound to one molecule of the enzyme. In addition, the effect of divalent cations was studied, results indicated that the enzyme has a requirement for manganese. An absolute requirement for cocarboxylase and manganese was not established. Two crystalline forms of the enzyme were studied by electron microscopy using negative staining technique. The plate‐shaped crystals were composed of rows of particles measuring approximately 100 × 60 Å. These particles consisted of sub‐units, and in some areas a helical organization of the crystal could be seen. The molecular organization was different in the two different crystalline forms of the enzyme.

No abstract