Effect of antioxidant vitamins A, C, E and their analogues on azo-dye binding protein in liver of rats treated with p-dimethylaminoazobenzene

p-Dimethylaminoazobenzene (DAB) is an azo-dye and known to cause liver tumour in rats. Azo-dye binding protein is a specific cytosolic protein involved in the translocation of azo-dye carcinogen metabolites from liver cytoplasm into the nucleus. Administration of vitamin A (40,000 and 50,000 IU), L-ascorbic acid (500 and 1000 mg) and vitamin E succinate (200-500 mg) reduced the amount of azo-dye binding protein in liver of rats treated with DAB. Supplementation of high doses of vitamin A acetate, vitamin A palmitate, sodium ascorbate, ascorbyl palmitate and vitamin E acetate had no effect on the quantity of azo-dye binding protein in liver. When the vitamin mixture was given, the level of azo-dye binding protein decreased in the liver at all the studied doses, which may be due to their synergistic effect.

Drug metabolizing enzyme system and heme pathway in hepatocarcinogenesis

Chemically induced and spontaneous liver tumors share some metabolic alterations. The decline in hemoprotein levels during hepatocarcinogenesis may result from a diminution of the intracellular heme pool. To elucidate if the onset of the pre-initiation stage alters the natural regulation mechanism of heme pathway, animals were fed with p-dimethylaminoazobenzene (DAB) and treated or not with 2-allylisopropylacetamide (AIA). The induction of 6-Aminolevulinic acid synthase (ALA-S) activity and the diminution in microsomal heme oxygenase (MHO) did not change when DAB fed animals were treated with AIA. Cytochrome P-450 (P-450) levels and glutathione S-transferase activity were increased in all the groups tested. Tryptophan pyrrolase, sulphatase and beta-glucuronidase activities were altered in DAB fed animals but AIA treatment did not produce any effect. Changes in drug metabolizing enzymes in livers of DAB fed animals could be the result of a primary deregulation of heme metabolism. These results give additional support to our hypothesis about a mechanism for the onset of hepatocarcinogenesis.

Formation and 32P-postlabeling of DNA and tRNA adducts derived from peroxidative activation of carcinogenic azo dye N,N-dimethyl-4-aminoazobenzene

Peroxidase in the presence of hydrogen peroxide catalyzes in vitro the activation of carcinogenic N,N-dimethyl-4-aminoazobenzene (DAB) to DNA-, tRNA- and homopolydeoxyribonucleotide-bound products. tRNA is the most susceptible to modification by the activated DAB. Binding of DAB products to macromolecules is inhibited by methyl viologen, nitrosobenzene, ascorbate, glutathione, NADH and MgCl2. The mechanism of these inhibitions was studied. The nuclease P1 version of the 32P-postlabeling assay was employed for detection and quantitation of some major DNA or tRNA adducts formed with DAB activated by a peroxidase system. tRNA modified by activated DAB shows a significantly increased acceptance for L-methionine.

Properties of an N,N-dimethyl-p-aminoazobenzene oxide reductase purified from rat liver cytosol

Homogenates of all rat tissues examined, except brain, catalyze reduction of N,N-dimethyl-p-aminoazobenzene N-oxide (DMAB N-oxide) to N,N-dimethyl-p-aminoazobenzene by NADPH. Liver is the most active, and about one third of the homogenate activity of this tissue is recovered in the cytosol fraction. The purified cytosol enzyme has the properties of a tetrameric protein (Mr 370,000) consisting of identical subunits free from chromophores that absorb in the visible spectrum and from metals or other detectable prosthetic groups. The purified reductase is also free from NADPH oxidase and from cytochrome c or azo reductase activities. The enzyme is quite specific for NADPH as reductant and DMAB N-oxide as the electron acceptor. Reduction of other N,N-dimethyl-arylamine or alkylamine oxides as well as N-methylheterocyclicamine oxides could not be detected. Analysis of kinetic data indicate that, at saturating concentrations of the other substrate, 21 microM NADPH and 700 microM DMAB N-oxide are required for half maximal velocity. At infinite concentrations of both substrates the turnover is 150 min-1 at 37 degrees C.