Metabolism and DNA binding of aflatoxicol and aflatoxin B1 in vivo and in isolated hepatocytes from rainbow trout (Salmo gairdneri).

The purpose of this study was to compare the metabolism and DNA binding of aflatoxicol (AFL) with that of aflatoxin B1 (AFB1) in vivo and in isolated hepatocytes from Mt Shasta strain rainbow trout (Salmo gairdneri). Maximum total binding of [3H]AFL to liver DNA from trout exposed by intraperitoneal injection was 38-47% of that of [3H]AFB1 over a 1-7 day period. The average AFL/AFB1 DNA binding ratio in 1-h incubations with isolated hepatocytes was 0.67 +/- 0.36 (n = 13). In freshly isolated hepatocytes, substantial interconversion between AFB1 and AFL via reductase and dehydrogenase enzymes was observed. Total in vivo excretion of conjugates in bile over 4 days was greater for [3H]AFL substrate than for [3H]AFB1. To determine if AFL binding was due to direct activation or to prior metabolism to AFB1 followed by activation, AFL with a tritium atom on the carbon containing the cyclopentenol function [1-3H]AFL, was synthesized and incubated with hepatocytes. Binding of [1-3H]AFL was 3% that of [3H]AFB1 and represents only direct binding of the intact cyclopentenol epoxide molecule before transformation to AFB1 and consequent loss of 3H. H.p.l.c. analysis of DNA hydrolyzed after incubation with [1-3H]AFL resulted primarily in production of non-radioactive 8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 (AFB1-N7-guanine). A radioactive peak estimated to be 1% as abundant as the AFB1-N7-guanine was also observed. The overall binding of generally labeled [3H]AFL to trout liver DNA in vivo and in freshly prepared hepatocytes correlates well with available tumor incidence and mutagenicity data. Conclusions from these findings are that direct interaction of AFL-8,9-epoxide with DNA is of relatively minor quantitative importance in rainbow trout hepatocytes and that the major adduct results from conversion of AFL to AFB1 prior to epoxide formation.

Enhancement of carcinogenesis by the natural anticarcinogen indole-3-carbinol.

Indole-3-carbinol (I3C), a natural constituent of cruciferous vegetables, is an inhibitor in several experimental animal models of carcinogenesis by polynuclear aromatic hydrocarbons or aflatoxin B1 (AFB1) when administered prior to or during carcinogen exposure. For assessment of the postinitiation effects of I3C, rainbow trout were exposed to dietary I3C in two different protocols--before and during AFB1 exposure or after AFB1 exposure only. Preinitiation exposure to I3C reduced AFB1-initiated hepatocellular carcinomas in trout as previously reported, but post-initiation I3C exposure strongly enhanced the tumor incidence above the positive AFB1 control. These results reveal the need for additional research to elucidate the overall effect of I3C on chemical carcinogenesis.

Altered lipid metabolism and impaired clearance of plasma cholesterol in mice fed cyclopropenoid fatty acids.

Swiss-Webster mice fed a diet containing 0.5% cyclopropenoid fatty acids (CPFA) for 6 weeks showed depressed growth rates and developed hypercholesteremia and increased concentrations of serum phospholipid and free cholesterol compared to control mice. No depression of cytochromes P-450 and b5 or microsomal mixed-function oxidase activities occurred to indicate impaired oxidative catabolism of serum cholesterol. Elimination of intragastrically administered [3H]cholesterol from blood was biphasic; there was no significant difference in first-order rate constants for absorption, distribution, and elimination processes between control and CPFA-fed animals. However, the area under the blood clearance curve for CPFA-fed animals was significantly increased (p less than or equal to 0.01) by 29% over controls, demonstrating a net increase in clearance time for exogenous cholesterol in CPFA-fed animals, thus contributing to their hypercholesteremia. In the CPFA-fed mice, the percentage of saturated fatty acid residues increased at the expense of monounsaturates in the cholesterol ester, triglyceride, and phosphatidyl choline fractions of serum lipids. Total polyene content of serum lipid was not altered; however, CPFA-fed animals demonstrated increased linoleic acid at the expense of arachidonic acid in all serum lipid fractions. Excessively saturated serum lipids may impede clearance of serum cholesterol in CPFA-fed animals by inhibited plasma lecithin-cholesterol acyltransferase (LCAT) and hepatic cholesterol esterase activities.

Inhibition of aflatoxin B1 carcinogenesis in rainbow trout by flavone and indole compounds.

Several compounds such as flavonoids, selenium, antioxidants and retinoids reportedly reduce the induction of cancer in experimental animals, and some have been suggested to function by affecting the mixed-function oxidase (MFO) system. The following compounds: 50 and 500 p.p.m. beta-naphthoflavone (BNF), 1000 p.p.m. flavone, 1000 p.p.m. of a tangeretin - nobilitin mixture, 1000 p.p.m. beta- ionone , 1000 p.p.m. indole-3-carbinol ( I3C ) and 2000 p.p.m. quercetin were examined for protection against aflatoxin B1 (AFB1) hepatocarcinogenesis, induction of the MFO system and metabolism of AFB1 in rainbow trout. These compounds were fed to fingerling rainbow trout for 8 weeks. At that time the activity of several MFO enzymes and cytochrome P450 content were measured and the trout were exposed for 2 weeks to 20 p.p.b. AFB1 in the same diets. After feeding the test diets without AFB1 for another 6 weeks and basal diet for another 52 weeks, the tumor incidence was determined. The effect of BNF and I3C on in vivo binding of AFB1 to DNA was also measured in separate groups of trout. BNF induced the trout MFO system in a dose-dependent manner, tangeretin - nobilitin was less effective and I3C did not induce. BNF showed significant alterations in the metabolism of AFB1 to aflatoxicol and aflatoxin M1 using cell fractions from pretreated fish. None of the other compounds, including I3C showed such an effect. Despite the apparent lack of in vitro effect of I3C , both BNF and I3C reduced AFB1 - DNA binding in vivo. I3C and BNF provided marked protection against AFB1-induced hepatocarcinogenesis, while the other compounds were less effective. The 58 weeks tumor incidences were 4% for 1000 p.p.m. I3C , 6% for 500 p.p.m. BNF and 18% for 50 p.p.m. BNF, compared to 38% for the AFB1-positive control. These data demonstrate that gross induction of the MFO system was not necessarily required for alterations in DNA adduct formation in vivo or protection against AFB1 carcinogenesis. Both BNF and I3C provided marked protection but only BNF induced the MFO system.

The sensitivity of rainbow trout and other fish to carcinogens.

Systematic design of replacement chemicals with reduced toxicities will require knowledge of mechanisms of action of parent compounds, especially in species which occupy the environment of most likely exposure. For aquatic systems, the rainbow trout has proven a valuable model for studying mechanisms of carcinogenicity. By comparison, small aquarium species show great potential as in situ field monitors of aquatic contamination by toxic chemicals but are less developed for mechanism studies. Fish species, especially rainbow trout, have also proven useful alternatives to traditional rodent models for comparative studies on mechanisms of action of nonaquatic carcinogens. These kinds of comparative studies form an essential basis for extrapolation of animal studies to man. Carcinogenicity testing of individual compounds and their replacements can provide only limited information on the expected impact of such chemicals on natural populations, since these populations are unavoidably exposed to potent modulators of the carcinogenic response. Hence any program which aims at redesign of commercial chemicals with reduced toxicities must have as a prior aim the full understanding of the mechanisms of joint carcinogen-inhibitor-promotor interactions. Because of their high sensitivity, low cost per individual, and low background tumor incidences, fish models such as the rainbow trout may be the only vertebrate models in which it is economically practical to initiate such complex studies.

Identification and mutagenicity of aflatoxicol-M1 produced by metabolism of aflatoxin B1 and aflatoxicol by liver fractions from rainbow trout (Salmo gairdneri) fed beta-naphthoflavone.

beta-Naphthoflavone (beta NF) fed to rainbow trout (Salmo gairdneri) at 50 or 500 ppm in the diet, modified the in vitro metabolism of aflatoxin B1 (AFB1) by the postmitochondrial fraction (PMF) of the liver. Production of aflatoxicol (AFL) was significantly less in the 500 ppm beta NF-fed group (33.9 ng/mg protein) than in the control group (45.7 ng/mg protein), aflatoxin M1 production was dependent on the dose of beta NF, being greatest in the 500 ppm beta NF-fed group (48.9 ng/mg protein), intermediate in the 50 ppm beta NF-fed group (3.7 ng/mg protein), and was not detected in controls. A new trout metabolite, 4-hydroxyaflatoxicol (aflatoxicol M1, AFLM1) was also detected in small amounts from in vitro metabolism by liver PMF from beta NF-fed trout. Sufficient quantities of AFLM1 for confirmation of identity by ultraviolet spectra, mass spectra and nuclear magnetic resonance spectra were prepared by biotransformation of AFL using liver microsomes and isolation by HPLC. In a modified Ames mutagen assay with Salmonella typhimurium TA98, ALFM1 was 4.1% as mutagenic as AFB1 in a previous determination. The carcinogenicity of AFLM1 to rainbow trout is expected to be considerably less than that of AFB1.

Aflatoxicol-induced hepatocellular carcinoma in rainbow trout (Salmo gairdneri) and the synergistic effects of cyclopropenoid fatty acids.

Aflatoxicol (AFL), a major metabolite of aflatoxin B1 (AFB1) in the Mt. Shasta rainbow trout (Salmo gairdneri), was found to produce hepatocellular carcinoma in trout. It was administered in a casein diet to duplicate groups of 120 fingering trout. In the same manner, additional duplicate groups received one of the following: no toxicant; AFB1; the diastereomer of AFL (AFL'); cyclopropenoid fatty acids (CPFA); and CPFA plus AFB1, AFL, and AFL'. Eight months after the initiation of the study, the following incidences of carcinoma were observed: control (0%); 20 ppb AFB1 (56%); 29 ppb AFL (26%); 61 ppb AFL' (0%); 50 ppm CPFA (3%); 20 ppb AFB1 plus 50 ppm CPFA (96%); 29 ppb AFL plus 50 ppm CPFA (94%); and 61 ppb AFL' plus 50 ppm CPFA (55%), showing both the carcinogenicity of AFL and the synergistic effects of CPFA. Twelve-month incidences were correspondingly higher in all cases. Aflatoxin M1, another metabolite of AFB1 in rainbow trout, was reported previously to be carcinogenic in trout. These results support the hypothesis that metabolism in rainbow trout does not effectively detoxify AFB1, but rather the formation of AFL extends the carcinogenicity of AFB1 and may contribute to the high sensitivity of rainbow trout to AFB.

Hepatocarcinogenicity of glandless cottonseeds and cottonseed oil to rainbow trout (Salmo gairdnerii).

Glandless cottonseed kernels are available for purchase and consumption by the general public. These kernels contain no gossypol but still have a full complement of naturally occurring cyclopropenoid fatty acids, which in rainbow trout are active as synergists with aflatoxins and primary liver carcinogens. Diets containing glandless cottonseed kernels or a lightly processes cottonseed oil produced significant numbers of hepatocellular carcinomas in rainbow trout after 1 year. The much greater incidence of cancer induced by the kernel than by the oil indicates that synergists or other carcinogens may be present in the kernel in addition to the cyclopropenoid fatty acids.

Aflatoxin B1 and aflatoxicol metabolism in rainbow trout (Salmo gairdneri) and the effects of dietary cyclopropene.

Aflatoxin M1 was identified as a minor metabolite formed by the in vitro incubation of aflatoxin B1 with the postmitochondrial fraction or microsomes from rainbow trout (Salmo gairdneri) liver. Fresh, whole trout liver converted perfused aflatoxin B1 to aflatoxicol and aflatoxin M1, and converted perfused aflatoxicol to aflatoxin B1 and aflatoxin M1. AFB1 reductive activity was found in the 105,000 xg supernatant, while activity converting AFL to AFB1 was distributed about equally in the 105,000 xg supernatant and in the microsomal pellet. Transformation of AFL to AFB1 was not inhibited by CO, but formation of AFM1 was completely blocked. Cyclopropene fatty acids (CPFA)-fed fish produced only one-half to one-third as much AFL from AFB1 as controls, and they produced no detectable AFM1. Most of the unreacted AFB1 was recovered by extraction of the incubation medium by organic solvent whereas, in controls, much of it remained bound to the protein. There was no difference in conversion of AFL to AFB1 when results were expressed in terms of postmitochondrial protein levels. CPFA-fed fish had lower microsomal protein and cytochrome P-450 levels and NADPH-cytochrome c reductase and aldrin epoxidation activities than did controls.

Effects of dietary cyclopropene fatty acids on the mixed function oxidase system of the rainbow trout (Salmo gairdneri).

Dietary cyclopropene fatty acids (CPFA) in rainbow trout (Salmo gairdneri) decreased cytochrome P-450 and b5 content and NADPH-cytochrome c reductase activity 67.8 percent, 22.0 percent, and 38.4 percent respectively compared to control-fed trout after 22 days. Dietary CPFA induced benzo(alpha)pyrene aryl hydrocarbon hydroxylase 37.5 percent over controls for the same time period. Microsomal protein levels were consistently lower in CPFA-fed trout. This pattern remained established in CPFA-fed trout for the duration of the experiment (74 days). The data suggest a possible mechanism by which dietary CPFA act as co-carcinogens.

Rat urinary metabolites of [9,10-methylene-14C] sterculic acid.

1. The metabolism of [9,10-methylene-14C] sterculic acid was studied in corn oil and Stercula foetida oil fed rats. The majority of the radioactivity was excreted into the urine as short chain dicarboxylic acids. The main urinary metabolites were cis-3,4-methylene adipic acid, cis-3,4-methylene suberic acid, trans-3,4-methylene adipic acid, cis-3,4-methylene pimelic acid, and cis-3,4-methylene azelic acid. 2. Formation of these urinary metabolites requires alpha-, beta-, and omega-oxidation plus reduction of the cyclopropene ring to a cyclopropane ring. Sterculic acid must be transported through both mitochondrial and microsomal systems. 3. Other non-radioactive urinary compounds were also identified. A proposed pathway for the metabolism of sterculic acid and possible detrimental effects caused by these metabolites is discussed.

Aflatoxin B1 metabolism to aflatoxicol and derivatives lethal to Bacillus subtilis GSY 1057 by rainbow trout (Salmo gairdneri) liver.

Aflatoxicol, R0, was isolated from Mt. Shasta strain rainbow trout (Salmo gairdneri), and liver homogenates were incubated with aflatoxin B1. Its identity was confirmed by mass, infrared, and ultraviolet spectrometry. The structure was identical to one of the diastereomers prepared by chemical reduction of aflatoxin B1. Aflatoxicol was apparently formed by a reduced nicotinamide adenine dinucleotide phosphate-dependent soluble enzyme of the 105,000 x g supernatant from rainbow trout. Aflatoxicol was not lethal in phosphate buffer to Bacillus subtilis GSY 1057 (metB4, hisA1, uvr-1) nor were aflatoxins B1, Q1, and B2. In the presence of reduced nicotinamide adenine dinucleotide phosphate and trout liver microsomes, aflatoxicol reduced the viability of B. subtilis. Aflatoxin B2, which lacks the vinyl ether present in the other compounds, could not be activated. The product of aflatoxin B1 activation by trout liver microsomes was sought after incubation of 14C-labeled aflatoxin B1. The radioactivity was found in unaltered aflatoxin B1 and in three extremely polar metabolites. The quantity of the new metabolites and the level of microbial lethality was reduced by addition of cytosine and cysteine to the incubation medium. The vinyl ether configuration was a structural requirement for activation, and this finding and the nature of the enzymatic reaction were consistent with the hypothesis that the compounds were metabolized to highly reactive and unstable electrophilic products which bound to nucleophiles such as cytosine and were lethal to B. subtilis. The formation of aflatoxicol as the major product of trout liver metabolism is of great significance considering that it could be activated to a lethal compound and that rainbow trout are one of the most sensitive species to aflatoxin B1-induced carcinoma.