No abstract

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.

Nuclear magnetic resonance (NMR) method for the quantitative analysis of cyclopropenoid fatty acids (CPFA) in lipids is described. Good accuracy is obtained at CPFA concentrations of 1% to 100%. At a CPFA concentration of 10%, NMR is accurate to 0.5%. The position of absorption of the two ring methylene hydrogens is solvent dependent.

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.

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.