G. Keith Elmund, Thomas C. Brewster, and Anthony T. Tul
Colorado State University Fort Collins, Colorado 80521
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Aflatoxins Potent food poisons and carcinogenic compounds
In 1960, large numbers of turkeys, pheasants, and ducklings in England, Kenya, and Uganda, and trout in the United States died in the course of a few months from a disease which inflicted damage to the liver and kidney. Because the disease was first noted in turkeys, it was designated Turkey X. Sargeant, et al. (1) isolated a pure culture from heavily contaminated sources of meal in Uganda and identified the fungus as Aspergillusflaws, a common contaminant of grains. Numerous foods and feeds are capable of supporting growth and toxin production by A . flaws. The most common substrates are grains, cocoanuts, and oil seeds. Aflatoxins not only cause acute lethal effectto a wide range of animals, but they also produce hyperplasia and neoplasia of the vital organs. Chemistry
Structures of various aflatoxins are summarized in the figure. Aflatoxins consist of the pentacyclic structure which can be classified as highly substituted coumarins. Aflatoxin B1 is the most toxic, carcinogenic, and prevalent of the eight confirmed structures. Aflatoxins exhibit blue fluorescence upon exposure to ultraviolet light. Different aflatoxins can he identified from the d i e r e n t R, values on thin layer chromatograms. Chemical synthesis or racemic aflatoxin B1 was achieved by Buchi and coworkers (%, 3) using phloroacetophenone as a starting material. Early reports on possible pathways of aflatoxin biosynthesis proposed that mevalonate (4), sterigmatocystin (5, 6), or kojic acid (7), could he intermediates or precursors. Recently, Basappa, et al. (8) reported that kojic acid and aflatoxin B1 are derived from separate metabolic pathways and that the former is not an intermediate in the biosynthesis of aflatoxin. In 1965, Thomas (6) proposed a hiosynthetic pathway wherein the anthraquinone, averufin, derived from acetate precursors, was an intermediate in the formation of sterigmatocystin. Evidence has accumulated that aflatoxins are synthesized from acetate precursors. Toxicology and Carcinogenicity
The effect of feeding aflatoxin to vertebrates is liver damage and especially the proliferation of bile duct cells. With chronic feeding, malignant liver tumors develop. The main site of retention of aflatoxin in the body is the liver and the remainder is excreted in the feces, urine, and milk as either unaltered aflatoxin Address reprint requests: A. T. Tu, Department of Biochemistry.
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or as a derivative. The immediate effect of aflatoxin is suppression of protein synthesis and reduction of the RNA content in the liver cells. Aflatoxins possess potent carcinogenicity as well as toxicity. The subcutaneous injection of rats with 2 pg of a mixture of B1 and G1 produced tumors in 5 out of 6 rats at the site of the injection (9, 10). When aflatoxin was administered to rats in their diet over a period of 9 weeks, the incidence of liver tumors was 100% (11). The liver, although the primary target organ, is not the only organ (in the rat) affected by aflatoxin. Butler and Barnes (14) also found (in the rat) neoplasma of the stomach, kidneys, lungs, salivary glands, mesenchymal tissue, brain, lymphatics, skin, pituitary, adrenal cortex, mammary glands, testis, and the uterus. The tumors of the liver tend to he solid. The rest of the liver usually appears normal. Kidney tumors may develop in rats treated with aflatoxin (14). The larger tumors were all, at least in part, localized by an apparent capsule. However, the bigger neoplasms were cystic and, when sectioned, hemorrhagic
OCH,
and necrotic loci, as well as foci distended with serous fluid, were evident. Thus, it can be seen from these studies that aflatoxin is a remarkably versatile as well as a very potent carcinogen.
following sequence of events: decreased nucleolar RNA synthesis, decreased DNA synthesis, and finally, decreased total nuclear RNA synthesis. Aflatoxin or an active metabolite (19) may bind to discrete positions on the DNA molecule preventing DNA replication and transcription of large RNA molecules.
Inferadion with Nucleic Acid
The strong carcinogenic action of ailatoxins may result from their ability to bind with DNA. Aflatoxin is reported to affect the processes of DNA replication, its transcription into RNA, and subsequent translation of the messenger into protein. Equilibrium dialysis and other physical techniques indicated that aflatoxin B1 binds to calf-thymus DNA in d r o (12, 15). DeRecondo, et al. (14) reported that aflatoxin inhibited DNA replication in regenerating rat liver cells as well as in an in vitro system utilizing calf-thymus DNA as a template. The inhibition of RNA polymerase activity upon exposure of DNA to aflatoxin has been demonstrated by various workers (12, 16, 16). Recently, Friedman and Wogan (17) reported that aflatoxin-DNA interaction inhibited RNA polymerase activity in rat liver and resulted in both decreased nucleolar RNA content and reduced incorporation of labeled precursors into 18s and 28s nuclear RNA. Upon injecting aflatoxin into the partially hepatectomized rats, LaFarge and Frayssinet (18) noted the
Liferoture Cited S A R ~ E A K. N TSHBRIDAN A,. O'KELLI, J.; A N D CARNAGRAN, R. B. A,.
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