Luke K. T. Lam

Inhibitors of colon carcinogenesis

Disulfiram (tetraethylthiuram disulfide, Antabuse) and sodium diethyldithio‐carbamate, when added to the diet, inhibit 1,2‐dimethylhydrazine (DMH)‐induced neoplasia of the large bowel in female CF1 mice. Ethylene bis(dithiocarbamato)manganese (Maneb) and bis(ethylxanthogen), two pesticides with structural similarities to disulfiram, produce comparable inhibition of DMH. In other work, disulfiram was found to inhibit the carcinogenic effect of azoxymethane (AOM) on the large bowel. Under comparable conditions the inhibition of AOM was considerably less than that obtained with DMH as the carcinogen. The data suggest that disulfiram inhibits DMH metabolism at more than one oxidative step.

Inhibition of Chemical Carcinogenesis by Phenols, Coumarins, Aromatic Isothiocyanates, Flavones, and Indoles

An increasing number and diversity of compounds have been found to inhibit the neoplastic effects of chemical carcinogens when administered prior to and/ or simultaneously with the carcinogen. These inhibitors include naturally occurring constituents of foods, particularly vegetables and fruit, and also synthetic compounds, some of which are food additives (Wattenberg, 1979a,b). The inhibitors encompass a wide range of chemical structures (Fig. 1). The time relationship between administration of the inhibitors and carcinogens as well as direct studies of mechanisms of inhibition indicate that these inhibitors can act by preventing carcinogenic species from reaching or reacting with critical cellular targets. In essence, they exert a barrier function.

Characterization of bacterial mutagenicity mediated by 13-hydroxy-ent-kaurenoic acid (steviol) and several structurally-related derivatives and evaluation of potential to induce glutathione S-transferase in mice

Stevioside is a sweet-tasting diterpene glycoside that is derived from Stevia rebaudiana (Bertoni) Bertoni (Compositae). It is used commercially in Japan and other parts of the world as a sucrose substitute. Whereas stevioside demonstrates no mutagenic activity in a variety of test systems, the aglycone, steviol (13-hydroxy-ent-kaurenoic acid), is mutagenic toward Salmonella typhimurium strain TM677 in the presence of a metabolic activating system derived from the liver of Aroclor 1254-pretreated rats. The required activating component is localized in the microsomal fraction of rat liver, suggestive of a cytochrome P-450-mediated reaction. Partially purified epoxide hydrolase does not inhibit steviol-induced mutagenicity, indicating that an active metabolite is not an epoxide that serves as a substrate for this enzyme preparation. The 13-hydroxy group of steviol is required for the expression of mutagenicity since ent-kaurenoic acid is nonmutagenic, and acetylation of steviol at this position negates mutagenicity. Similarly, diterpenes bearing a strong structural resemblance to steviol, cafestol and kahweol, were found to demonstrate no mutagenic activity toward Salmonella typhimurium TM677, as were their respective acetates and palmitic acid esters. Conversely, 19-O-beta-D-glucopyranosyl steviol, a potential hydrolysis product of stevioside, is mutagenic and bactericidal in the presence of a metabolic activating system. Additionally, in contrast to the nonmutagenic diterpenes cafestol and kahweol that are effective as inducers of glutathione S-transferase activity, evaluation by administration to mice proved steviol, isosteviol and various steviol glycosides to be inactive in this process. Thus, structural differences among these naturally occurring and semi-synthetic diterpenes appear to impart major differences in biological activity that may relate to human health upon dietary ingestion.

Effects of cyclic 12-, 8-, and 6-carbon compounds on glutathione S-transferase activity

The effects of feeding ICR/Ha mice cyclic 12-, 8-, and 6-carbon compounds on glutathione S-transferase (GST) activity in the liver, intestinal mucosa, and the forestomach were determined. The compounds used for this study were 1,5,9-trans,trans,cis- cyclododecatriene , 1,2-trans-5,6-trans-9,10-cis- cyclododecatriene -1,2-oxide, cyclododecanol , cyclododecene oxide, cyclododecane , 1,5- cyclooctadiene , cyclooctene oxide, cyclohexene, and cyclohexene oxide. The unsaturated cyclic 12-carbon compounds elicited the greatest increase in GST activity. Thus, feeding 1,5,9-trans,trans,cis- cyclododecatriene increased this activity almost 4-fold in the livers and the intestinal mucosa of experimental animals. Cyclic 8-carbon compounds were less effective and feeding the cyclic 6-carbon compounds did not result in any significant increase in GST activity. None of the compounds elicited increased GST activity in the fore-stomach. Previous studies have shown that compounds inducing increased GST activity can protect against chemical carcinogens. It remains to be determined whether the compounds identified in the present investigation as inducers of this enzyme system will have such protective capacities.