a 1,4-Phenylene bis (methylene) selenocyanate. Source: Data from References 160-162.
Selenobetaine and SeMCYS are good precursors for generating monometh-ylated selenium.57 160 Selenobetaine tends to lose a methyl group before scission of the Se-methylene carbon bond to form methylselenol. SeMCYS is converted to methylselenol directly when cleaved by beta-lyase, and unlike Semet it cannot be incorporated nonspecifically into proteins. That these selenocompounds can be converted directly to methylselenol is presumably the reason they are more efficacious than other forms of selenium. Dimethylselenoxide and selenobetaine methyl ester are converted to dimethylselenide but are less effective for reduction of tumors.57 Trimethylselenonium is essentially not effective in tumor reduction. Thus, there is a negative correlation between the effectiveness of these seleno-compounds and the degree of methylation.
Even though Semet is effective against mammary tumors, one disadvantage as noted above is that it can be incorporated directly into general proteins instead of converted to compounds that most effectively reduce tumors.57 When this occurs, its efficacy for tumor reduction is reduced. For example, when a low-methionine diet is fed, there is significant reduction in the protective effect of Semet even though the tissue selenium was actually higher in animals as compared to those given an adequate amount of methionine.163 When methionine is limiting, a greater percentage of Semet is incorporated nonspecifically into body proteins in place of methionine because the methionine-tRNA cannot distinguish between methionine and Semet. Feeding diets with Semet to animals as the main selenium source will result in greater tissue accumulation of selenium than other forms of selenium.164 165 It is not known whether this stored selenium can serve as a reserved pool of this element, but the evidence indicates that it is metabol-ically active.166
With the knowledge of the effects of these selenocompounds as anti-carcinogenic agents, it was of interest to investigate the most appropriate methods for delivery to the general population. One obvious approach was to investigate additional methods for expeditious ways to deliver these protective agents through the food system. This appears to be a logical approach because it is estimated that an intake of five servings of fruits and vegetables per day would reduce cancer by as much as 20%,167 and the enrichment with selenium should make them more effective. One strategy in this direction was the investigation of enriching garlic with selenium.168 The addition of selenium-enriched garlic to yield 3 |g selenium/g diet significantly reduced the mammary tumor incidence in rats from 83 to 33%. Similar to garlic, selenium-enriched broccoli also reduced mammary tumors from 90 to 37%.155
Selenium-enriched garlic was shown to be twice as effective as selenium-enriched yeast in the reduction of mammary tumors.153 Chemical analysis of selenium in these two products indicated that Semet was the predominant form of selenium in enriched yeast, whereas SeMCYS (as the glutamyl derivative) was the predominant form of selenium in enriched garlic.153 The glutamyl derivative is considered a carrier of SeMCYS and both of these compounds were shown to be equally effective in the reduction of mammary tumors.169 These results are consistent with those in Table 10.1, where SeMCYS was more effective than Semet for reduction of mammary tumors. The chemical composition of seleno-compounds in these two sources of selenium is apparently responsible for this difference in efficacy. However, it is not known whether doubling the amount of selenium as selenium-enriched yeast will be as effective as enriched garlic. Neither is it known whether the combination of enriched yeast and enriched garlic would be more effective than either alone.
Even though Semet was shown to be the major selenocompound in enriched yeast,153 this has not always been the observation. As low as 30% of the selenium as Semet was found in enriched yeast that had been stored for several years.57 The relative amounts of selenocompounds in enriched yeast from several sources varied markedly, ranging from 27 to 60% for Semet.169a In one source, Semet (together with three unidentified selenium compounds) was predominant in the sample hydrolysates. One batch of yeast that had been stored for 10 years contained only 27% of the selenium as Semet. Thus, it is concluded that the speciation of selenium changes with storage, even when kept at cool temperatures. Loss of selenium has been noted in selenium-enriched broccoli powder even though it was stored at -10°C (P.D. Whanger, unpublished data). The total selenium decreased by 60% in this enriched broccoli powder over a period of 4 years at -10°C, and thus the forms of selenium are likely to have changed. It is suggested that this also happens with enriched yeast stored for extended periods of time even at very low temperatures.
Using another model, selenium enriched broccoli florets,155 170 171 as well as enriched broccoli sprouts,155 significantly reduced colon tumors in rats. This is intriguing because colon cancer is the third most common newly diagnosed cancer in the U.S., resulting in about 55,000 deaths per year due to this type of cancer,152 which is the second leading cause of cancer-related deaths in men in the U.S.84
Selenium-enriched broccoli was more effective than selenite, selenate, or Semet in the reduction of induced colon carcinogenesis.171173 In contrast, selenite, selenate, and Semet were more effective for induction of GPX activity than selenium-enriched broccoli.171 This indicates that the plant converts the selenium to more effective forms for reduction of these tumors and these results emphasize the need to study the effects of selenium in food forms. Similar to chemically induced colon tumors, there were significantly fewer intestinal tumors when mice with a genetic defect for development of intestinal tumors were fed selenium-enriched broccoli.174 These results along with previous data indicate that selenium-enriched broccoli is effective against both chemically and genetically induced intestinal tumors. Data from work with another strain of mice that develop spontaneous intestinal tumors is consistent with these results where selenium deficiency resulted in activation of genes involved in DNA damage.175
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