Chemical speciation influences comparative activity of selenium-enriched garlic and yeast in mammary cancer prevention

A recent human intervention trial showed that daily supplementation with se lenized yeast (Se-yeast) led to a decrease in the overall cancer morbidity and mortality by nearly 50%; past research has also demonstrated that selen ized garlic (Se-garlic) is very effective in mammary cancer chemoprevention in the rat model. The goal of this study was to compare certain biological activities of Se-garlic and Se-yeast and to elucidate the differences base d on the chemical forms of selenium found in these two natural products. Ch aracterization of organic selenium compounds in yeast (1922 mu g/g Se) and garlic (296 mu g/g Se) was carried out by high-performance liquid chromatog raphy with inductively coupled plasma mass spectrometry or with electrospra y mass spectrometry. Analytical speciation studies showed that the bulk of the selenium in Se-garlic and Se-yeast is in the form of gamma-glutamyl-Se- methylselenocysteine (73%) and selenomethionine (85%), respectively. The ab ove methodology has the sensitivity and capability to account for >90% of t otal selenium. In the rat feeding studies, supplementation of Se-garlic in the diet at different levels consistently caused a lower total tissue selen ium accumulation when compared to Se-yeast. On the other hand, Se-garlic wa s significantly more effective in suppressing the development of premaligna nt lesions and the formation of adenocarcinomas in the mammary gland of car cinogen-treated rats. Given the present finding on the identity of selenome thionine and gamma-glutamyl-Se-methylselenocysteine as the major form of se lenium in Se-yeast and Se-garlic, respectively, the metabolism of these two compounds is discussed in an attempt to elucidate how their disposition in tissues might account for the differences in cancer chemopreventive activi ty.