Urinary trimethylselenonium excretion by the rat: effect of level and source of selenium-75

The purpose of this study was to explore in rats the urinary metabolites of selenium (Se), by using [75Se]selenomethionine, [75Se]selenocystine, and [75Se]selenite, and to assess the effects of low and high levels of Se intake on trimethylselenonium ion (TMSe) excretion in urine. Male adult rats were adapted for 6 weeks to a commercial rat laboratory stock diet (0.25 ppm Se). They were then starved for 24 hours and given an oral dose of either low (16 micrograms Se/kg body weight) or high (1500 micrograms Se/kg body weight) Se as the test Se compounds. Appearance of radioactivity in TMSe and non-TMSe Se metabolites in urine was monitored for 48 hours. About 40% of the 75Se dose was excreted in urine. TMSe was the major urinary Se metabolite (57-69% of urinary 75Se and 16-25% of oral 75Se dose) at high, and a minor urinary Se metabolite (10% of urinary 75Se and 3-4% of oral 75Se dose) at low dose levels of Se and for all three Se test compounds. At least 80% of urinary 75Se and 26-42% of the orally administered 75Se were excreted as non-TMSe Se metabolites in urine under the latter condition. It is hypothesized that at a requirement intake of Se either a trace or no TMSe is excreted in urine, and it becomes a major excretory metabolite of Se when the dietary trace mineral intake exceeds a requirement level, probably serving as a means of detoxification.