Metabolism of Selenium and its Interaction with Mercury: Mechanisms by a Speciation Study

Abstract

Metabolic, nutritional and toxicological aspects of selenium (Se) were studied using a hyphenated technique. Se in biological samples was separated by HPLC and Se in the eluate was detected in-line by mass spectrometry with ionization by inductively coupled argon plasma (HPLC-ICP MS). The distributions of Se in the soluble fractions of various organs and body fluids were determined after ingestion or injection of naturally occurring Se or Se enriched with a stable isotope in the form of selenite, selenate or selenomethionine. Metabolic pathways specific to each Se species were discussed based on the results of speciation of each Se metabolite.

Selenite in the bloodstream was taken up by red blood cells and reduced to selenide, and then the reduced form of Se (selenide) was transported to the plasma, where it was bound selectively to albumin and was then transported to the liver. On the other hand, intravenous selenate was taken up directly by the liver. Excess Se derived from any nutritional Se species is mainly excreted in the urine after being methylated in the liver.

The mechanisms underlying the interaction between Se and mercuric ions in the bloodstream were explained by the formation of a ternary complex, {(HgSe)_n}_m-selenoprotein P (n is the number of (HgSe) complexes and m is the number of the binding sites for the (HgSe)_n complex on selenoprotein P). The complex between Hg and Se in the bloodstream was thus explained by the interaction between their specific chemical species in each metabolic pathway.

The sensitivity of the HPLC-ICP MS method was enhanced with the use of enriched stable isotopes by the simultaneous detection and speciation of both endogenous and exogenous Se.

Key Words:

• selenium
• metabolism of selenium
• speciation of selenium
• mercury
• interaction between selenium and metals
• selenoprotein P