Kazuya Ishiwata

Selenosugars are key and urinary metabolites for selenium excretion within the required to low-toxic range

Essential micronutrient selenium is excreted into the urine and/or expired after being transformed to methylated metabolites. Monomethylated selenium is excreted into the urine in response to a supply within the required to low-toxic range, whereas tri- and dimethylated selenium increase with excessive supply at a toxic dose. Here we show that the major urinary selenium metabolite within the required to low-toxic range is a selenosugar. The structure of 1β-methylseleno-N-acetyl-d-galactosamine was deduced from the spectroscopic data and confirmed by chemical synthesis. This metabolite was also detected in the liver, and an additional metabolite increased with inhibition of methylation. The latter metabolite was again a selenosugar conjugated with glutathione instead of a methyl group and was assumed to be a precursor for methylation to the former metabolite. A metabolic pathway for the urinary excretion of selenium, i.e., from the glutathione-S-conjugated selenosugar to the methylated one, was proposed. Urinary monomethylated (selenosugar) and trimethylated selenium can be used as specific indices that increase within the required to low-toxic range and with a distinct toxic dose, respectively.

Identification of a novel selenium metabolite, Se-methyl-N-acetylselenohexosamine, in rat urine by high-performance liquid chromatography--inductively coupled plasma mass spectrometry and--electrospray ionization tandem mass spectrometry.

The major urinary metabolite of selenium (Se) in rats was identified by HPLC-inductively coupled argon plasma mass spectrometry (ICP-MS) and--electrospray tandem mass spectrometry (ESI-MS/MS). As the urine sample was rich in matrices such as sodium chloride and urea, it was partially purified to meet the requirements for ESI-MS. The group of signals corresponding to the Se isotope ratio was detected in both the positive and negative ion modes at m/z 300 ([M+H]+) and 358 ([M+CH3COO]-) for 80Se, respectively. These results suggested that the molecular mass of the Se metabolite was 299 Da for 80Se. The Se metabolite was deduced to contain one methylselenyl group, one acetyl group and at least two hydroxyl groups from the mass spectra of the fragment ions. The spectrum of the Se metabolite was completely identical to that of the synthetic selenosugar, 2-acetamide-1,2-dideoxy-beta-D-glucopyranosyl methylselenide. However, the chromatographic behavior of the Se metabolite was slightly different from that of the synthetic selenosugar. Thus, the major urinary Se metabolite was assigned as a diastereomer of a selenosugar, Se-methyl-N-acetyl-selenohexosamine.

Identification of selenohomolanthionine in selenium-enriched Japanese pungent radish

Sulfur (S) and selenium (Se) are same-group elements that share, in part, a common metabolic pathway. Because a radish variant having pungent flavor (Raphanus sativus L. cv. ‘Yukibijin’) has a unique S metabolic pathway, we expected that it would also produce unique Se compounds through that metabolic pathway. Se metabolites in the radish were assigned by speciation techniques, i.e., HPLC-inductively coupled plasma mass spectrometry (ICP-MS) and HPLC-electrospray ionization tandem mass spectrometry (ESI-MS-MS). 68.5% of total Se in the selenized Japanese pungent radish was water-extractable, and the water extract contained selenate (around 45% of total Se), Se-methylselenocysteine (around 11%), an unknown Se compound tentatively named KD-1 (around 5%), and others (around 1.2%). KD-1 was assigned by ESI-MS-MS as 4,4′-selenobis[2-aminobutanoic acid] or Se-(3-amino-3-carboxypropyl)-homocysteine, commonly known as selenohomolanthionine (SeHLan). This Se compound was first identified in nature. The metabolic pathway of SeHLan was suggested to involve the phosphorylation of homoserine to produce O-phosphohomoserine, and the subsequent reaction of O-phosphohomoserine with selenohomocysteine (SeHCys).

Incorporation of selenium into selenoprotein P and extracellular glutathione peroxidase: HPLC-ICPMS data with enriched selenite.

The metabolic turnover of selenoprotein P (Sel P) and extracellular glutathione peroxidase (eGPx) in plasma was examined by HPLC-ICPMS, with the use of enriched Se, [82Se]selenite. [82Se]selenite was injected intravenously into rats at a dose of 25 micrograms 82Se kg-1 body weight, and the concentrations of labeled 82Se and naturally occurring 77Se in the serum, liver and kidneys were determined in samples obtained 1, 3, 6, 9, 12, 18 and 24 h after the injection. The distributions of both exogenous (labeled) 82Se and endogenous (naturally occurring) 77Se in serum, and supernatant fractions of the liver and kidneys were determined on a gel filtration column by HPLC-ICPMS. This dose was shown not to affect the constitutive levels of cellular GPx (cGPx), eGPx and Sel P. The labeled Se in Sel P increased soon after the injection, peaked at 6-9 h and then decreased, whereas that in eGPx continued increasing after 6 h post-injection and then throughout the remaining observation period in the bloodstream. These observations demonstrated the rapid and efficient incorporation of Se into Sel P in the liver and excretion into the plasma followed by the slow and steady incorporation of Se into eGPx in the kidneys and excretion into the plasma, with a minimal response of cGPx to selenite injection.

Comparison of distribution and metabolism between tellurium and selenium in rats

Tellurium (Te) has shown recent increase in use as a component of optical magnetic disks having phase-change property, such as digital versatile disk-random access memory (DVD-RAM) and DVD-rewritable (DVD-RW). However, the toxicity and metabolic pathway of Te remain unclear despite its being known as a non-essential and harmful metalloid. This study was performed to gain an insight into Te metabolism in the body. The mechanism for the distinction of Te from selenium (Se), an essential metalloid belonging to the same group as Te, was also clarified. Rats were given drinking water containing tellurite and (82)Se-labeled selenite at the same concentration, and the concentrations of these metalloids in organs, body fluid and excreta were determined 2 days later. The results demonstrate that urinary and fecal excretion of Te was, respectively, lower and higher than that of exogenous (labeled) Se, suggesting that Te was less absorbed than Se. The ingested Te was transformed, i.e., methylated in organs and effluxed into bloodstream, and the effluxed Te was highly accumulated in rat red blood cells (RBCs) in the form of dimethylated Te. In contrast, Se was not accumulated in RBCs. Finally, Te was excreted in urine as trimethyltelluronium and might be exhaled as dimethyltelluride. The results suggest that the metabolism of Te was distinct from that of Se in rats.

Speciation and identification of low molecular weight selenium compounds in the liver of sea turtles

It is known that marine mammals and seabirds co-accumulate selenium (Se) and mercury (Hg) in their organs in the insoluble form called mercury selenide. In this study, we found that two sea turtles, hawksbill turtle (Eretmochelys imbricata) and green turtle (Chelonia mydas), accumulated Se but not Hg in their livers. Se speciation by HPLC-ICP-MS demonstrated that the livers contained low molecular weight selenometabolites in addition to selenoproteins. Two of the selenometabolites existed in relatively small amounts and were identified as selenosugar (1β-methylseleno-N-acetyl-D-galactosamine) and trimethylselenonium (TMSe) based on their chromatographic behavior. This suggests that selenosugar and TMSe are Se metabolites common to marine and terrestrial animals. The chromatographic behavior of the major hepatic selenometabolite in sea turtles was unique and did not match that of any authentic Se standards. Further analysis using HPLC-ESI-MS-MS revealed it to be selenoneine (2-selenyl-N,N,N-trimethyl-L-histidine), a metabolite that was recently identified in the blood of bluefin tuna. The results suggest that sea turtles possess specific mechanisms for Se metabolism to result in the sole accumulation of Se.

Identification of urinary tellurium metabolite in rats administered sodium tellurite

Tellurium (Te) is one of the important metalloids used in industry, and is ubiquitously found in the environment. However, the biological and toxicological effects of Te are still unclear despite its being recognized as a hazardous element. In this study, we attempted to identify urinary Te metabolites (UTMs) in rats by HPLC-ICP-MS and electrospray ionization (ESI)-MS. To unambiguously identify UTMs, two different chromatographic mechanisms, i.e., multi-mode gel filtration and cation exchange columns, were employed. The major UTM detected after ingestion of tellurite was trimethyltelluronium, and no urinary sugar metabolites containing Te were detected despite the fact that the major urinary selenometabolite was a selenosugar (methyl-2-acetamido-2-deoxy-1-seleno-β-D-galactopyranoside). Interestingly, the ingestion of tellurite enhanced the excretion of selenometabolites in urine. These results suggest that Te is discretely metabolized from selenium (Se), an essential element belonging to the same group, although it affects the metabolism of Se in rats. Thus, the disturbance of Se metabolism, i.e., the induction of Se deficiency, may be one of the potential toxic effects of Te.

Speciation of selenomethionine metabolites in wheat germ extract

Selenometabolites transformed from selenomethionine (SeMet) in wheat germ extract (WGE) were identified by complementary use of HPLC-ICP-MS and HPLC-ESI-MS/MS. Three selenium (Se)-containing peaks tentatively named WGE1, WGE2, and WGE3 were detected by HPLC-ICP-MS. WGE1 had [M]+ at m/z 212 on HPLC-ESI-MS analysis, and its fragment ions indicated that WGE1 is selenomethionine methylselenonium (MeSeMet). WGE2 and WGE3 exhibited absorption at 254 nm and molecular ions at m/z 433 and 447, respectively. Their fragment ions revealed that WGE2 and WGE3 are Se-adenosylselenohomocysteine (AdoSeHcy) and Se-adenosylselenomethionine (AdoSeMet), respectively. Their structures were coincident with the absorption of WGE2 and WGE3 at 254 nm. In addition, a trace amount of AdoSeMet was suggested to also exist in rabbit reticulocyte lysate, a mammalian in vitro translation system, because the transformation of AdoSeMet from SeMet was completely inhibited by (2S)-2-amino-4,5-epoxypentanoic acid (AEPA), a potent inhibitor of AdoMet synthetase. These results suggest that SeMet and methionine (Met) share a common metabolic pathway, i.e., SeMet is not only incorporated into proteins in place of Met but also metabolized to AdoSeMet in higher eukaryotes and MeSeMet in plants.