Chethaka Kahakachchi

Element selective characterization of stability and reactivity of selenium species in selenized yeast

The concerted application of element specific atomic spectral detection for chromatographic eluent monitoring allows previously unexploited qualitative and quantitative analytical concepts to be developed for the determination of selenium species. Selenium speciation is vital in order to better understand its metabolism and biological significance in clinical chemistry, biology, toxicology, and nutrition. Fluoroacid ion pair HPLC with ICP-MS detection and GC derivatization with atomic emission detection (AED) together aid analysis and elucidation of reaction pathways of selenium compounds in high selenium enriched yeast, as used widely in nutritional and clinical cancer preventative studies. Comparisons between currently produced and archived selenized yeasts show major differences in speciation. The formation of selenomethionine selenoxide and the identification of Se–S bonded S-(selenomethyl)-cysteine in archived nutritional yeast may be important for short and long term stability and nutritional activity studies.

Extraction of arsenic species from spiked soils and standard reference materials

The abilities of various extractants to recover four arsenic species [As(iii), As(v), dimethylarsinic acid (DMA), and monomethylarsonic acid (MMA)] from soils spiked with 20 micro g g(-1) As were investigated. The extractants were water, buffer solutions (citrate and ammonium dihydrogen phosphate), acidic solutions (phosphoric acid and acetic acid), a basic solution (sodium hydroxide) and household chemicals (vinegar and Coca Cola). Gentle shaking at room temperature with each extractant for 24 h gave different recoveries for the different arsenic species. With 0.1 M NaOH solution 46% As(iii), 53% DMA, 100% MMA and 84% As(v) were recovered. A rapid extraction procedure using a sonicator probe has been developed to obtain higher extraction efficiencies. Extracts of arsenic-spiked soil, SRM 2711 Montana soil and SRM 2709 San Joaquin soil were analyzed by HPLC-ICP-MS. In the SRM water extracts, DMA and MMA were identified in addition to inorganic arsenic. The solution detection limits (3s) were 0.1, 0.12, 0.13 and 0.15 ng mL(-1) for As(iii), DMA, MMA and As(v), respectively for HPLC-ICP-MS.

Speciation, Quantification and Stability of Selenomethionine, S-(methylseleno)cysteine and selenomethionine Se-oxide in yeast-based nutritional supplements

The speciation and stability of organoselenium compounds present in the selenized yeast evaluated in National Cancer Institute human intervention trials were examined along with those of related supplements. Total selenium was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES), ICP-mass spectrometry (ICP-MS) and/or electrothermal atomic absorption spectrometry (ET-AAS) after microwave digestion. Enzymatic extraction enabled selenium speciation profiles to be obtained by ion-pair reversed phase high performance liquid chromatography (HPLC) with ICP-MS detection. Extracts were also subjected to chloroformate derivatization followed by Se and S specific GC analysis with atomic emission detection (GC-AED). Currently available and archived supplements show major differences in speciation for selenomethionine, selenomethionine Se-oxide and S-(methylseleno)cysteine with respect to time and conditions of storage. The formation of the latter compound is reported under different synthetic and elevated temperature conditions.

Speciation of gadolinium in gadolinium-based magnetic resonance imaging agents by high performance liquid chromatography inductively coupled plasma optical emission spectrometry

A highly sensitive and robust method for the determination of six gadolinium species by reversed-phase liquid chromatography coupled with inductively coupled plasma optical emission spectrometry (HPLC-ICP-OES) has been developed. The gadolinium(III) complexes, Gd–DTPA2−, Gd–DOTA1−, Gd–HP–DO3A, Gd–DTPA–BMA and Gd–DTPA–BMEA were analyzed by reversed-phased chromatography with a mobile phase of 10 mM ammonium acetate at pH 7.0. The Gd–BOPTA2− gadolinium(III) complex was separated on a C18 reversed-phase column with a mobile phase of 10 mM ammonium acetate and 5% acetonitrile at pH 7.0. The total time for the analysis took less than 15 minutes for each gadolinium(III) complex with isocratic elution at a flow rate of 1.0 mL min−1. The calibration curves for the gadolinium(III) complexes were linear over the concentration range of 0.393 to 78.625 µg mL−1 (2.5 to 500 µM). The solution detection limits of the six gadolinium(III) complexes obtained by HPLC-ICP-OES were between 8 and 35 ng Gd mL−1 (0.05 to 0.2 µM) using an injection volume of 50 µL. The developed HPLC-ICP-OES method was successfully applied for the determination of gadolinium species in magnetic resonance contrast media solutions and spiked human serum solutions. The precision for 3 replicate determinations at the 3.93 µg mL−1 (25 µM) Gd level was less than 5% RSD for the six gadolinium(III) complexes in water and human serum. The recoveries were between 95 and 103% for the gadolinium(III) complexes in human serum solutions. Chromatographic profiles were obtained for Magnevist™, Dotarem™, ProHance™, Omniscan™, Optimark™ and MultiHance™ contrast media solutions.

Insights into the Chemical Biology of Selenium

The long-sought pathway by which selenocysteyl-tRNA[Ser]Sec is synthesized in eukaryotes has been revealed. Seryl-tRNA[Ser]Sec is O-phosphorylated and SecS, a pyridoxal phosphate-dependent protein, catalyzes the reaction of O-phosphoseryl-tRNA[Ser]Sec with monoselenophosphate to give selenocysteyl-tRNA[Ser]Sec . 1 H- 77 Se HMQC-TOCSY NMR spectroscopy has been developed to detect the selenium-containing amino acids present in selenized yeast after protease XIV digestion. An archived selenized yeast sample is found to contain the novel amino acid S-(methylseleno)cysteine in addition to selenomethionine. Arsenite and selenite react with GSH to form (GS) 2 AsSe−. The structure of this compound has been determined by EXAFS, 77 Se NMR and Raman spectroscopic and chromatographic studies. Its formation under biological conditions has been demonstrated.