Integrated strategies for identification of selenometabolites in animal and plant samples

Selenium (Se) is an essential micronutrient in animals as it is required by certain redox-regulating enzymes, such as glutathione peroxidases and thioredoxin reductase. In plants, Se is not essential and exists as a “bystander”mineral. However, its beneficial effects on plant growth have been reported [1]. Some Se-containing compounds (selenocompounds), such as methylselenocysteine (MeSeCys) and γ-glutamylmethylselenocysteine (GluMeSeCys), have anti-tumor activity, and selenomethionine (SeMet), a naturally occurring selenoamino acid, has cancer-preventing effects. Indeed, SeMet has been used in the selenium and vitamin E cancer prevention trial (SELECT) in the United States, Puerto Rico, and Canada to determine if taking selenium (as SeMet) and/or vitamin E supplements can prevent prostate cancer [2]. These pharmacologically available selenoamino acid derivatives are biosynthesized in selenium-enriched (selenized) plants and yeast. Therefore, novel selenized food materials are being actively developed for use in cancer chemotherapy and chemoprevention. On the other hand, Se is widely used in industry, such as glass and ceramic manufacturing and electronics, and is thus also known as an environmental contaminant. From the viewpoint of environmental chemistry, Se-hyperaccumulating plants are applicable to the phytoremediation of Secontaminated water and soil. As a non-metallic element, Se is utilized in the metabolic pathways of animals and plants to form Se-containing compounds having carbon–Se covalent bond(s) (organic selenometabolites). Therefore, it is necessary to identify selenocompounds to determine the metabolic pathway of Se and understand the beneficial or toxicological effects of these compounds. However, as Se is a micronutrient, selenometabolites exist at extremely low concentrations in animals. The difficulty of detecting Se in each selenometabolite after separation on the basis of chemical properties (chemical speciation) has been overcome with the emergence of inductively coupled plasma-mass spectrometry (ICP–MS) as the most sensitive and robust Se detector available to date. Easily hyphenated with HPLC, HPLC– ICP–MS is the technique of choice for speciation of Se in biological samples. However, there remains a critical disadvantage in the identification of selenometabolites by HPLC–ICP–MS. Although ICP–MS is sensitive to target elements and is robust to matrices, it provides little molecular information about selenometabolites. Thus, identification by HPLC– ICP–MS is limited to the situation where certified or authentic Se species are available. As an alternative to HPLC–ICP–MS, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)–tandem mass spectrometry (MS–MS) are used to identify unknown selenometabolites. The fact that Se has a characteristic cluster of signals that reflect its isotopic pattern (Se, 0.89 %; Se, 9.36 %; Se, 7.63 %; Se, 23.8 %; Se, 49.6 %, and Se, 8.73 %) has facilitated the scanning of Secontaining species by MS. Because of this, Se is the preferred target of ESI–MS–MS analyses. ESI is a softer technique than ICP, and can provide molecular information. In addition, MS–MS enables structure elucidation. However, ESI–MS–MS has a number of weak points compared with ICP–MS. First, the detection limit of ESI–MS–MS for selenocompounds is inferior to that of ICP–MS. Second, ESI is severely affected by the sample matrix. To counter this Anal Bioanal Chem (2008) 390:1685–1689 DOI 10.1007/s00216-007-1796-8