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).