Identification and Distribution of Selenium-Containing Glucosinolate Analogues in Tissues of Three Brassicaceae Species

The Brassicaceae are known for their capacity to produce and accumulate the sulfur (S)-rich glucosinolates, which have appreciable human health benefits. Selenium (Se) is chemically very similar to S and many plant enzymes appear unable to distinguish between these two elements. Thus Se may be metabolized through many of the S uptake and assimilation pathways. We were interested in the effect of Se-fertilization on the production of glucosinolate compounds in Brassicaceae. We fertilized broccoli, cauliflower and forage rape with Na2SeO4 and examined the glucosinolates produced in four tissues (tap root, stem, leaf and floret) of these plants using liquid chromatography-mass spectrometry (LC-MS). Several Se-containing glucosinolates were identified and measured. In each case, the Se atom substituted for the S atom normally found in the methylthioalkyl moiety of the glucosinolate and was presumably donated by selenomethionine. The highest concentration of these new Se-containing glucosinolates was in broccoli florets and forage rape roots. In forage rape leaves the majority of the methylthio class of glucosinolates was selenized. Se fertilization also appeared to increase the concentration of the non-selenized methylthioglucosinolates in the shoot tissues of these Brassica species. Our results show that Se and S metabolism of Brassica tissues vary in their responses to Se fertilization and that several enzymes of the glucosinolate biosynthetic and metabolism pathways can use Se in place of S to generate Se-containing glucosinolates.