Submitted to: Phytochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 2, 2007
Publication Date: April 5, 2007
Citation: Li, L., Lyi, S.M., Zhou, X., Kochian, L.V. 2007. Biochemical and molecular characterization of the homocysteine s-methyltransferase from broccoli (brassica oleracea var. italica). Phytochemistry. 68:1112-1119. Interpretive Summary: Broccoli is known for its anti-cancer properties because it contains high levels of certain sulfur compounds. In addition, when broccoli is grown in selenium-containing soil, it accumulates high levels of Selenium (Se) compounds, such as Se-methylselenocysteine, that show superior anticarcinogen properties. To gain a better understanding of sulfur and selenium metabolism in broccoli, we isolated two genes that encode homocysteine S-methyltransferase from broccoli and functionally characterized them. While one gene product exhibits only homocysteine S-methyltransferase activity, the other shows both homocysteine S-methyltransferase and selenocysteine Se-methyltransferase activity, indicating the gene has evolved to include an additional function in selenium metabolism. The cloning and characterization of these two genes provide information for the further improvement of the health-promoting properties of broccoli and other crops via engineering of metabolism of the health beneficial compounds.
Technical Abstract: Plants are known for their unique ability to synthesize methionine from S-methylmethionine (SMM) and homocysteine using the enzyme SMM: homocysteine S-methyltransferase (HMT) in the SMM cycle. Two cDNAs encoding HMTs were cloned from broccoli and functionally expressed in E. coli. One of them was designated as BoHMT1 and the other was the BoSMT gene that we previously characterized as a selenocysteine methyltransferase (Lyi et al., 2005). BoHMT1 exhibited high substrate specificity for homocysteine and had over 2-fold higher specific enzyme activity than BoSMT. Both of BoHMT and the HMT activity of BoSMT were methionine-sensitive and their genes represent single copy sequences in the broccoli genome. While the transcript of BoHMT1 was readily detected in most tissues of the plants except old leaves, BoSMT transcript level was extremely low or undetectable when broccoli plant was not exposed to selenium. Sulfate deficiency resulted in a reduced level of BoHMT1 transcript and the metal ion cadmium increased its abundance. In contrast to BoSMT whose expression was dramatically upregulated by treating the plant with selenium, selenium exposure did not affect BoHMT1 expression in broccoli. Also, the expression of BoHMT1 was not significantly affected by altered plant levels of methionine or SMM. The differences in the substrate specificity, enzyme activity, and expression responses to different factors between BoHMT1 and BoSMT suggest the unique roles of these two proteins in sulfur and selenium metabolism in broccoli.