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Metabolism and Enzymology

Purification and Characterization of Methionine:Glyoxylate Aminotransferase from Brassica carinata and Brassica napus¹

Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1

The first step in the biosynthesis of allylglucosinolate from methionine in Brassica is thought to be the transamination of methionine to 2-keto-4-methylthiobutyrate. By using Q-Sepharose and Red Agarose, followed by high resolution anion exchange chromatography and chromatofocussing, a methionine:glyoxylate aminotransferase (MGAT) was purified to homogeneity from leaves of Brassica carinata var R-4218, and approximately 5000-fold from leaves of Brassica napus var Topas. The final purification was accomplished using nondenaturing polyacrylamide gel electrophoresis. The enzyme has a pl of 4.3, a native molecular mass of 230 to 290 kilodaltons, and a subunit molecular mass of approximately 50 kilodaltons. Four isozymes of the enzyme were identified in the six species of Brassica commonly cultivated. Nonglucosinolate producing species had only low levels of MGAT or an MGAT isozyme which was distinctly different from that in Brassica.

³ Present address: Department of Biochemistry, McMaster University, Hamilton, Ontario, Canada L8N 3Z5.

⁴ Present address: Department of Plant Science, University of British Columbia, Vancouver, British Columbia, Canada V6T 2A2.

¹ Supported by the Natural Sciences and Engineering Research Council of Canada.

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