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Applied and Environmental Microbiology, December 1998, p. 4670-4675, Vol. 64, No. 12
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Use of 13C Nuclear Magnetic Resonance and Gas Chromatography To Examine Methionine Catabolism by Lactococci

Song Gao,1 Ed S. Mooberry,2 and James L. Steele1,*

Department of Food Science,1 and Department of Biochemistry,2 University of Wisconsin-Madison, Madison, Wisconsin 53706

Received 16 June 1998/Accepted 18 September 1998

Formation of methanethiol from methionine is widely believed to play a significant role in development of cheddar cheese flavor. However, the catabolism of methionine by cheese-related microorganisms has not been well characterized. Two independent methionine catabolic pathways are believed to be present in lactococci, one initiated by a lyase and the other initiated by an aminotransferase. To differentiate between these two pathways and to determine the possible distribution between the pathways, 13C nuclear magnetic resonance (NMR) performed with uniformly enriched [13C]methionine was utilized. The catabolism of methionine by whole cells and cell extracts of five strains of Lactococcus lactis was examined. Only the aminotransferase-initiated pathway was observed. The intermediate and major end products were determined to be 4-methylthio-2-oxobutyric acid and 2-hydroxyl-4-methylthiobutyric acid, respectively. Production of methanethiol was not observed in any of the 13C NMR studies. Gas chromatography was utilized to determine if the products of methionine catabolism in the aminotransferase pathway were precursors of methanethiol. The results suggest that the direct precursor of methanethiol is 4-methylthiol-2-oxobutyric acid. These results support the conclusion that an aminotransferase initiates the catabolism of methionine to methanethiol in lactococci.

* Corresponding author. Mailing address: Department of Food Science, University of Wisconsin-Madison, 1605 Linden Drive, Madison, WI 53706. Phone: (608) 262-5960. Fax: (608) 262-6872. E-mail: jlsteele{at}facstaff.wisc.edu.

Applied and Environmental Microbiology, December 1998, p. 4670-4675, Vol. 64, No. 12
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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