This study examined methionine degradation in Pseudomonas fluorescens UKI and a mutant capable of using methionine as sole carbon and nitrogen source. Using radioactively labeled methionine, researchers traced the metabolic pathway and found that 95% of consumed methionine was recovered as 2-oxomethionine, 2-oxobutyric acid, and carbon dioxide. Both strains demonstrated demethiolating activity (methioninase) when grown with methionine, releasing methanethiol. The critical finding was that growth on methionine depended on decarboxylation of 2-oxobutyric acid, as the wild-type strain could not utilize this intermediate. When additional carbon sources like glucose were present, decarboxylation activity decreased significantly. Chromatographic analysis found no evidence for the cystathionine pathway operating in these bacteria, unlike in mammals and fungi. The demethiolating enzyme was relatively constitutive but showed highest activity in methionine-containing media. These results indicate P. fluorescens degrades methionine through a transamination/demethiolation pathway distinct from the well-characterized cystathionine pathway used by eukaryotes.

Key findings

• Methionine degradation in P. fluorescens proceeds via demethiolation to release methanethiol, followed by decarboxylation of 2-oxobutyric acid rather than via the cystathionine pathway
• The ability to grow on methionine as sole carbon source depends specifically on the capacity to decarboxylate 2-oxobutyric acid; wild-type bacteria lack this enzyme
• Demethiolating activity is present in both parent and mutant strains but is highest when methionine is the primary carbon source
• Additional carbon sources suppress 2-oxobutyric acid decarboxylation while maintaining demethiolating activity, indicating differential regulation of pathway enzymes
• No intermediates of the cystathionine pathway were detected despite sensitive chromatographic analysis, ruling out trans-sulfuration as the catabolic route