Validating the inhibitory effects of d- and l-serine on the enzyme activity of d-3-phosphoglycerate dehydrogenases that are purified from Pseudomonas aeruginosa, Escherichia coli and human colon

BACKGROUND: We previously demonstrated that the serA gene is associated with bacterial pathogenicity, including bacterial penetration through the Caco-2 cell monolayers, bacterial motility, bacterial adherence, and fly mortality. l-Serine is known to inhibit the d-3-phosphoglycerate dehydrogenase (PGDH) activity of the SerA protein, and it significantly reduced the bacterial pathogenicity as described above. We also demonstrated that in a PGDH assay using crude extracts isolated from overnight cultures of E. coli overexpressing the P. aeruginosa serA gene, l-serine inhibited the PGDH activity of the SerA protein. The basal PGDH activity of the negative control strain was high, presumably due to contamination of unknown proteins in the crude extracts. Therefore, to further confirm the direct inhibition of PGDH activity of P. aeruginosa SerA by l-serine, we purified and characterized the PGDH from P. aeruginosa and compared it with the previously characterized PGDHs from E. coli, and the human colon as controls. RESULTS: Optimum pH and ionic strength of the purified PGDHs were different depending on the three species; optimal activity of P. aeruginosa PGDH was at pH 7.5 with 50–100 mM Tris–HCl, E. coli PGDH was at pH 8.5 with 100–200 mM Tris–HCl, and human PGDH was at pH 9.0 with 100–200 mM Tris–HCl. The addition of l-serine reduced the activity of PGDH from P. aeruginosa and E. coli, but not the PGDH from human colon. The median inhibitory concentration (IC(50)) of l-serine was 630 μM for P. aeruginosa and 250 μM for E. coli, while IC(50) of d-serine was much higher than that of l-serine; 76 mM in P. aeruginosa PGDH and 45 mM in E. coli PGDH. CONCLUSIONS: These results suggest that l-serine significantly repressed P. aeruginosa pathogenicity through direct inhibition of the PGDH activity, but was not able to inhibit the human PGDH activity. Oral administration of l-serine to compromised hosts might interfere with bacterial translocation and prevent gut-derived sepsis caused by P. aeruginosa through inhibition of the function of the serA gene product.