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

Background: We previously demonstrated that the 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 . overexpressing the . 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 . SerA by l-serine, we purified and characterized the PGDH from and compared it with the previously characterized PGDHs from , 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 PGDH was at pH 7.5 with 50-100 mM Tris-HCl, 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 and , but not the PGDH from human colon. The median inhibitory concentration (IC) of l-serine was 630 μM for and 250 μM for , while IC of d-serine was much higher than that of l-serine; 76 mM in PGDH and 45 mM in PGDH.

Conclusions: These results suggest that l-serine significantly repressed 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 through inhibition of the function of the gene product.