Zinc chloride inhibits Eikenella corrodens lysine decarboxylase production in vitro

Objectives: In healthy adults and patients after periodontal therapy, zinc-containing dentifrices prevent gingivitis better than dentifrices without zinc (PMID:10371876). During experimental gingivitis, Eikenella corrodens lysine decarboxylase (LdcE) increases in biofilms, depletes lysine from gingival crevicular fluid (GCF), and compromises dentally-attached (DAT) cell protection of the gingiva from bacterial products (PMID:27620654). Gingivitis development is retarded in LdcE immunized dogs whose antibodies inhibit LdcE activity (PMID:22975025). This study examined whether zinc-mediated protection from gingivitis results from inhibiting LdcE directly or its production by Eikenella corrodens.
Methods: In cell extracts of E. corrodens after growth with or without ZnCl₂ for 48 h. LdcE activity was measured as cadaverine produced in 20 min from 0 – 15 mM lysine (Levine et al., 2001, PMID:11312612). Extracts of cells grown without ZnCl₂ were assayed in 0.0 – 77.0 mM ZnCl₂. Cultures grown in 0.0 – 1.0 mM ZnCl₂ were assayed for mean cell yield, protein content, and LdcE activity per mg protein. Activity was regressed against ZnCl₂ concentration (Xlstat; Addinsoft Corp, NY).
Results: LdcE activity fell to 25% of controls as assay-ZnCl₂ concentration increased from zero to 0.77 mM (R²=0.93; p<0.031). ZnCl₂ in the growth medium had no significant effect on cell yield or extracted protein content, but LdcE activity/mg protein fell to 15% as ZnCl₂ increased from zero to 1.0 mM (R²=0.90, p<0.001). ZnCl₂ inhibits LdcE production indirectly by preventing enzyme expression at the cell surface.
Conclusions: The unique N-terminal amino acid sequence of LdcE contains a hydrophobic sequence ILLGCG that may explain the loose adherence of LdcE to the E. corrodens surface. The positively charged zinc ion could link the cysteine (C) residue electrostatically to the E. corrodens cell surface (PMID:24970223), preventing initial dimer and subsequent double-ring pentamer formation required for enzyme activity (PMID:27080013). This process could explain the better control of gingivitis by zinc dentifrices.