5.3 Salivary Buffering Systems and Protective Macromolecules

3. Salivary Buffering Systems and Protective Macromolecules

Saliva protects teeth against chemical demineralization and mechanical wear while executing broad-spectrum antimicrobial surveillance.

Salivary Buffering Systems

Salivary buffering counteracts acidic challenges from dietary intake and microbial carbohydrate fermentation, preventing the oral pH from falling below the critical pH of enamel (pH 5.5) and dentine (pH 6.2–6.7).

1. The Carbonic Acid-Bicarbonate Buffer System (H₂CO₃ ⇌ H⁺ + HCO₃⁻):
   • The single most important buffer in stimulated saliva.
   • Operates with an effective pKa of approximately 6.1.
   • When acid (H⁺) enters saliva, it reacts with bicarbonate to form carbonic acid, which is rapidly converted into dissolved carbon dioxide and water: H⁺ + HCO₃⁻ ⇌ H₂CO₃ → (Carbonic Anhydrase VI) → H₂O + CO₂↑
   • The reaction is actively accelerated in the oral cavity by salivary carbonic anhydrase VI (gustin), a zinc-metalloenzyme secreted exclusively by serous acinar cells. As stimulated flow increases, the 10-fold rise in HCO₃⁻ concentration massively expands buffer capacity.
2. The Inorganic Phosphate Buffer System (H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻):
   • The dominant buffer in unstimulated (resting) saliva.
   • Operates with a pKa of 6.8, which precisely matches the physiological resting pH of the oral cavity (~6.7–7.0).
   • Monohydrogen phosphate (HPO₄²⁻) accepts an excess hydrogen ion to form dihydrogen phosphate (H₂PO₄⁻). However, because its concentration remains static (~2–4 mmol/L) and does not increase upon stimulation, it is rapidly overwhelmed by heavy acid challenges.
3. Protein and Peptide Buffers:
   • Salivary proteins contain ionizable histidine, glutamate, and lysine side-chains that contribute minor amphoteric buffering.
   • Sialin: A salivary basic tetrapeptide (Gly-Gly-Lys-Arg) that is rapidly metabolized by oral bacteria into alkaline basic products.
   • Salivary Urea: Normal concentrations (~3–10 mmol/L) are hydrolysed by bacterial urease (Streptococcus salivarius) into carbon dioxide and ammonia (NH₃): (NH₂)₂CO + H₂O → (Urease) → 2 NH₃ + CO₂ Ammonia immediately consumes free protons (NH₃ + H⁺ → NH₄⁺), generating an alkaline rebound that drives the classic recovery phase of the Stephan curve.

Protective Macromolecular Components

Saliva contains an array of specialized proteins delivering non-immune and immune protection:

• Mucins (MG1 and MG2):
  • High Molecular Weight Mucin 1 (MG1 / MUC5B): Oligomeric, heavily O-glycosylated glycoprotein (>1,000 kDa). Hydrophilic carbohydrate chains trap water, providing a lubricious, viscoelastic mucosal barrier resistant to proteolytic degradation, shear forces, and thermal trauma. MG1 binds tenaciously to the tooth surface as an essential structural constituent of the acquired enamel pellicle.
  • Low Molecular Weight Mucin 2 (MG2 / MUC7): Monomeric glycoprotein (~150–200 kDa). Lacks gel-forming properties; functions primarily in the fluid phase by binding, aggregating, and clumping microbial pathogens (Streptococcus mutans, Candida albicans) to accelerate physical swallowing clearance.
• Salivary Alpha-Amylase (Ptyalin):
  • An endo-enzyme (1,4-alpha-D-glucan glucanohydrolase) requiring calcium and chloride cofactors.
  • Cleaves internal α-(1→4) glycosidic bonds of starch and glycogen into maltose, maltotriose, and α-limit dextrins. Inactivated in the stomach when gastric pH drops below 3.0. Amylase also binds to Streptococcus viridans strains, integrating them into the pellicle.
• Lysozyme (Muramidase):
  • Cationic antimicrobial enzyme that hydrolyses the β-(1→4) glycosidic bond between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in bacterial peptidoglycan cell walls.
  • Highly lethal against Gram-positive bacteria; also activates endogenous bacterial autolysins and disrupts negatively charged microbial membranes via its cationic charge.
• Lactoferrin:
  • Iron-binding glycoprotein with extremely high affinity for free ferric ions (Fe³⁺).
  • Bacteriostatic mechanism: Sequesters free iron, starving obligate iron-requiring pathogens (e.g., Streptococcus mutans, Porphyromonas gingivalis) essential for their metabolic cytochromes and growth.
  • Bactericidal mechanism: Direct interaction of its apoprotein domain with bacterial cell surfaces, disrupting outer membrane permeability.
• Histatins:
  • A family of small, histidine-rich, cationic peptides secreted exclusively by parotid and submandibular serous cells.
  • Histatin 5 exhibits potent, specialized antifungal activity against Candida albicans. It enters fungal cells via the Dur3 transporter, accumulates in the mitochondria, induces the generation of reactive oxygen species (ROS), causes non-lytic potassium and ATP efflux, and terminates cell cycle progression. Histatins also promote oral epithelial wound closure by stimulating cell spreading.
• Secretory Immunoglobulin A (sIgA):
  • The principal antibody isotype in human saliva, synthesized by mucosal plasma cells as a dimer linked by a J-chain.
  • During transepithelial transport across salivary acinar/ductal cells via the polymeric immunoglobulin receptor (pIgR), it cleaves and retains the secretory component.
  • The Secretory Component shields sIgA from degradation by bacterial proteases present in the septic oral cavity.
  • Protective Function: Mediates immune exclusion by binding to microbial surface adhesins, agglutinating pathogens, neutralizing toxins and viruses, and preventing bacterial adhesion to enamel and mucosal surfaces—all without activating the classical complement cascade, thereby preventing destructive host tissue inflammation.

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