Carbonic anhydrase isoenzyme VI: distribution, catalytic properties and biological significance

Abstract Secretory carbonic anhydrase isoenzyme VI (CA VI) catalyses the reversible hydration of carbon dioxide (CO2 + H2O ↔ HCO3- + H+). Low concentrations of salivary CA VI are associated with high decayed, missing or filled teeth (DMFT) index scores and a high incidence of acid injury in the upper gastrointestinal tract plus lowered taste and smell perception. Two mechanisms of action for CA VI have been proposed: acid neutralisation and growth factor function. In the present study the distribution and catalytic properties of CA VI have been examined in order to further clarify its mechanisms of action and biological significance. CA VI was found to be present and secreted by the alveolar epithelium of the mammary gland, serous acinar cells of lingual von Ebner’s glands, serous demilune cells of posterior lingual mucous glands and serous cells of submucosal tracheobronchial glands. CA VI was also found in the serous cells in the tracheobronchial mucosal epithelium, taste pore, taste bud, base of the tracheobronchial cilia, bronchiolar Clara cells and enamel pellicle. An immunofluorometric assay showed that the mean concentration of CA VI in colostral milk was eight times higher than that in mature milk (35 mg/l vs. 4.5 mg/l). Stopped-flow spectroscopy measurements revealed that the dehydration activity of CA VI is moderate (maximum kcat = 3.0 × 105 · s-1). The finding that CA VI is a potent catalyst of acid neutralisation emphasizes the possible role of the pellicle bound CA VI in local neutralisation of the acidic metabolic products of dental biofilm. The function of CA VI in von Ebner’s glands’ saliva is likely taste stimuli modification via CA activity although other functions may exist. Its role in milk or respiratory tract mucus remains open, however, as these secretions do not have significant acid predispositions that would need enzymatic catalysis for removal