PubMed · 7448885
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K Saman, V Holecek, J Komers, V Hradecká. 1980. [Biochemical problems in glaucoma].. https://pubmed.ncbi.nlm.nih.gov/7448885/
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Direct metal ligands to transition metals in metalloproteins exert a profound effect on protein-metal affinity and function. Indirect ligands, i.e., second-shell residues that hydrogen bond to direct metal ligands, typically exert more subtle effects on the chemical properties of the protein-metal complex. However, E117 of human carbonic anhydrase II (CAII), which is part of the E117-119-Zn(2+) triad, is a notable exception: E117-substituted CAIIs exhibit dramatically increased kinetics of zinc complexation, and the E117Q variant exhibits enormously diminished catalytic activity and sulfonamide affinity. The three-dimensional structures of zinc-bound and zinc-free E117Q CAII reveal no discrete structural changes in the active site that are responsible for enhanced zinc equilibration kinetics and decreased activity. Additionally, the structure of the acetazolamide complex is essentially identical to that of the wild-type enzyme despite the 10(4)-fold loss of enzyme-inhibitor affinity. We conclude, therefore, that the functional differences between E117Q and wild-type CAIIs arise from electrostatic and not structural differences in the active site. We propose that the E117Q substitution reverses the polarity of the residue 117-H119 hydrogen bond, thereby stabilizing H119 as a histidinate anion in the E117Q CAII holoenzyme. The additional negative charge in the first coordination sphere of the metal ion increases the pK(a) of the zinc-water ligand, destabilizes the transition state for CO(2) hydration, and facilitates the exchange of a zinc-histidine ligand with an additional water molecule by decreasing the stability of the tetrahedral zinc complex. These novel properties engineered into E117Q CAII facilitate the exploitation of CAII as a rapid and sensitive Zn(2+) biosensor.
Amiloride (0.5 mM) inhibited the rate of entry of Na+ into corneal endothelial cells by more than half ((0.76 +/- 0.10) to (0.21 +/- 0.10) microEq cm(-2)h(-1)). The same concentration of amiloride caused only minimal disturbance to corneal hydration control by the endothelium (range 0-12%). Amiloride (0.5 mM) and acetazolamide (1 mM) reversibly inhibited trans-endothelial short circuit current by about a half. Their combined effect was not additive. Acetazolamide (1 mM) reduced net HCO3- flux across the short-circuited endothelium by about the same amount ((0.50 +/- 0.11) microEq cm(-2)h(-1)) that amiloride (0.5 mM) reduced Na+ entry into the cells ((0.55 +/- 0.14) microEq cm(-2)h(-1)). Low concentrations of amiloride (10 microM) had little effect on the transport characteristics of the endothelium, indicating that Na+ entry into the endothelial cells under physiological conditions is not primarily through Na+ channels. The data are consistent with an Na+/H+ exchanger acting in tandem with carbonic anhydrase through a pathway which could have a regulatory role on endothelial transport via its effect on Na+ re-entry. Residual trans-endothelial HCO3- transport, apparently unaffected by amiloride or acetazolamide inhibition, is calculated to be of sufficient magnitude to maintain corneal hydration.
BACKGROUND: The antitussive activity of inhaled furosemide has been attributed to its blocking effect on the Na(+)-2Cl(-)-K+ cotransporter. It is likely that the antitussive activity of inhaled diuretics is more complex because amiloride, a diuretic that has no effect on the Na(+)-2Cl(-)-K+ cotransporter, also shows a significant effect against cough induced by low-chloride-ion solutions. Apart from pharmacokinetics of inhaled diuretics, this activity could also depend on the inhibition of carbonic anhydrase. OBJECTIVES: We therefore studied the effect of inhaled acetazolamide, a selective inhibitor of carbonic anhydrase activity, on cough induced by the inhalation of different chloride ion solutions in a group of normal subjects. This was compared with the antitussive effect of furosemide. In addition, we attempted to determine whether the effect of acetazolamide is dose-dependent. METHODS: Cough challenge consisted of consecutive inhalations of four solutions having decreasing concentrations of chloride ions (150, 75, 37.5 and 0 mmol/L). Nine normal subjects underwent the cough challenge 5 minutes after the inhalation of saline placebo, acetazolamide (500 mg), and furosemide (30 mg) according to a randomized, double-blind study design. A group of six subjects were challenged according to the same procedure and study design, after the inhalation of saline placebo and of two doses of acetazolamide (250 mg and 500 mg). RESULTS: Inhaled acetazolamide significantly reduced cough response to 37.5 and 0 mmol/L chloride solutions compared with placebo (p less than 0.015 and p less than 0.015, respectively). Furosemide showed a similar protective effect (p less than 0.015 and p less than 0.025, respectively). Acetazolamide provided a significantly better protective effect than furosemide (p less than 0.025 and p less than 0.015, respectively). The antitussive activity of the two doses of acetazolamide was not statistically different. CONCLUSION: These results demonstrate that inhaled acetazolamide, a selective inhibitor of carbonic anhydrase, attenuates cough induced by low-chloride-ion solutions in normal subjects. At the applied doses, its antitussive activity is slightly greater than furosemide. This finding suggests that the inhibition of carbonic anhydrase activity is likely involved in modulating changes caused by absence of a chloride ion in the airway microenvironment of human beings.