Carbonic anhydrase, absorptivity

Food containing salicylate (curry powder, paprika, licorice, prunes, raisins, and tea) may increase the risk of adverse reactions. Coadministration of the salicylates with activated charcoal decreases the absorption of the salicylates. Antacids may decrease the effects of the salicylates. Coadministration with the carbonic anhydrase inhibitors increases the risk of salicylism. Aspirin may increase the risk of bleeding during... 

As examples. Table 8 records some observations on d—d and charge transfer absorption bands in metal/protein systems. The examination of the spectrum of cobalt carbonic anhydrase (d—d) and of iron conalbumin (charge-transfer) permitted a prediction of the ligands from the protein to the metal. The predictions have now been substantiated by other methods. 

In turn, the concentration of C02 in the atmosphere depends on the mass of the biosphere and its rate of decay after death, and on the carbonic-anhydrase concentrations in the sea surface. In future predictions of the rate of increase of C02 partial pressure in the atmosphere due to burning fossil fuels, it will be important to include the interaction of the atmospheric C02 with the bio-organic reservoir and the catalyzation of its absorption into the sea by means of the action of carbonic-anhydrase dissolved in sea water, considerations which have not been taken into account in past computations. 

Carbonic anhydrase influences the tubular reabsorption of sodium in proximal tubule where biocarbonate absorption occurs and in the distal tubule where sodium is exchanged for potassium or hydrogen ion and bicarbonate is formed as the accompanying anion. The hydration of carbon dioxide takes place under the influence of enzyme carbonic anhydrase which forms carbonic acid which dissociates and breaks into hydrogen and carbonate ions. 

Acetazolamide inhibits the enzyme carbonic anhydrase, and interferes with the ability of the renal tubules to produce and secrete hydrogen ions. And, the diuretic action is due to the decreased sodium biocarbonate absorption in proximal tubules and diminished hydrogensodium exchange in the distal tubules. 

The benzothiadiazides and the newer thiazide-like diuretics antagonise the absorption of Na-i- and Cl- in the DCT by inhibition of the Na+/CI- symport mechanism in the luminal membrane (Figure 12.4). They do not have any effect on the LoH. Some of the thiazides are thought to have a minor action on the PCT but this is minimal and is not due to inhibition of the Na-i-/CI- symporter. It may be due to carbonic anhydrase inhibition which is a feature of sulphonamide-derived drugs. 

Loop diuretics selectively inhibit NaCI reabsorption in the TAL. Because of the large NaCI absorptive capacity of this segment and the fact that the diuretic action of these drugs is not limited by development of acidosis, as is the case with the carbonic anhydrase inhibitors, loop diuretics are the most efficacious diuretic agents currently available. 

Roberts et al. reported a 27 kDa monomeric carbonic anhydrase, TWCAl, from the marine diatom Thalassiosira weissflogii (221). X-ray absorption spectroscopy indicated that the catalytic zinc is coordinated by three histidines and a water molecule, similar to the active sites of the a- and y-CAs (222). Also, the active site geometry is similar to that of a-CAs. Based on these results the catalytic mechanism is expected to be similar to that of the -class carbonic anhydrases. Tripp et al. (223) proposed that this TWCAl is the prototype of a fourth class carbonic anhydrase designated as 8-class CAs. In the... 

The purpose of the present review is to summarize how cobalt-linked absorption spectra and other physical properties have been utilized in attempts to elucidate relations between structure and function in these enzymes. The emphasis will be on carbonic anhydrase not only because it reflects the author s own interests, but mainly because it is the most extensively studied cobalt enzyme. Its environmentally-sensitive absorption spectrum has furnished essential information as to the role of the metal ion in the catalytic reaction. For other enzymes, the probe properties of cobalt are just beginning to be explored, but significant advances have recently been reported (7). 

Fig. 4. H-dependence of the visible absorption spectrum of bovine Co(II) carbonic anhydrase. The broken curve represents the basic spectral form, from ref. (58) (near infrared) and at pH 11.6 (visible). The solid curves were obtained in imidazole-sulfate buffers, ionic strength 0.1, pH 7.80, 7.00, 6.55, 6.10, respectively. The spectrum of lowest intensity was obtained by extrapolation and represents the acidic spectral form. Insert Spectrophotometric titration at 640 nm. The curve was calculated for the titration of a single group with pKa = 6.6...

Fig. 7. Absorption spectra of sulfonamide complexes of bovine Co(II) carbonic anhydrase. Solid curve sulfanilamide. Broken curve acetazolamide, only the visible spectrum shown...

The circular dichroic spectrum of cobalt alkaline phosphatase (Fig. 16) shows more clearly the complexity of the visible absorption. Although it can not be ruled out that the spectrum of this Co (I I) enzyme represents two slightly different Co(II) sites, there are striking similarities with Co(II) carbonic anhydrase, which has only one metal-binding site. At high pH, cobalt carbonic anhydrase and cobalt alkaline phosphatase have several spectral features in common, and both may have a similar kind of irregular coordination. It should be noted, however, that the absorption coefficient for Co(II) alkaline phosphatase per equivalent of activity-linked metal ion is only half of the value for Co(II) carbonic anhydrase. 

The most important aspect of the study of Co(II) metalloenzymes is the possibility of using the metal ion as a functional, built-in reporter of the dynamics of the active site. The spectral and magnetic properties of Co (II) carbonic anhydrase have given valuable clues to the catalytic function of this enzyme. The recent studies of Co(II) alkaline phosphatase and Co (II) carboxypeptidase A indicate the general applicability of this approach to enzymes where the probe properties of the constitutive metal ion are poor. The comparison of the absorption spectra of these enzymes and low-molecular weight models have shown that the proteins provide irregular, and in some cases nearly tetrahedral environments. It is obvious, however, that a knowledge of the crystal structures of the enzymes is necessary before the full potential of this method can be exploited. 

Carbon dioxide absorption by an aqueous huffer in the presence of an enzyme (carbonic anhydrase) 116... 

Maren TH, Conroy CW, Wynns GC, Levy NS. Ocular absorption, blood levels, and excretion of dorzolamide, a topicaUy active carbonic anhydrase inhibitor. J Ocul Pharmacol Ther 1997 13 23-30. 

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