Red cells genetically deficient in carbonic anhydrase II have elevated levels of a carbonic anhydrase indistinguishable from muscle CA III

A distinct carbonic anhydrase in the mucus of the colon of humans and other mammals

We have collected gastrointestinal, mainly colonic, mucus from humans,guinea pigs, rats, and normal and carbonic anhydrase II (CAII)-deficient mice. In the mucus of all species, substantial CA activity was present. Using antibodies against human CA isoforms we found that the human mucus CA differs from cytosolic CAI and CAII, membrane-bound CAIV, and the secreted CAVI of saliva. The high sensitivity of mucus CA to acetazolamide rules out its identity with cytosolic CAIII. Partial sequences obtained from the purified human mucus CA show similarity, but not identity, with human CAI, and clear differences from the other known CAs. Additional evidence concerning the CA isoform present in mucus was obtained for the mucus CA of other species and was derived from: (1) the mucus of CAII-deficient mice, whose high CA activity confirms that it is not CAII that is responsible; (2) the inhibitory effect of iodide, which shows that mucus CA from mice, guinea pig and humans does not have the high anion sensitivity of CAI; (3) the inactivating effect of 0.2%SDS on guinea pig, mouse and human mucus CA, ruling out the SDS-resistant CAIV; and (4) the partitioning of guinea-pig mucus CA into the water phase in Triton X114 phase separation experiments, which also argues against its identity with membrane-bound CAs, such as CAIV. A comparison of colonic mucus CA activity in normal and germ-free rats indicates that the mucus CA is not of bacterial origin but is produced by the gastrointestinal tissues. We conclude(from its immunoreactivity, from iodide inhibition and from partial amino acid sequences) that mucus CA of human origin probably represents an isozyme, which is specific for mucus and is not identical with the known CA isozymes. The results obtained for mucus CA of other species collectively point in the same direction.

A different structural feature for carbonic anhydrases in human erythrocytes

This study presents a different structural feature for carbonic anhydrase in human erythrocytes. Carbonic anhydrase isozymes (CA-I and CA-II) were purified from an erythrocyte pool of 20 healthy subjects. For purification, Sepharose-4B-L-tyrosine-sulfanilamide affinity column was used. Resnets from 3-10% discontinuous SDS-polyacrylamide gel electrophoresis (SDS-PAGE) showed a single band for CA-I and two distinct bands for CA-II. The molecular weights of the two bands were similar. One peak for CA-I and two peaks for CA-II were obtained in gel filtration. The enzymatic activities of the bands in question were also of different value. Native electrophoresis showed two bands for CA-I, and it showed three bands for CA-II. It can be concluded that CA-I is a polymer composed of a single promoter and CA-II has three different polymers composed of two distinct promoters, suggesting a new structural feature of human erythrocyte carbonic anhydrase isozymes.

Interaction between red cell membrane band 3 and cytosolic carbonic anhydrase

We have previously proposed that a membrane transport complex, centered on the human red cell anion transport protein, band 3, links the transport of anions, cations and glucose. Since band 3 is specialized for HCO3-/Cl- exchange, we thought there might also be a linkage with carbonic anhydrase (CA) which hydrates CO2 to HCO3-. CA is a cytosolic enzyme which is not present in the red cell membrane. The rate of reaction of CA with the fluorescent inhibitor, dansylsulfonamide (DNSA) can be measured by stopped-flow spectrofluorimetry and used to characterize the normal CA configuration. If a perturbation applied to a membrane protein alters DNSA/CA binding kinetics, we conclude that the perturbation has changed the CA configuration by either direct or allosteric means. Our experiments show that covalent reaction of the specific stilbene anion exchange inhibitor, DIDS, with the red cell membrane, significantly alters DNSA/CA binding kinetics. Another specific anion exchange inhibitor, benzene sulfonate (BSate), which has been shown to bind to the DIDS site causes a larger change in DNSA/CA binding kinetics; DIDS reverses the BSate effect. These experiments show that there is a linkage between band 3 and CA, consistent with CA interaction with the cytosolic pole of band 3.

Antibodies to carbonic anhydrase in systemic lupus erythematosus and other rheumatic diseases

OBJECTIVE

Autoantibodies to CA were demonstrated in the sera of patients with systemic lupus erythematosus (SLE) and some other rheumatic diseases. This study was undertaken to define the isoform and species specificity of these reactions, as well as to develop a method for detecting immune complexes.

METHODS

Antibodies to CA were sought by enzyme-linked immunosorbent assay (ELISA) and by Western immunoblotting.

RESULTS

An increased prevalence of CA autoantibodies was detected, by both methods, in patients with SLE, scleroderma, and polymyositis, compared with controls. In SLE patients, CA autoantibodies occurred preferentially in those with anti-U1 RNP or anti-U1 RNP and Ro/SS-A. Some sera reacted with only the CA I or CA II isoform, while approximately 50% of sera that were CA positive reacted with both isoforms. The autoantibodies reacted preferentially with the human enzymes, rather than the bovine CA, both on Western blot and by ELISA: Selected IgG F(ab')2 fragments from anti-CA-containing sera specifically inhibited the enzyme activity of CA, and the CA inhibitor acetazolamide partially inhibited the binding of anti-CA to CA. Thus, at least a part of autoanti-CA is directed toward the active site of CA. Finally, CA molecules were detected as immune complexes in sera from selected anti-CA-positive patients.

CONCLUSION

Autoantibodies to CA represent a previously unrecognized autoantibody to an abundant intracellular protein of the human erythrocyte.

The enzyme-inhibitor approach to cell-selective labelling--I. Sulphonamide inhibitors of carbonic anhydrase as carriers for red cell labelling: in vitro uptake of pIBS by human red blood cells

Red cell carbonic anhydrase is identified as an ideal target in an enzyme-inhibitor approach to radiolabel localisation. Current problems in blood pool labelling could be overcome by using selective sulphonamide inhibitors as carriers. p-Iodobenzenesulphonamide (pIBS) was selected as the choice reagent for red blood cell labelling. Rapid uptake of [125I]-pIBS was found in vitro, consistent with passive diffusion across the cell membrane. The intracellular binding could be attributed to interaction with two specific acceptor sites, with dissociation constants of 4.9 +/- 1.0 and 0.10 +/- 0.05 mumol dm-3, and maximum binding capacities of 166 +/- 5 and 19.9 +/- 1.0 mumol dm-3, respectively under the experimental conditions. These data correlate with the two major carbonic anhydrase isozymes; acceptor assignments were confirmed by gel chromatography of the red cell lysate.

Comparative immunochemical studies of carbonic anhydrase III in horses and other mammalian species

1. Carbonic anhydrase III (CA-III) from different mammalian species (horse, cow, dog, cat, rat and rabbit) has been analyzed by the immunodiffusion technique with anti-equine CA-III serum. 2. Immunodiffusion demonstrated the absence of cross-reactivity between isozyme CA-I, CA-II, and CA-III. 3. Cross-reactions were observed between the CA-III from all the species examined except the rabbit. 4. Molecular weights and isoelectric points of CA-III from different species were determined by Western blotting.

Carbonic anhydrases

Some of the current studies of carbonic anhydrases are directed to the genetic mechanisms underlying their synthesis. Determination of the structure of their genes will probably most readily resolve the question of whether the membrane bound forms of the enzyme represent products of additional loci other than those of the three well-known soluble forms. Extensions of our knowledge of the sequences of these isozymes as well as those from lower animals and from plants will make possible a more precise evaluation of the extent of the multigene aspects of these proteins and their evolutionary backgrounds. Studies of the interrelationships of the regulation of the transcriptional and translational processes of the well-known isozymes and in particular the effects of hormones will be of interest. Insights into modifications of the isozymes' synthetic processes occurring in various diseases should also be forth-coming from these studies. In addition to the above the applications of what are perhaps today somewhat classical methods of protein chemistry will be needed to explore the reasons for the changes in activity accompanying the sequence variations of the different isozymes, the decreases or increases in activity accompanying derivatizations of specific residues and the reasons for the differences in the activity of different inhibitors on the various isozymes. The broad specificity of these enzymes for different substrates and the ability of CA-III to hydrolyze various phenyl esters and in some cases to become derivatized also present problems in protein structural chemistry. In terms of the latter reactions, the meaning of the relationships of these activities to those of the protein ubiquitin, which is homologous to CA-III, needs clarification. It would appear that various of the protein structural studies will be aided by crystallographic investigations of not only CA-III but of various of its derivatives which undergo either increases or decreases in activity. The above areas of studies present a wide variety of problems for workers in various disciplines and backgrounds who are interested in the carbonic anhydrases.

Acylation and carbamylation of equine muscle carbonic anhydrase (CA-III) upon reaction with p-nitrophenyl esters and carbamoyl phosphate

Equine muscle carbonic anhydrase (CA-III) behaves like ubiquitin in undergoing extensive acylation of N epsilon-lysine residues upon reacting with p-nitrophenyl esters. The enzyme undergoes extensive carbamoylation of lysine residues when reacted with carbamoyl phosphate. The modification of from 6 to 7 lysine residues results in the production of a series of more anodic electrophoretic components. The derivatization of the lysine residues leads to a marked decrease in the enzyme's ability to hydrate CO2. The equine CA-III possesses both acid and alkaline phosphatase activities in contrast to the rabbit which possesses only the former type.

Carbonic anhydrase is aberrantly and constitutively expressed in both human and murine erythroleukemia cells

The levels of the erythrocyte proteins carbonic anhydrase (CA) and hemoglobin (Hb) change coordinately during human ontogeny. To further probe the coordinate gene expression of these two proteins in vitro, we used an immunoblotting technique to measure their levels during erythroid differentiation in normal human and murine erythroid progenitors, in human and murine erythroleukemia cells, and in normal murine erythroid progenitors infected with Friend virus. Levels of CA and Hb seem to gradually increase in normal differentiating stem cells. In contrast, both human and murine erythroleukemia cells show high levels of CA, but not of Hb, prior to induction of differentiation. Friend virus infection of normal murine progenitors appears to stimulate CA synthesis as an initial and integral step in transformation. In addition, both the erythroleukemia cells and the erythroid progenitors transformed with Friend virus seem to contain much higher levels of CA than Hb during the early stages of differentiation. This relationship is in marked contrast to normal erythroid differentiation, in which Hb levels are always higher than CA levels. Thus, neoplastic transformation seems to be associated with aberrant production of CA that does not correspond to a maturation arrest of the normal differentiation sequence.

The value of inherited deficiencies of human carbonic anhydrase isozymes in understanding their cellular roles

Very little light has been shed on the role of the low-activity CA I isozyme in humans by studies on CA I-deficient individuals. On the other hand, CA II-deficient individuals exhibit abnormalities of bone, kidney and brain, implicating a functional role for the high-activity CA II isozyme in cells from these tissues and organs. It also appears that the CA II-deficient red cell is capable of normal respiratory function under unstressed conditions. In addition, there is some preliminary evidence that those organs such as the eye which primarily contain the CA II isozyme, may be able to function effectively in the absence of CA II.