Carbonic Anhydrase Domain Research Articles

Computational analysis of physico-chemical properties and homology modeling of carbonic anhydrase from Cordyceps militaris

In the present study, the protein sequence of carbonic anhydrase enzyme from Cordyceps militaris (accession no. EGX89555.1) was retrieved from GenPept database and subjected to computation for various physico-chemical properties, transmembrane segment prediction, homology modeling, domain identification and structural alignment. Five potential transmembrane segments of various lengths were predicted by transmembrane prediction server. The 3D structure of this protein was determined by homology modeling using 3d jigsaw server. A single eukaryotic-type carbonic anhydrase domain belonging to Carb_anhydrases family domain was identified at two positions from 45 to 153 and 232 to 302 residues region. This modeled structure showed 100 % structural similarity with human carbonic anhydrase III (PDB ID: 1z93-A) in PDB database.

Feasibility and constraints of particle targeting using the antigen–antibody interaction

This work is concerned with the surface modification of fluorescent silica nanoparticles by a monoclonal antibody (M75) and the specific bioadhesion of such particles to surfaces containing the PG domain of carbonic anhydrase IX (CA IX), which is a trans-membrane protein specifically expressed on the surfaces of several tumor cell lines. The adhesion strength of antibody-bearing silica nanoparticles to antigen-bearing surfaces was investigated under laminar flow conditions in a microfluidic cell and compared to the adhesion of unmodified silica nanoparticles and nanoparticles coupled with an unspecific antibody. Adhesion to cancer cells using flow cytometry was also investigated and in all cases the adhesion strength of M75-modified nanoparticles was significantly stronger than for the unmodified or unspecific nanoparticles, up to several orders of magnitude in some cases. The specific modification of nano- and microparticles by an antibody-like protein therefore appears to be a feasible approach for the targeting of tumor cells.

Increased expression level and catalytic activity of internally-duplicated carbonic anhydrase from Dunaliella species by reconstitution of two separate domains

Although internally-duplicated, α-type carbonic anhydrase (CA) from Dunaliella species (Dsp-CA) can be expressed in Escherichia coli system, the produced amount is not sufficient for practical application. Here, we demonstrated to express the N- and C-half CA domains of Dsp-CA as distinct proteins to increase the expression levels further. The purified N-half CA domain (Dsp-CA-n) showed very low detectable activities of esterase or CO2 hydratase. In contrast, the purified C-half CA domain (Dsp-CA-c) retained both activities, which were enhanced by the presence of Dsp-CA-n. The expression levels of both domains were increased by 2–5-fold as compared to that of Dsp-CA. The CA activity was successfully reconstituted by mixing the two domains, N- and C-half domains, and more activity level was achieved than dimeric intact CA. These results newly suggest that the produced amount and activity of a duplicated CA are enhanced successfully by expressing each half CA domain individually and in vitro reconstitution.

Characterization of Carbonic Anhydrase Isozyme Specific Inhibition by Sulfamated 2-Ethylestra Compounds

Sulfamated 2-ethylestra compounds have demonstrated strong anticancer activity, high bioavailability and an ability to bypass liver metabolism by reversibly binding carbonic anhydrase (CA) II in erythrocytes. Another CA isoform, CA IX, is overexpressed in many cancers. The CA domain of CA IX is oriented extracellularly, which may permit targeting inhibitors to tumors. Presented here is the characterization of three 2-ethylestra compounds bound to both CA II and a CA IX mimic protein. Inhibition by 18O exchange showed that compound 16 demonstrated an approximately 12-fold higher affinity for CA II over CA IX mimic. Structurally, compounds 15 and 16 showed alternate binding modes between CA II and CA IX mimic, apparently due to a water-mediated hydrogen bond to the isozyme-specific residue 67. Though the specificity was demonstrated for CA II over CA IX, this study reveals insights that may be key to developing isozyme specific CA inhibitors for novel anticancer therapies. Keywords: Carbonic anhydrase, Isozyme specific, Structure based drug design, 2-Ethylestra compounds, Anti-cancer therapy, Steroid sulfatase inhibitors, Sulfamated 2-ethylestra compounds, erythrocytes, liver metabolism, estrogen, colchicine binding site, apoptosis, anti-angiogenic, PISA server, CA isoforms, cancer

A New Peptide Ligand for Targeting Human Carbonic Anhydrase IX, Identified through the Phage Display Technology

Carbonic anhydrase IX (CAIX) is a transmembrane enzyme found to be overexpressed in various tumors and associated with tumor hypoxia. Ligands binding this target may be used to visualize hypoxia, tumor manifestation or treat tumors by endoradiotherapy.MethodsPhage display was performed with a 12 amino acid phage display library by panning against a recombinant extracellular domain of human carbonic anhydrase IX. The identified peptide CaIX-P1 was chemically synthesized and tested in vitro on various cell lines and in vivo in Balb/c nu/nu mice carrying subcutaneously transplanted tumors. Binding, kinetic and competition studies were performed on the CAIX positive human renal cell carcinoma cell line SKRC 52, the CAIX negative human renal cell carcinoma cell line CaKi 2, the human colorectal carcinoma cell line HCT 116 and on human umbilical vein endothelial cells (HUVEC). Organ distribution studies were carried out in mice, carrying SKRC 52 tumors. RNA expression of CAIX in HCT 116 and HUVEC cells was investigated by quantitative real time PCR.Results In vitro binding experiments of 125I-labeled-CaIX-P1 revealed an increased uptake of the radioligand in the CAIX positive renal cell carcinoma cell line SKRC 52. Binding of the radioligand in the colorectal carcinoma cell line HCT 116 increased with increasing cell density and correlated with the mRNA expression of CAIX. Radioligand uptake was inhibited up to 90% by the unlabeled CaIX-P1 peptide, but not by the negative control peptide octreotide at the same concentration. No binding was demonstrated in CAIX negative CaKi 2 and HUVEC cells. Organ distribution studies revealed a higher accumulation in SKRC 52 tumors than in heart, spleen, liver, muscle, intestinum and brain, but a lower uptake compared to blood and kidney.ConclusionsThese data indicate that CaIX-P1 is a promising candidate for the development of new ligands targeting human carbonic anhydrase IX.

Crystal structure of the catalytic domain of the tumor-associated human carbonic anhydrase IX

Carbonic anhydrase (CA) IX is a plasma membrane-associated member of the alpha-CA enzyme family, which is involved in solid tumor acidification. It is a marker of tumor hypoxia and a prognostic factor in several human cancers. An aberrant increase in CA IX expression in chronic hypoxia and during development of various carcinomas contributes to tumorigenesis through at least two mechanisms: pH regulation and cell adhesion control. Here we report the X-ray structure of the catalytic domain of CA IX in complex with a classical, clinically used sulfonamide inhibitor, acetazolamide. The structure reveals a typical alpha-CA fold, which significantly differs from the other CA isozymes when the protein quaternary structure is considered. Thus, two catalytic domains of CA IX associate to form a dimer, which is stabilized by the formation of an intermolecular disulfide bond. The active site clefts and the PG domains are located on one face of the dimer, while the C-termini are located on the opposite face to facilitate protein anchoring to the cell membrane. A correlation between the three-dimensional structure and the physiological role of the enzyme is here suggested, based on the measurement of the pH profile of the catalytic activity for the physiological reaction, CO(2) hydration to bicarbonate and protons. On the basis of the structural differences observed between CA IX and the other membrane-associated alpha-CAs, further prospects for the rational drug design of isozyme-specific CA inhibitors are proposed, given that inhibition of this enzyme shows antitumor activity both in vitro and in vivo.

A structural investigation of complex I and I + III 2 supercomplex from Zea mays at 11–13 Å resolution: Assignment of the carbonic anhydrase domain and evidence for structural heterogeneity within complex I

The projection structures of complex I and the I + III 2 supercomplex from the C 4 plant Zea mays were determined by electron microscopy and single particle image analysis to a resolution of up to 11 Å. Maize complex I has a typical L-shape. Additionally, it has a large hydrophilic extra-domain attached to the centre of the membrane arm on its matrix-exposed side, which previously was described for Arabidopsis and which was reported to include carbonic anhydrase subunits. A comparison with the X-ray structure of homotrimeric γ-carbonic anhydrase from the archaebacterium Methanosarcina thermophila indicates that this domain is also composed of a trimer. Mass spectrometry analyses allowed to identify two different carbonic anhydrase isoforms, suggesting that the γ-carbonic anhydrase domain of maize complex I most likely is a heterotrimer. Statistical analysis indicates that the maize complex I structure is heterogeneous: a less-abundant “type II” particle has a 15 Å shorter membrane arm and an additional small protrusion on the intermembrane-side of the membrane arm if compared to the more abundant “type I” particle. The I + III 2 supercomplex was found to be a rigid structure which did not break down into subcomplexes at the interface between the hydrophilic and the hydrophobic arms of complex I. The complex I moiety of the supercomplex appears to be only of “type I”. This would mean that the “type II” particles are not involved in the supercomplex formation and, hence, could have a different physiological role.

Carbonic Anhydrase in Calcified Endoskeleton: Novel Activity in Biocalcification in Alcyonarian

Carbonic anhydrase (CA) is a key enzyme in the chemical reaction of living organisms and has been found to be associated with calcification in a number of invertebrates including calcareous sponges, but until now no direct evidence has been advanced to show CA activity in alcyonarian corals. However, it is essential to understand the role of CA in the process of biocalcification in alcyonarian. Here we describe the novel activity of CA and its relationship to the formation of calcified hard tissues in alcyonarian coral, Lobophytum crassum. We find that two CA proteins, which were partially purified by electro-elution treatment, can control the morphology of CaCO(3) crystals and one of them is potentially involved in the process of biocalcification. Previously, we isolated CA from the total extract of alcyonarian, and further, we report here a single protein, which has both calcium-binding and CA activities and is responsible for CaCO(3) nucleation and crystal growth. This matrix protein inhibited the precipitation of CaCO(3) from a saturated solution containing CaCl(2) and NaHCO(3), indicating that it can act as a negative regulator for calcification in the sclerites of alcyonarians. The effect of an inhibitor on the enzyme activity was also examined. These findings strongly support the idea that carbonic anhydrase domain in alcyonarian is involved in the calcification process. Our observations strongly suggest that the matrix protein in alcyonarian coral is not only a structural protein but also a catalyst.

Carbonic Anhydrase Inhibitors: Inhibition of Isozymes I, II, and IX with Triazole-Linked O-Glycosides of Benzene Sulfonamides

We report the synthesis of a series of benzene sulfonamides containing triazole-O-glycoside tails for evaluation as carbonic anhydrase (CA) inhibitors. These glycoconjugates were synthesized by the 1,3-dipolar cycloaddition reaction of 4-azidobenzenesulfonamide with O-propynyl glycosides. Compounds were assessed for their ability to inhibit the enzymatic activity of the physiologically dominant isozymes hCA I and II and the tumor-associated isozyme hCA IX (h = human). Against hCA I these compounds were either micromolar or low-nanomolar inhibitors, while against hCA II and IX inhibition in the range of 6.8-53 and 9.7-107 nM, respectively, was observed. The most potent inhibitor against hCA IX was the galactose derivative 8 (Ki = 9.7 nM); this is so far the most potent glycoconjugate inhibitor reported for the tumor-associated hCA IX. These carbohydrate-tethered sulfonamides may prove interesting lead candidates to target tumor-associated CA isozymes, wherein the CA domain is located extracellularly.

Inhibition of membrane-associated carbonic anhydrase isozymes IX, XII and XIV with a library of glycoconjugate benzenesulfonamides

A library of glycoconjugate benzenesulfonamides that contain diverse carbohydrate-triazole tails were investigated for their ability to inhibit the enzymatic activity of the three human transmembrane carbonic anhydrase (CA) isozymes hCA IX, hCA XII and hCA XIV. These isozymes have their CA domains located extracellularly, unlike the physiologically dominant hCA II, and are of immense current interest as druggable targets. Elevated expression of isozymes IX and XII is a marker for a broad spectrum of hypoxic tumors—this physiology may facilitate a novel approach to discriminate between healthy cells and cancerous cells. Many of these glycoconjugates were potent inhibitors (low nM), but importantly exhibited different isozyme selectivity profiles. The most potent hCA IX inhibitor was the glucuronic acid derivative 20 ( K i = 23 nM). This compound was uniquely hCA IX selective cf. all other isozymes (16.4-, 16.8- and 4.6-fold selective against hCA II, XII, and XIV, respectively). At hCA XII there were many inhibitors with K is < 10 nM that also demonstrated excellent selectivity (up to 344-fold) against other isozymes. Potent hCA XIV inhibitors were also identified, several with K is ∼ 10 nM, however no hCA XIV-selective derivatives were evidenced from this library. The sugar tails of this study have shown promise as a valuable approach to both solubilize the aromatic sulfonamide CA recognition pharmacophore and to deliver potent inhibition and isozyme differentiation of the transmembrane CAs.

A novel carbonic anhydrase from the giant clam Tridacna gigas contains two carbonic anhydrase domains

This report describes the presence of a unique dual domain carbonic anhydrase (CA) in the giant clam, Tridacna gigas. CA plays an important role in the movement of inorganic carbon (Ci) from the surrounding seawater to the symbiotic algae that are found within the clam's tissue. One of these isoforms is a glycoprotein which is significantly larger (70 kDa) than any previously reported from animals (generally between 28 and 52 kDa). This alpha-family CA contains two complete carbonic anhydrase domains within the one protein, accounting for its large size; dual domain CAs have previously only been reported from two algal species. The protein contains a leader sequence, an N-terminal CA domain and a C-terminal CA domain. The two CA domains have relatively little identity at the amino acid level (29%). The genomic sequence spans in excess of 17 kb and contains at least 12 introns and 13 exons. A number of these introns are in positions that are only found in the membrane attached/secreted CAs. This fact, along with phylogenetic analysis, suggests that this protein represents the second example of a membrane attached invertebrate CA and it contains a dual domain structure unique amongst all animal CAs characterized to date.

Carbonic anhydrase IX (CA IX) mediates tumor cell interactions with microenvironment

Expression of CA IX is normally restricted to the mucosa of alimentary tract, but on the other hand, it takes place in a high percentage of human cancers derived from tissues which are normally CA IX-negative. It is a transmembrane protein with two extracellular domains: carbonic anhydrase (CA) with a high catalytic activity and a proteoglycan-like segment (PG), mediating cell-cell adhesion. Both CA and PG domains interact with the microenvironment and they could play a role in tumorigenesis, but their roles are poorly understood. The present work characterizes some newly recognized properties of the PG. One of them is a prevalently negative charge, caused by a high proportion of dicarboxylic amino acids. This is reflected by easy dissociation of complexes formed by PG either with monoclonal antibody M75 or with the cell surface receptor already at slightly acidic pH. This property might facilitate separation of cells from the primary tumor. Released cells may subsequently attach elsewhere in the organism and eventually start metastatic growth. Another aim of the present study was to identify human tumor cell lines which are expressing the presumed CA IX receptor molecule. The same cell lines were also tested for the presence of CA IX protein; we found that expression of CA IX and of the receptor is independent of each other. In addition, we examined the species specificity of CA IX receptors. The PG domain, which contains the epitope of mAb M75 -PGEEDLP- overlapping with the binding site for putative receptor is relatively conserved in evolution: human and rat CA IX cross-react with M75 antibody on western blots. Consistently with this, human and rat cells can attach to purified human CA IX protein. On the other hand, murine CA IX contains an entirely different equivalent of PG sequence and it does not react with M75 antibody or attach to human CA IX protein. This is suggestive of the co-evolution of CA IX protein together with its receptor.

The Carbonic Anhydrase Domain Protein Nacrein is Expressed in the Epithelial Cells of the Mantle and Acts as a Negative Regulator in Calcification in the Mollusc Pinctada fucata

Signals and organic matrix proteins secreted from the mantle are critical for the development of shells in molluscs. Nacrein, which is composed of a carbonic anhydrase domain and a Gly-X-Asn repeat domain, is one of the organic matrix proteins that accumulates in shells. In situ hybridization revealed that nacrein was expressed in the outer epithelial cells of the mantle of the pearl oyster Pinctada fucata. The recombinant nacrein protein inhibited the precipitation of calcium carbonate from a saturated solution containing CaCl2 and NaHCO3, indicating that it can act as a negative regulator for calcification in the shells of molluscs. Because deletion of the Gly-X-Asn repeat domain of nacrein had a significant effect on the ability of nacrein to inhibit the precipitation of calcium carbonate, it is conceivable that the repeat domain has a primary role in the inhibitory function of nacrein in shell formation. Together these studies suggest that nacrein functions as a negative regulator in calcification in the extrapallial space between the shell and the mantle by inhibiting the precipitation of CaCO3.

Contactin-Associated Protein (Caspr) and Contactin Form a Complex That Is Targeted to the Paranodal Junctions during Myelination

Specialized paranodal junctions form between the axon and the closely apposed paranodal loops of myelinating glia. They are interposed between sodium channels at the nodes of Ranvier and potassium channels in the juxtaparanodal regions; their precise function and molecular composition have been elusive. We previously reported that Caspr (contactin-associated protein) is a major axonal constituent of these junctions (Einheber et al., 1997). We now report that contactin colocalizes and forms a cis complex with Caspr in the paranodes and juxtamesaxon. These proteins coextract and coprecipitate from neurons, myelinating cultures, and myelin preparations enriched in junctional markers; they fractionate on sucrose gradients as a high-molecular-weight complex, suggesting that other proteins may also be associated with this complex. Neurons express two contactin isoforms that differ in their extent of glycosylation: a lower-molecular-weight phosphatidylinositol phospholipase C (PI-PLC)-resistant form is associated specifically with Caspr in the paranodes, whereas a higher-molecular-weight form of contactin, not associated with Caspr, is present in central nodes of Ranvier. These results suggest that the targeting of contactin to different axonal domains may be determined, in part, via its association with Caspr. Treatment of myelinating cocultures of Schwann cells and neurons with RPTPbeta-Fc, a soluble construct containing the carbonic anhydrase domain of the receptor protein tyrosine phosphatase beta (RPTPbeta), a potential glial receptor for contactin, blocks the localization of the Caspr/contactin complex to the paranodes. These results strongly suggest that a preformed complex of Caspr and contactin is targeted to the paranodal junctions via extracellular interactions with myelinating glia.

Mitochondrial carbonic anhydrase CA VB: differences in tissue distribution and pattern of evolution from those of CA VA suggest distinct physiological roles.

A cDNA for a second mouse mitochondrial carbonic anhydrase (CA) called CA VB was identified by homology to the previously characterized murine CA V, now called CA VA. The full-length cDNA encodes a 317-aa precursor that contains a 33-aa classical mitochondrial leader sequence. Comparison of products expressed from cDNAs for murine CA VB and CA VA in COS cells revealed that both expressed active CAs that localized in mitochondria, and showed comparable activities in crude extracts and in mitochondria isolated from transfected COS cells. Northern blot analyses of total RNAs from mouse tissues and Western blot analyses of mouse tissue homogenates showed differences in tissue-specific expression between CA VB and CA VA. CA VB was readily detected in most tissues, while CA VA expression was limited to liver, skeletal muscle, and kidney. The human orthologue of murine CA VB was recently reported also. Comparison of the CA domain sequence of human CA VB with that reported here shows that the CA domains of CA VB are much more highly conserved between mouse and human (95% identity) than the CA domains of mouse and human CA VAs (78% identity). Analysis of phylogenetic relationships between these and other available human and mouse CA isozyme sequences revealed that mammalian CA VB evolved much more slowly than CA VA, accepting amino acid substitutions at least 4.5 times more slowly since each evolved from its respective human-mouse ancestral gene around 90 million years ago. Both the differences in tissue distribution and the much greater evolutionary constraints on CA VB sequences suggest that CA VB and CA VA have evolved to assume different physiological roles.

Carbonic anhydrase (CA)-related proteins (CA-RPs), and transmembrane proteins with CA or CA-RP domains.

Members of the a-carbonic anhydrase (α-CA) gene family encode not only proteins that exhibit the characteristic catalytic activity of CA (i.e., the reversible hydration of CO2), but also CA-related proteins that are apparently devoid of this activity. For a recent summary of the activity mechanisms and functions of the mammalian CA isozymes, see Sly and Hu (1995). In amniotes (reptiles, birds, and mammals), the active CA isozymes have been designated, CA I — CA VII, CA IX, CA XII, and CA XIII, and the presumed inactive isoforms, CA-RP VIII, CA-RP X, and CA-RP-XI. In non-amniotes, apparently inactive α-CA-related proteins (CAH-RPs) have been identified in both the nematode, C. elegans, and in the yam (Dioscorea). Additional CA-related proteins are present as transmembrane proteins in several pox viruses, and as the CA-RP N-terminal domains of the extracellular regions of the transmembrane proteins of receptor protein tyrosine phosphatases β and γ (RPTPβ and γ). All of these presumably “acatalytic” CA-related isoforms probably either have no, or greatly diminished, CO2 hydration activity due to the substitution of one or more of the three histidine residues that are required to bind the zinc ion that is essential for efficient CA activity.KeywordsPurkinje CellCarbonic AnhydraseCarbonic Anhydrase ActivityChondroitin Sulfate ProteoglycanRenal Cell Carcinoma Cell LineThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Isolation and Characterization of CA XIV, a Novel Membrane-bound Carbonic Anhydrase from Mouse Kidney

Carbonic anhydrase (CA) is involved in various physiological processes such as acid-base balance and transport of carbon dioxide and ions. In this study, we have succeeded in the isolation of a novel CA from the mouse kidney by use of the signal sequence trap method. It is a 337-amino acid polypeptide with a calculated molecular mass of 37.5 kDa, consisting of a putative amino-terminal signal sequence, a CA domain, a transmembrane domain, and a short hydrophilic carboxyl terminus, which we designated CA XIV. The CA domain of CA XIV is highly homologous with those of known CAs, especially extracellular CAs including CA XII, IX, VI, and IV. The expression study of an epitope-tagged protein has suggested that CA XIV is located on the plasma membrane. When expressed in COS-7 cells, CA XIV exhibits CA activity that is predominantly associated with the membrane fraction. By Northern blot analysis, the gene expression of CA XIV is most abundant in the kidney and heart, followed by the skeletal muscle, brain, lung, and liver. In situ hybridization has revealed that, in the kidney, the gene is expressed intensely in the proximal convoluted tubule, which is the major segment for bicarbonate reabsorption and also in the outer border of the inner stripe of the outer medulla. In conclusion, we have cloned a functional cDNA encoding a novel membrane-bound CA. This study will bring new insights into our understanding of carbon dioxide metabolism and acid-base balance.

Sequence and Tissue Expression of a Novel Human Carbonic Anhydrase-Related Protein, CARP-2, Mapping to Chromosome 19q13.3

In this study, we report the identification and characterisation of a novel carbonic anhydrase related-protein. We have determined that the full length coding sequence of an anonymous expressed sequenced tag, D19S799E, encodes a novel carbonic anhydrase related-protein (CARP-2) that is 328 amino acids in length. This peptide exhibits between 23.1-28.8% amino acid identity with the seven active human carbonic anhydrase (CA) isozymes. Four substitutions of key amino acids in the catalytic domain of CAs (equivalent to His94Arg, His96Leu, His119Gln, and Thr199Ser) are likely to render CARP-2 inactive as a carbonic anhydrase. Northern blot analysis of 23 human tissues indicates that CARP2 is expressed abundantly in the brain with moderate expression also present in spinal cord and thyroid. D19S799E (and thus CARP2) has previously been localised close to the polymorphic marker D19S412 and the genes DBP and FUT1/FUT2 on 19q13.3.

Human carbonic anhydrase XII: cDNA cloning, expression, and chromosomal localization of a carbonic anhydrase gene that is overexpressed in some renal cell cancers.

We report the cloning and characterization of a tumor-associated carbonic anhydrase (CA) that was identified in a human renal cell carcinoma (RCC) by serological expression screening with autologous antibodies. The cDNA sequence predicts a 354-amino acid polypeptide with a molecular mass of 39,448 Da that has features of a type I membrane protein. The predicted sequence includes a 29-amino acid signal sequence, a 261-amino acid CA domain, an additional short extracellular segment, a 26-amino acid hydrophobic transmembrane domain, and a hydrophilic C-terminal cytoplasmic tail of 29 amino acids that contains two potential phosphorylation sites. The extracellular CA domain shows 30-42% homology with known human CAs, contains all three Zn-binding histidine residues found in active CAs, and contains two potential sites for asparagine glycosylation. When expressed in COS cells, the cDNA produced a 43- to 44-kDa protein in membranes that had around one-sixth the CA activity of membranes from COS cells transfected with the same vector expressing bovine CA IV. We have designated this human protein CA XII. Northern blot analysis of normal tissues demonstrated a 4.5-kb transcript only in kidney and intestine. However, in 10% of patients with RCC, the CA XII transcript was expressed at much higher levels in the RCC than in surrounding normal kidney tissue. The CA XII gene was mapped by using fluorescence in situ hybridization to 15q22. CA XII is the second catalytically active membrane CA reported to be overexpressed in certain cancers. Its relationship to oncogenesis and its potential as a clinically useful tumor marker clearly merit further investigation.

Immunohistochemistry of carbonic anhydrase isozyme IX (MN/CA IX) in human gut reveals polarized expression in the epithelial cells with the highest proliferative capacity.

MN/CA IX is a recently discovered member of the carbonic anhydrase (CA) gene family that has been identified in the plasma membranes of certain tumor and epithelial cells and found to promote cell proliferation when transfected into NIH3T3 cells. This study presents localization of MN/CA IX in human gut and compares its distribution to those of CA I, II, and IV, which are known to be expressed in the intestinal epithelium. The specificity of the monoclonal antibody for MN/CA IX was confirmed by Western blots and immunostaining of COS-7 cells transfected with MN/CA IX cDNA. Immunohistochemical stainings of human gut revealed prominent polarized staining for MN/CA IX in the basolateral surfaces of the enterocytes of duodenum and jejunum, the reaction being most intense in the crypts. A moderate reaction was also seen in the crypts of ileal mucosa, whereas the staining became generally weaker in the large intestine. The results indicate isozyme-specific regulation of MN/CA IX expression along the cranial-caudal axis of the human gut and place the protein at the sites of rapid cell proliferation. The unique localization of MN/CA IX on the basolateral surfaces of proliferating crypt enterocytes suggests that it might serve as a ligand or a receptor for another protein that regulates intercellular communication or cell proliferation. Furthermore, MN/CA IX has a completely conserved active site domain of CAs suggesting that it could also participate in carbon dioxide/bicarbonate homeostasis.