Purification and characterization of a high-Mr carbonic anhydrase from sheep parotid gland

Molecular Cloning and Characterization of Carbonic Anhydrase XII from Pufferfish (Takifugu rubripes)

In this study, an 1888-bp carbonic anhydrase XII (CA XII) sequence was cloned from the brain of the pufferfish, Takifugu rubripes. The cloned sequence contained a coding region of 1470-bp, which was predicted to translate into a protein of 490 amino acid residues. The predicted protein showed between 68–56% identity with the large yellow croaker (Larimichthys crocea), tilapia (Oreochromis niloticus), and Asian arowana (Scleropages formosus) CA XII proteins. It also exhibited 36% and 53% identity with human CA II and CA XII, respectively. The cloned sequence contained a 22 amino acid NH₂-terminal signal sequence and three Asn-Xaa-Ser/Thr sequons, among which one was potentially glycosylated. Four cysteine residues were also identified (Cys-21, Cys-201, Cys-355, and Cys-358), two of which (Cys-21 and Cys-201) could potentially form a disulfide bond. A 22-amino acid COOH-terminal cytoplasmic tail containing a potential site for phosphorylation by protein kinase A was also found. The cloned sequence might be a transmembrane protein, as predicted from in silico and phylogenetic analyses. The active site analysis of the predicted protein showed that its active site residues were highly conserved with tilapia CA XII protein. Homology modeling of the pufferfish CA XII was done using the crystal structure of the extracellular domain of human carbonic anhydrase XII at 1.55 Å resolution as a template. Semi-quantitative reverse transcription (RT)-PCR, quantitative PCR (q-PCR), and in situ hybridization confirmed that pufferfish CA XII is highly expressed in the brain.

Cross-species comparison of mammalian saliva using an LC-MALDI based proteomic approach

Despite the importance of saliva in the regulation of oral cavity homeostasis, few studies have been conducted to quantitatively compare the saliva of different mammal species. Aiming to define a proteome signature of mammals' saliva, an in-depth SDS-PAGE-LC coupled to MS/MS (GeLC-MS/MS) approach was used to characterize the saliva from primates (human), carnivores (dog), glires (rat and rabbit), and ungulates (sheep, cattle, horse). Despite the high variability in the number of distinct proteins identified per species, most protein families were shared by the mammals studied with the exception of cattle and horse. Alpha-amylase is an example that seems to reflect the natural selection related to digestion efficacy and food recognition. Casein protein family was identified in all species but human, suggesting an alternative to statherin in the protection of hard tissues. Overall, data suggest that different proteins might assure a similar role in the regulation of oral cavity homeostasis, potentially explaining the specific mammals' salivary proteome signature. Moreover, some protein families were identified for the first time in the saliva of some species, the presence of proline-rich proteins in rabbit's saliva being a good example.

Isolation and sequencing of swine carbonic anhydrase VI, an enzyme expressed in the swine kidney

Background

Carbonic anhydrase VI (CA-VI) is produced by the salivary gland and is secreted into the saliva. Although CA-VI is found in the epithelial cells of distal straight tubule of swine kidneys, the exact function of CA-VI in the kidneys remains unclear.

Results

CA-VI was located in the epithelial cells of distal straight tubule of swine kidneys.

A full-length cDNA clone of CA-VI was generated from the swine parotid gland by reverse transcription polymerase chain reaction, using degenerate primers designed based on conserved regions of the same locus in human and bovine tissues.

The cDNA sequence was 1348 base pairs long and was predicted to encode a 317 amino acid polypeptide with a putative signal peptide of 17 amino acids. The deduced amino acid sequence of mature CA-VI was most similar (77.4%) to that of human CA-VI. CA-VI expression was confirmed in both normal and nephritic kidneys, as well as parotid. As the primers used in this study spanned two exons, the influence of genomic DNA was not detected. The expression of CA-VI was demonstrated in both normal and nephritic kidneys, and mRNA of CA-VI in the normal kidneys which was the normalised to an endogenous β–actin was 0.098 ± 0.047, while it was significantly lower in the diseased kidneys (0.012 ± 0.007). The level of CA-VI mRNA in normal kidneys was 19-fold lower than that of the parotid gland (1.887).

Conclusions

The localisation of CA-VI indicates that it may play a specialised role in the kidney.

Identification of an inflammation-inducible serum protein recognized by anti-disialic acid antibodies as carbonic anhydrase II

Acute-phase proteins are an important marker of inflammation and sometimes have a role in the general defense response towards tissue injury. In the present study, we identified a 32-kDa protein that was immunoreactive with monoclonal antibody 2-4B (mAb.2-4B), which is specific to di/oligoNeu5Gc structures, and that behaved as an acute-phase protein following stimulation with either turpentine oil or lipopolysaccharides. The 32-kDa protein was identified as carbonic anhydrase II (CA-II), based on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analyses of the purified protein. Mouse and human CA-II was immunoreactive and immunoprecipitated with mAb.2-4B, but contained no sialic acid. In addition to mAb.2-4B, the mAb. S2-566 an antibody specific for diNeu5Ac-containing glycans, recognized the CA-II, whereas an anti-oligo/polysialic acid antibody did not. These results indicate that a part of the CA-II protein structure mimics the disialic acid structure recognized by the monoclonal antibodies. This is the first report that CA-II circulates in the serum following inflammation.

Measurement of Carbonic Anhydrase Isozyme VI (CA-VI) in Bovine Sera, Saliva, Milk and Tissues

Concentrations of bovine carbonic anhydrase isozyme VI (CA-IV) in bovine serum, saliva, normal milk, colostrum, submandibular gland, liver, and mammary gland were determined. CA-VI was purified from bovine saliva and an antibody to CA-VI was generated. The concentrations of CA-VI in the saliva (7.8 +/- 7.9 microg/ml), serum (2.1+/- 5.7 ng/ml), milk (7.9 +/- 12.1 ng/ml), submandibular gland (284.7 microg/g protein), liver (921.0 +/- 180.7 ng/g protein) and mammary gland (399.6 +/- 191.2 ng/g protein) were determined by ELISA. No seasonal change in CA-VI levels was observed in normal milk. The concentration of CA-VI in colostrum (day 1 post partum) was 119 ng/ml and decreased rapidly by 1 month following birth. Mammary gland contained much smaller amounts than the submandibular gland. CA-VI mRNA was detected in the liver and mammary gland of cow by RT-PCR. The ELISA used in this study proved to be a precise and sensitive method for determining CA-VI concentrations in saliva, serum, milk and tissue specimens from cows. The ELISA may enable the study of changes in CA-VI associated with hereditary or metabolic disorders of the salivary gland, mammary gland and liver using small samples of saliva, serum or milk.

A role of salivary carbonic anhydrase VI in dental plaque

OBJECTIVE

Carbonic anhydrase (CA) VI is a unique secreted isozyme of CA, which catalyzes the reversible reaction CO2 +H2O<-->H+ +HCO3-. CA VI has been thought to provide a greater buffering capacity to fluids into which it is secreted. This study was performed to confirm this in saliva.

DESIGN

Nine healthy subjects participated in the study. The pH of the dental plaque from each subject was monitored after a mouth rinse with 10% sucrose with or without 10(-5)M acetazolamide, a specific inhibitor of CA. Also CA was examined in plaque by enzyme histochemistry, immunohistochemistry and Western blot analysis.

RESULTS

Though sucrose and sucrose plus inhibitor yielded Stephan curves with a similar temporal pattern, the pH values of the latter were significantly lower than those of the former. Plaque exhibited CA activity by enzyme histochemistry. Immunohistochemistry and Western analysis demonstrated that the activity was due to CA VI but not to CA I or CA II.

CONCLUSIONS

The results indicate that CA VI in saliva penetrates plaque and facilitates acid neutralization by salivary bicarbonate. Therefore, CA VI may be considered an anti-caries protein in saliva.

Carbonic anhydrase VI in the mouse nasal gland

Western blotting analysis of mouse nasal tissue using a specific anti-mouse secreted carbonic anhydrase (CA VI) antibody has shown that CA VI is present in this tissue. A single immunoreactive band of 42 kD was observed, as has been found previously for salivary tissues. RT-PCR analysis has shown that nasal mucosa expressed CA VI mRNA. By immunohistochemistry (IHC), CA VI was observed in acinar cells, in duct contents of the anterior gland of the nasal septum, and in the lateral nasal gland. The Bowman's gland, the posterior gland of the nasal septum, and the maxillary sinus gland were negative. Immunoreactivity was also observed in the mucus covering the respiratory and olfactory mucosa and in the lumen of the nasolacrimal duct. In contrast, an anti-rat CA II antibody (that crossreacts with the mouse enzyme) stained only known CA II-positive cells and an occasional olfactory receptor neuron. These results indicate that CA VI is produced by the nasal gland and is secreted over the nasal mucosa. By reversible hydration of CO(2), CA VI is presumed to play a role in mucosal functions such as CO(2) sensation and acid-base balance. It may also play a role in olfactory function as a growth factor in maturation of the olfactory epithelial cells.

Characterization of lacrimal gland carbonic anhydrase VI

We have previously demonstrated by immunohistochemistry the presence of secreted carbonic anhydrase (CA VI) in the acinar cells of the rat lacrimal glands. In this study we purified the sheep lacrimal gland CA VI to homogeneity and demonstrated by Western analysis that it has the same apparent subunit molecular weight (45 kD) as the enzyme isolated from saliva. RT-PCR analysis showed that CA VI mRNA from the lacrimal gland was identical to that of the parotid gland CA VI mRNA. An RIA specific for sheep CA VI showed the lacrimal gland tissue concentration of the enzyme to be 4.20 +/- 2.60 ng/mg protein, or about 1/7000 of the level found in the parotid gland. Immunohistochemistry (IHC) and in situ hybridization (ISH) showed that lacrimal acinar cells expressed both immunoreactivity and mRNA for CA VI. Moreover, CA VI immunoreactivity was occasionally observed in the lumen of the ducts. Unlike the parotid gland, in which all acinar cells expressed CA VI immunoreactivity and mRNA, only some of the acinar cells of the lacrimal gland showed expression. These results indicate that the lacrimal gland synthesizes and secretes a very small amount of salivary CA VI. In tear fluid, CA VI is presumed to have a role in the maintenance of acid/base balance on the surface of the eye, akin to its role in the oral cavity.

Purification of carbonic anhydrase isozyme VI (CA-VI) from swine saliva

Salivary or secreted carbonic anhydrase (CA), which constitutes a new class of CA, designated CA-VI, was isolated. Swine CA-VI purified from swine saliva by inhibitor-affinity chromatography and ion exchange chromatography had a specific activity of 5,468 units/mg. The molecular weight was 250,000, as determined by gel filtration under non-denaturing conditions, and the subunit molecular weight was found to be 37,000 by sodium dodecyl sulfate polyacrylamide gel electrophoresis, indicating that swine CA-VI consists of 7 subunits. The treatment of the enzyme with endo-N-acetylglucosaminidase F reduced its subunit molecular weight from 37,000 to 35,000 and 32,000. We raised a rabbit antibody against purfied swine CA-VI. Double immunodiffusion showed that anti-swine CA-VI serum reacted with swine CA-VI and swine saliva, but not with hemolysate (containing CA-I and CA-Il) or muscle extracts (containing CA-III). The concentration of CA-VI in swine saliva, measured using single radial immunodiffusion, was 0.027 +/- 0.017 mg/mg total protein.

Molecular cloning and characterization of GalNAc 4-sulfotransferase expressed in human pituitary gland

We have previously cloned chondroitin-4-sulfotransferase (C4ST) cDNA from mouse brain. In this paper, we report cloning and characterization of GalNAc 4-sulfotransferase (GalNAc4ST), which transfers sulfate to position 4 of the nonreducing terminal GalNAc residue. The obtained cDNA contains a single open reading frame that predicts a type II transmembrane protein composed of 424 amino acid residues. Identity of the amino acid sequence between GalNAc4ST and human C4ST was 30%. When the cDNA was transfected in COS-7 cells, sulfotransferase activity toward carbonic anhydrase VI was overexpressed but no sulfotransferase activity toward chondroitin or desulfated dermatan sulfate was increased over the control. Sulfation of carbonic anhydrase VI by the recombinant GalNAc4ST occurred at position 4 of the GalNAc residue of N-linked oligosaccharides. The recombinant GalNAc4ST transferred sulfate to position 4 of GalNAc residue of p-nitrophenyl GalNAc, indicating that this sulfotransferase transfers sulfate to position 4 at the nonreducing terminal GalNAc residue. Dot blot analysis showed that the message of GalNAc4ST was expressed strongly in the human pituitary, suggesting that the cloned GalNAc4ST may be involved in the synthesis of the nonreducing terminal GalNAc 4-sulfate residues found in the N-linked oligosaccharides of pituitary glycoprotein hormones.

Chromatographic and electrophoretic methods related to the carbonic anhydrase isozymes

There are three gene families that encode zinc metalloenzymes that catalyze the reversible hydration of CO2. The encoded enzymes are termed carbonic anhydrases (CAs). The CA isozymes have been purified from representatives of all types of organisms. Most CAs are strongly inhibited by aromatic sulfonamides. Several chromatographic and electrophoretic methods have been devised to determine binding constants for sulfonamides to CAs, and these compounds have been extensively used for, often single-step, affinity chromatographic separation of CAs from complex matrixes. The purification of different CA isozymes from different organisms is reviewed, as are methods for detection of CAs during chromatography and electrophoresis.

Sequence of bovine carbonic anhydrase VI: potential recognition sites for N-acetylgalactosaminyltransferase

Carbonic anhydrases (CAs I-VII) are products of a gene family that encodes seven isoenzymes and several CA-related proteins. We report the cloning and sequencing of the cDNA clones encoding one of these isoenzymes, CA VI, from bovine submaxillary gland. The translated polypeptide consists of 319 amino acids, including a signal peptide (14 amino acids) typical of secreted proteins. The predicted mature protein contains 305 amino acids including a 13-amino-acid C-terminal sequence that is also present in the sheep but absent in human CA VI. The deduced mature bovine protein is 87% and 68% identical to that of sheep and human CA VI, respectively. Active-site residues of the enzyme, as well as the three zinc-binding histidines and the two cysteines involved in an intra-chain disulphide bond, are all conserved in the three species. Two potential Asn-glycosylation sites are also conserved, both of which appear to be glycosylated in sheep and bovine CA VI. Two potential peptide recognition sequences are present in bovine CA VI for the glycoprotein hormone: N-acetylgalactosaminyltransferase (GalNAc-transferase), which is one of the two transferases required to form GalNAc-4-SO4 in bovine CA VI-linked oligosaccharides. Specifically, these two sequences are Asp-Leu-Lys-Met-Lys-Lys and Ile-Thr-Lys-Arg-Lys-Lys. Comparison of these sequences with sheep and human CA VI sequences indicates that distinct glycoforms of CA VI could exist in submaxillary gland from different species.

Purification and characterization of the GalNAc-4-sulfotransferase responsible for sulfation of GalNAc beta 1,4GlcNAc-bearing oligosaccharides

The pituitary glycoprotein hormone lutropin is characterized by its pulsatile appearance in the bloodstream which is important for the expression of its biological activity in the ovary. We have previously shown that lutropin bears unique Asn-linked oligosaccharides terminating with GalNAc-4-SO4 which allow the hormone to be rapidly cleared from the bloodstream via a specific receptor in the liver, thus contributing to its pulsatile appearance in the circulation. Furthermore, we have found that carbonic anhydrase VI, synthesized by the submaxillary gland and secreted into the saliva, also bears Asn-linked oligosaccharides terminating with GalNAc-4-SO4, suggesting that this unique sulfated structure mediates other biological functions in addition to rapid clearance from the circulation. We report here the purification of a GalNAc-4-sulfotransferase which transfers sulfate to terminal beta 1,4-linked GalNAc on Asn-linked oligosaccharides. We show that the purified submaxillary gland enzyme has kinetic parameters identical to the pituitary enzyme, indicating that the same sulfotransferase is responsible for the sulfation of lutropin oligosaccharides in pituitary and carbonic anhydrase VI oligosaccharides in submaxillary gland. This GalNAc-4-sulfotransferase has an apparent molecular mass of 128 kDa and can be specifically photoaffinity radiolabeled with 3',5'-ADP, a competitive inhibitor of sulfotransferase activity. The acceptor specificity of this GalNAc-4-sulfotransferase indicates that it is able to transfer sulfate to terminal GalNAc beta 1,4GlcNAc on both N- and O-glycosidically linked oligosaccharides, suggesting that this enzyme is also responsible for the sulfation of O-linked glycans on proopiomelanocortin.

Radioimmunoassay for salivary carbonic anhydrase in human parotid saliva

A specific radioimmunoassay for human carbonic anhydrase (CA) VI has been developed and used to determine the concentrations of the enzyme in saliva. The assay detected as little as 200 pg of CA VI and the antibody used did not cross-react with CA II or other salivary proteins. The method showed an intra-assay variation of 8.5% and an inter-assay variation of 16.9%. The concentration in parotid saliva varied over a wide range (from 9.7 micrograms/ml to 121 micrograms/ml) with an average value of 47.0 +/- 39.2 (SD) micrograms/ml (n = 50). The mean secretion rate of CA VI from the combined parotid glands was 42.8 +/- 37.9 micrograms/min. CA VI represented about 3% of the total protein in parotid saliva.

Differential expression of GalNAc-4-sulfotransferase and GalNAc-transferase results in distinct glycoforms of carbonic anhydrase VI in parotid and submaxillary glands

Differential expression of glycosyltransferases has the potential to generate functionally distinct glycoforms of otherwise identical proteins. We have previously demonstrated the presence of unique oligosaccharides terminating with GalNAc-4-SO4 on the pituitary glycoproteins lutropin (LH), thyroid stimulating hormone (TSH), and pro-opiomelanocortin (POMC). A glycoprotein hormone:GalNAc-transferase and a GalNAc-4-sulfotransferase are present in the pituitary and can account for the synthesis of these unique oligosaccharides on specific glycoproteins. Both transferases are coordinately expressed in a number of tissues in addition to pituitary, including submaxillary gland, lacrimal gland, and kidney, suggesting that additional glycoproteins bearing oligosaccharides terminating with GalNAc-4-SO4 are synthesized in these tissues. In this study we show that while the glycoprotein hormone:GalNAc-transferase and the GalNAc-4-sulfotransferase are coordinately expressed in bovine submaxillary gland, the GalNAc-transferase is expressed in the parotid gland in the absence of the GalNAc-4-sulfotransferase. The relative expression of these two transferases in submaxillary and parotid glands correlates with the presence of unique Asn-linked oligosaccharides on carbonic anhydrase VI (CA VI) synthesized in each of these tissues. The majority of Asn-linked oligosaccharides on CA VI synthesized in submaxillary gland terminate with GalNAc-4-SO4. In contrast, CA VI which is synthesized in bovine parotid gland bears oligosaccharides which terminate predominantly with beta 1,4-linked GalNAc which is not sulfated. The presence of different terminal residues on the Asn-linked oligosaccharides of submaxillary and parotid CA VI thus correlates with the complement of transferases in these glands and suggests differing biological roles for submaxillary and parotid CA VI.

Solvent hydrogen isotope effects and anion inhibition of CO2 hydration catalysed by carbonic anhydrase from Pisum sativum

Chloroplast carbonic anhydrase from Pisum sativum has been studied to elucidate the catalytic mechanism and to test if the mechanism proposed for human carbonic anhydrase II is also valid for pea carbonic anhydrase. The catalytic activity was found to depend on the chemical nature of the buffer. Barbital buffer gives the highest turnover number at infinite buffer concentration and the lowest Km value with respect to the buffer, while the kinetic parameters obtained in the imidazole-type buffer, 1-methylimidazole, do not differ from those obtained using the biological-type buffer Mops. The anion inhibition of CO2 hydration was investigated using SCN- at pH 6-9. The binding of the anion was found to be pH dependent with the strongest interaction at low pH. We obtained an uncompetitive inhibition pattern at high pH and noncompetitive inhibition patterns at pH 7 and low pH. The catalytic mechanism was further tested by measurements of the solvent hydrogen isotope effects on the kinetic parameters for CO2 hydration. The observed effects were comparatively small with a kcat value of approximately 2 irrespective of the pH. The effect on kcat/Km and on Km changes when going from high pH to pH 7 and low pH. At high pH, the solvent isotope effect in Km is at least 3, giving a value below 1 for kcat/Km, while at pH 7 and low pH the major effect is found in kcat/Km with values of 2.6 and 2.9. The dependence of the CO2-hydration activity on the buffer concentration is in agreement with a ping-pong mechanism with buffer acting as a second substrate. This is analogous to the behaviour of human carbonic anhydrase II. The inhibition patterns and the observed isotope effects at high pH can also be explained within the framework of the catalytic mechanism for human carbonic anhydrase II, with a rate-determining and buffer-dependent part. The results are consistent with a mechanism involving a proton transfer that contributes to rate limitation. However, the isotope effects found at pH 7 and low pH indicate that some part of the mechanism has changed. Moreover, we cannot decide whether the mechanism for pea carbonic anhydrase involves an internal proton-shuttle group, or if the buffer molecule acts in a direct proton transfer from the zinc-coordinated water.

Radioimmunoassay of carbonic anhydrase VI in saliva and sheep tissues

A specific and sensitive radioimmunoassay has been developed for the measurement of the secreted carbonic anhydrase isoenzyme (CA VI) in sheep saliva and tissues. The assay can detect as little as 75 pg of CA VI, and the antibody used does not cross-react with CA II or CA III. The intra-assay variation, measured using a saliva sample, was 3.0%, whereas the inter-assay variation was 10.5%. The concentration of CA VI in parotid saliva from normal, resting sheep was 5.6 +/- 3.0 micrograms.ml-1 (n = 42) or 79.4 +/- 35.7 micrograms.mg of total protein-1. With feeding, the CA VI concentrations increased an average of 6-fold. The secretion rate of CA VI from the vascularly isolated neurotomized parotid gland of the anaesthetized sheep was 0.62 +/- 0.40 micrograms.min-1, compared with a rate of 11.7 +/- 7.8 micrograms.min-1 from the parotid gland of normal conscious sheep. Stimulation of the parotid-gland preparation by the muscarinic agent bethanechol increased the secretion rate to 438 +/- 172 microgram.min-1 (n = 8), and electrical stimulation of the secretomotor Moussu nerve increased CA VI secretion rate to 634 +/- 330 micrograms.min-1 (n = 4). Submandibular saliva from anaesthetized sheep contained 6.9 +/- 2.1 micrograms of CA VI.ml-1 (n = 3). The only tissues found to contain measurable amounts of CA VI were the parotid (6.4 micrograms.mg of protein-1) and submandibular (1.8 micrograms.mg of protein-1) salivary glands. The sublingual salivary gland, kidney, lung, adrenal, brain, skeletal muscle, liver, heart, pancreas, small intestine and cerebrospinal fluid did not have a measurable CA VI content.

Immunohistochemical studies of the carbonic anhydrase isozymes in the bovine placenta

Placentae from 11 cows, ranging from about 80 to 270 days gestation, were studied for immunohistochemical localization of carbonic anhydrase isozymes. CA isozymes were localized using the avidin-biotin-peroxidase complex (ABC) method in the bovine placenta. CA-II was found in the fetal trophoblastic cells with single nuclei as well as in erythrocytes of maternal and fetal blood. The percentage of positive cells for anti-CA-II in the trophoblastic cells did not change during development of gestation in the fetal bovine. CA-I and CA-III were not detected in the bovine placenta.

Carbonic anhydrase III in the salivary glands and kidney of the Japanese monkey (Macaca fuscata)

Carbonic anhydrase III (CA-III) was found in muscles of the Japanese monkey by the double immunodiffusion test and western blotting using antiserum raised against equine CA-III. Immunocytochemical localization of CA-III in the salivary glands and kidney of the monkey was studied using an avidin-biotinylated glucose oxidase complex. CA-III was found mainly in the striated duct and interlobular duct cells of the parotid glands. In the submandibular glands, striated duct, interlobular duct, and excretory duct cells were strongly stained for CA-III. In the kidney of the monkey, CA-III was localized mainly in the dark cells of the collecting duct at the medulla and in the epithelial cells of thick limb of Henle's loop.

Tissue and species distribution of the secreted carbonic anhydrase isoenzyme

The secreted carbonic anhydrases, CA VI, are high molecular mass, oligomeric enzymes originally found in the sheep parotid gland and saliva. The enzymes have been purified from the saliva or parotid glands of several different species. All the CA VI enzymes studied have an apparent subunit Mr of about 45,000 as previously reported for the sheep enzyme. By Western analysis, CA VI from human, cow and dog cross-reacted with antibody raised against the purified sheep enzyme whereas that of the mouse did not. The N-terminal sequences of the sheep, human, cow and mouse enzymes are reported. The sheep, cow and human N-terminal sequences are similar to one another while the mouse sequence is substantially different. Nevertheless, the amino acids in the aromatic cluster I (Trp-5, Tyr-7, Trp-16 and Tyr/Phe-20) have all been conserved, as is the case with the cytoplasmic carbonic anhydrases. Eighteen tissues from the sheep have been examined for the presence of CA VI by Western analysis but it has been found only in the salivary glands. Northern analysis and hybridization histochemistry show that the mRNA for CA VI in sheep is expressed specifically in the acinar cells of the parotid and submandibular glands.

The immunohistolocalization of carbonic anhydrase III in the submandibular gland of rats and hamsters

Carbonic anhydrase III has been localized using the avidin-biotin-glucose oxidase complex (ABC) method in the submandibular gland of the rat and hamster. This isozyme, which is predominant in skeletal muscle, was observed in intercalated duct, striated duct and excretory duct cells in the rat submandibular glands. In contrast, only some striated duct cells in hamster submandibular glands were stained.