Erythrocyte carbonic anhydrase I: inherited deficiency in humans

Spontaneous regression of tumours. Possible cross reactivity of autoantibodies against carbonic anhydrase I

Spontaneous tumour regression in patients after high dose therapy and autologous stem cell transplantation or patients with standard therapy is accompanied with the presence of high titers autoantibodies against carbonic anhydrase I (CA I). The concomitant presence of aplastic anaemia-like syndrome in these patients points to parallel bone marrow suppression during this period. It seems that CA I, an 'obscure' enzyme, does not have any significant physiological role in humans. One possible explanation points to the fact that autoantibodies against CA I may target another antigen(s) which is(are) important in tumour growth as well as in normal haematopoiesis. One of the candidates for such a target is the DNA polymerase theta.

Carbonic Anhydrases in Metazoan Model Organisms: Molecules, Mechanisms, and Physiology

During the past three decades, mice, zebrafish, fruit flies, and Caenorhabditis elegans have been the primary model organisms used for the study of various biological phenomena. These models have also been adopted and developed to investigate the physiological roles of carbonic anhydrases (CAs) and carbonic anhydrase-related proteins (CARPs). These proteins belong to eight CA families and are identified by Greek letters: α, β, γ, δ, ζ, η, θ, and ι. Studies using model organisms have focused on two CA families, α-CAs and β-CAs, which are expressed in both prokaryotic and eukaryotic organisms with species-specific distribution patterns and unique functions. This review covers the biological roles of CAs and CARPs in light of investigations performed in model organisms. Functional studies demonstrate that CAs are not only linked to the regulation of pH homeostasis, the classical role of CAs but also contribute to a plethora of previously undescribed functions.

Overexpression of CA1 mRNA and the CA I Protein in Tumor Cells Does Not Change the Gene Expression of the ECM Proteins

In our study, we performed retroviral transduction to overexpress codon-optimized variant of gene encoding human carbonic anhydrase I (optiCA1) in two tumor cell lines PC3 and MDA-MB-231, derived from human prostatic and breast carcinoma respectively. We achieved significantly enhanced and stable overexpression of exogenous optiCA1 gene. The expression of endogenous, wild CA1 gene was found to be normally low (C_t 28.6 for PC3 cells) or below to the detection limit (C_t 35.5 for MDA-MB-231 cells). No morphological changes and no decreasing viability of tumor cells were observed upon stable overexpression of the optiCA1 gene. In our study we have shown that the overexpression of the optimized human CA1 in engineered PC3 and MDA-MB-231 cells did not induce similar changes as we observed in tumor cells cultivated in the presence of human sera containing extensively high titers of anti-CA I autoantibodies from patients with complete remission of malignant disease. In both optiCA1transduced cell lines, the expression of selected genes responsible for basal lamina assembly, cytoskeleton, extracellular matrix proteins and proto-oncogenes (COL1A1, COL4A4, LAMC2, CTHRC1, and WNT7B) was not changed.

A thylakoid-located carbonic anhydrase regulates CO2 uptake in the cyanobacterium Synechocystis sp. PCC 6803

Carbonic anhydrases (CAs) are involved in CO₂ uptake and conversion, a fundamental process in photosynthetic organisms. Nevertheless, the mechanism underlying the regulation of CO₂ uptake and intracellular conversion in cyanobacteria is largely unknown. We report the characterization of a previously unrecognized thylakoid-located CA Slr0051 (EcaB) from the cyanobacterium Synechocystis sp. PCC 6803, which possesses CA activity to regulate CO₂ uptake. Inactivation of ecaB stimulated CO₂ hydration in the thylakoids, suppressed by the classical CA inhibitor acetazolamide. Absence of ecaB increased the reduced state of the photosynthetic electron transport system, lowered the rate of photosynthetic O₂ evolution at high light (HL) and pH, and decreased the cellular affinity for extracellular inorganic carbon. Furthermore, EcaB was upregulated in cells grown at limiting CO₂ concentration or HL in tandem with CupA. EcaB is mainly located in the thylakoid membranes where it interacts with CupA and CupB involved in CO₂ uptake by converting it to bicarbonate. We propose that modulation of the EcaB level and activity in response to CO₂ changes, illumination or pH reversibly regulates its conversion to HCO₃ by the two CO₂ -uptake systems (CupA, CupB), dissipating the excess HCO₃^- and alleviating photoinhibition, and thereby optimizes photosynthesis, especially under HL and alkaline conditions.

The Human Carbonic Anhydrase II in Platelets: An Underestimated Field of Its Activity

Carbonic anhydrases constitute a group of enzymes that catalyse reversible hydration of carbon dioxide leading to the formation of bicarbonate and proton. The platelet carbonic anhydrase II (CAII) was described for the first time in the '80s of the last century. Nevertheless, its direct role in platelet physiology and pathology still remains poorly understood. The modulation of platelet CAII action as a therapeutic approach holds promise as a novel strategy to reduce the impact of cardiovascular diseases. This short review paper summarises the current knowledge regarding the role of human CAII in regulating platelet function. The potential future directions considering this enzyme as a potential drug target and important pathophysiological chain in platelet-related disorders are described.

Carbonic anhydrase activators

Mammalian carbonic anhydrases (CAs; EC 4.2.1.1) of which 16 isoforms are known, are involved in important physiological functions. Their inhibition is exploited pharmacologically for the treatment of many diseases (glaucoma, edema, epilepsy, obesity, hypoxic tumors, neuropathic pain, etc.) but the activators were less investigated till recently. A review on the CA activation is presented, with the activation mechanism, drug design approaches of activators and comparison of the various isoforms activation profiles being discussed. Some CAs, which are abundant in the brain, were recently demonstrated to be activatable by drug-like compounds, affording the possibility to design agents that enhance cognition, with potential therapeutic applications in aging and neurodegenerative diseases as well as tissue engineering.

Silencing of CA1 mRNA in tumour cells does not change the gene expression of the extracellular matrix proteins

We report the silencing of CA1 mRNA in PC3 and MDA cells. The levels of mRNA coding CA1 protein in the knock-down mRNA (CA1 siRNA) cells have been measured by RT-PCR and were approximately 5% (PC3) and 20% (MDA-MB-231), respectively, of the level of control (Mock siRNA) used during silencing. In PC3 and MDA-MB-231 cells, the mRNAs for COL1A1 and COL4A4 were up-regulated. The mRNAs for CTHRC1, LAMC2, and WNT7B were not changed when compared to the control. The morphology of the cells during the treatments remained the same. On the Western blots, the lysate from the silenced cells showed lower levels of CA I as well.

Targeting erythrocyte carbonic anhydrase and 18O-isotope of breath CO2 for sorting out type 1 and type 2 diabetes

The inability to envisage the acute onset and progression of type 1 diabetes (T1D) has been a major clinical stumbling block and an important area of biomedical research over the last few decades. Therefore there is a pressing need to develop a new and an effective strategy for early detection of T1D and to precisely distinguish T1D from type 2 diabetes (T2D). Here we describe the precise role of the enzymatic activity of carbonic anhydrase (CA) in erythrocytes in the pathogenesis of T1D and T2D. We show that CA activities are markedly altered during metabolism of T1D and T2D and this facilitates to the oxygen-18 (¹⁸O) isotopic fractionations of breath CO₂. In our observations, T1D exhibited considerable depletions of ¹⁸O-isotopes of CO_2, whereas T2D manifested isotopic enrichments of ¹⁸O in breath CO₂, thus unveiling a missing link of breath¹⁸O-isotopic fractionations in T1D and T2D. Our findings suggest that the alterations in erythrocytes CA activities may be the initial step of altered metabolism of T1D and T2D, and breath ¹⁸O-isotope regulated by the CA activity is a potential diagnostic biomarker that can selectively and precisely distinguish T1D from T2D and thus may open a potential unifying strategy for treating these diseases.

Oxygen-18 isotope of breath CO₂ linking to erythrocytes carbonic anhydrase activity: a biomarker for pre-diabetes and type 2 diabetes

Carbonic anhydrase (CA), a well-characterized metalloenzyme, is associated with oxygen-18 ( ¹⁸O)-isotopic fractionations of CO₂. To investigate how CA activity links the ¹⁸O of breath CO₂ to pre-diabetes (PD) and type 2 diabetes (T2D) during metabolism, we studied pre- and post-dose CA activities in erythrocytes with simultaneous monitoring of ¹⁸O/ ¹⁶O-isotope ratios of breath CO₂ and thereafter elucidated potential metabolic pathways underlying CA alteration in the pathogenesis of T2D. Here we show that the post-dose CA activity in both T2D and PD was markedly enhanced, whereas the non-diabetic controls (NDC) exhibited a considerable reduction in post-dose CA activity when compared with their basal CA activities. However, T2D and PD exhibited isotopic enrichments of ¹⁸O in breath CO₂, while a marked depletion of ¹⁸O in CO₂ was manifested in NDC. Thus, the isotopic enrichments and depletions of ¹⁸O in breath CO₂ were well correlated with the changes in CA activities for controls, PD and T2D. Our findings suggest the changes in CA activities in erythrocytes may contribute to the pathogenesis of T2D and the breath C ¹⁸O ¹⁶O regulated by the CA activity as a potential biomarker for non-invasive assessment of T2D, and thus may open a new method for treating T2D.

Expression of a novel carbonic anhydrase, CA XIII, in normal and neoplastic colorectal mucosa

Background

Carbonic anhydrase (CA) isozymes may have an important role in cancer development. Some isozymes control pH homeostasis in tumors that appears to modulate the behaviour of cancer cells. CA XIII is the newest member of the CA gene family. It is a cytosolic isozyme which is expressed in a number of normal tissues. The present study was designed to investigate CA XIII expression in prospectively collected colorectal tumor samples.

Methods

Both neoplastic and normal tissue specimens were obtained from the same patients. The analyses were performed using CA XIII-specific antibodies and an immunohistochemical staining method. For comparison, the tissue sections were immunostained for other cytosolic isozymes, CA I and II.

Results

The results indicated that the expression of CA XIII is down-regulated in tumor cells compared to the normal tissue. The lowest signal was detected in carcinoma samples. This pattern of expression was quite parallel for CA I and II.

Conclusion

The down-regulation of cytosolic CA I, II and XIII in colorectal cancer may result from reduced levels of a common transcription factor or loss of closely linked CA1, CA2 and CA13 alleles on chromosome 8. Their possible role as tumor suppressors should be further evaluated.

Pharmacogenomics of diuretic drugs: data on rare monogenic disorders and on polymorphisms and requirements for further research

This review summarizes the current status of our knowledge about the role of pharmacogenetic variation in response to diuretics and suggests future research topics for the field. Genes with a role in the pharmacokinetics of most diuretics are renal drug transporters, especially OAT1, OAT3 and OCT2 (genes SLC22A6, SLC22A8 and SLC22A2) whereas variants in carbonic anhydrase (CA), cytochrome P450 enzymes and sulfotransferases are relevant only for specific substances. Genes on the pharmacodynamic side include the primary targets of thiazide, loop, K+-sparing and aldosterone antagonistic diuretics: NCC, NKCC2, ENaC and the mineralocorticoid receptor (genes SLC12A3, SLC12A1, SCNN1A, B, G and NR3C2). Rare variants of these proteins cause Gitelman’s syndrome, Bartter’s syndrome, Liddle’s syndrome or pregnancy-induced hypertension. Polymorphisms in these and in associated proteins such as GNB3, α-adducin and angiotensin-converting enzyme (ACE) seem to be clinically relevant. In conclusion, first knowledge has evolved that efficacy of diuretic drugs may be determined by genetic polymorphisms in genes determining pharmacokinetics and pharmacodynamics of this drug class. In the future, the selection of a diuretic drug or the dosing schedules may be individually chosen based on pharmacogenetic parameters, however, many questions remain to be answered before this fantasy becomes reality.

A Membrane Metabolon Linking Carbonic Anhydrase with Chloride/Bicarbonate Anion Exchangers

The erythrocyte Cl-/HCO3- anion exchanger (AE1, Band 3) and the enzyme carbonic anhydrase (CA) catalyze interconnected processes involved in bicarbonate metabolism. The high activity form of carbonic anhydrase, CAII binds to an acidic motif located within the carboxyl-terminal tail of anion exchangers via its basic amino-terminal region. CAII is thereby positioned at the cytosolic surface of the membrane, ideally placed to catalyze CO2 hydration and to channel bicarbonate to or from the anion exchanger. This association of a soluble enzyme and a membrane transporter may be an example of a metabolon, a weakly associated complex of sequential metabolic enzymes.

Carbonic anhydrase IV activity is localized on the exterior surface of human erythrocytes

Carbonic anhydrase (CA) cytoplasmic isozymes CA I and CA II were found in human erythrocyte membrane ghosts, when prepared at pH 5.4 and pH 7.4, but not in ghosts prepared at pH 8.2. These findings could indicate that previously reported CA activity of ghosts was owing to contamination by CA I and CA II during the preparation of the ghosts. However, using a sensitive micro-assay, CA activity was also found in ghosts prepared at pH 8.2. This activity constitutes 0.2% of the erythrocytes' total CA-activity, and originates from a membrane-associated isoform of CA, located at the exterior membrane surface. It has immunochemical and kinetic properties like those of the membrane-bound CA IV, previously isolated from kidney, lung and small blood vessels. Its function is possibly to interact with the red cell membrane anionic transport protein, band 3, for the bicarbonate/chloride exchange.

Carbonic anhydrase II binds to the carboxyl terminus of human band 3, the erythrocyte C1-/HCO3- exchanger

In this study, we provide evidence that the 33-residue carboxyl-terminal (Ct) region of the human erythrocyte chloride/bicarbonate exchanger, band 3, binds carbonic anhydrase II (CAII). Immunofluorescence showed that tomato lectin-mediated clustering of band 3 in ghost membranes caused a similar clustering of CAII, indicating an in situ association. CAII cosolubilized and coimmunoprecipitated with band 3, suggesting that the two proteins form a complex. Band 3 (K1/2 = 70 nM) or the membrane domain of band 3 (K1/2 = 100 nM) bound saturably to immobilized CAII in a solid phase binding assay. The interaction with CAII was specifically blocked by an antibody to the Ct of band 3. Affinity blotting showed that a glutathione S-transferase (GST)-fusion protein (GST-Ct) containing the last 33 residues of human band 3 bound to CAII. The solid phase binding assay showed that binding of GST-Ct to immobilized CAII was saturable (K1/2 = 20 nM). The binding rate was slow (t1/2 = 12 h) at physiological ionic strength and pH but was enhanced at low ionic strength or acidic pH. Intact band 3 (Ki = 15 nM), the membrane domain of band 3 (Ki = 100 nM), or antibodies to the Ct of band 3 were able to block GST-Ct binding to CAII, confirming the specificity of the interaction. Affinity chromatography showed that CAII bound to immobilized GST-Ct with a 1:1 stoichiometry. This work indicates that CAII, the bicarbonate supplier, is directly coupled to band 3, the chloride/bicarbonate exchanger in red blood cells.

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.

Structure and mechanism of carbonic anhydrase

Carbonic anhydrase (CA; carbonate hydro-lyase, EC 4.2.1.1) is a zinc-containing enzyme that catalyzes the reversible hydration of carbon dioxide: CO2+ H2O<-->HCO3(-)+H+. The enzyme is the target for drugs, such as acetazolamide, methazolamide, and dichlorphenamide, for the treatment of glaucoma. There are three evolutionarily unrelated CA families, designated alpha, beta, and gamma. All known CAs from the animal kingdom are of the alpha type. There are seven mammalian CA isozymes with different tissue distributions and intracellular locations, CA I-VII. Crystal structures of human CA I and II, bovine CA III, and murine CA V have been determined. All of them have the same tertiary fold, with a central 10-stranded beta-sheet as the dominating secondary structure element. The zinc ion is located in a cone-shaped cavity and coordinated to three histidyl residues and a solvent molecule. Inhibitors bind at or near the metal center guided by a hydrogen-bonded system comprising Glu-106 and Thr-199. The catalytic mechanism of CA II has been studied in particular detail. It involves an attack of zinc-bound OH- on a CO2 molecule loosely bound in a hydrophobic pocket. The resulting zinc-coordinated HCO3- ion is displaced from the metal ion by H2O. The rate-limiting step is an intramolecular proton transfer from the zinc-bound water molecule to His-64, which serves as a proton shuttle between the metal center and buffer molecules in the reaction medium.

Mutation creates an open reading frame within the 5' untranslated region of macaque erythrocyte carbonic anhydrase (CA) I mRNA that suppresses CA I expression and supports the scanning model for translation

A variant allele at the CA I locus that produces a deficiency of erythrocyte-specific CA I occurs as a widespread polymorphism in pigtail macaques from southeast Asia. Sequence analyses revealed a C----G substitution 12 nucleotides downstream of the cap site in the variant erythrocyte CA I mRNA. This mutation forms a new AUG start site and an open reading frame coding for 26 amino acids that terminates 6 nucleotides before the normal AUG initiation codon for CA I. It appears that the presence of this upstream open reading frame greatly diminishes reinitiation of translation from the normal start site, resulting in trace levels of CA I in erythrocytes. Preferential use of the first AUG codon supports the scanning model for translation initiation in eukaryotes.

Ectopic expression of chloramphenicol acetyltransferase (CAT) in the cerebellum in mice transgenic for a carbonic anhydrase II promoter-CAT construct that is without apparent phenotypic effect

We have developed six transgenic lines of mice with constructs containing presumptive 5' regulatory regions of carbonic anhydrase II (CA II). Four of the lines contained 1,100 bases of the 5' flanking region of the human CA II gene, and two transgenic lines resulted from a construct containing 500 bases of the 5' flanking region of the mouse CA II gene. Tissue-specific expression of the chloramphenicol acetyltransferase (CAT) gene was not obtained in any of the transgenic lines. One of the transgenic lines was found to have high levels of expression of CAT in cerebellum. This expression persisted through multiple generations and was independent of the parental origin of the transgene. On the assumption that the expression was due to the insertion of the transgene in or near a gene expressed normally in cerebellum, homozygous mice were bred for the transgenic insert to see if a mutation might have been induced. Homozygous mice were found and seemed to be normal in all aspects of their phenotype studied. Thus, in this case, neither the insertion of the gene nor the ectopic expression of CAT seemed to be harmful to the animals.

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.

Nucleotide sequence, tissue-specific expression, and chromosome location of human carbonic anhydrase III: the human CAIII gene is located on the same chromosome as the closely linked CAI and CAII genes

The carbonic anhydrases (CA) are a class of metalloenzymes that catalyze the reversible hydration of carbon dioxide. The genes for the carbonic anhydrase isozymes are members of a multigene family that are differentially expressed in a number of cell types. We have isolated a full-length representative of a CAIII mRNA transcript from an adult human muscle cDNA library, and we present the complete nucleotide sequence of this cDNA clone. RNA blots demonstrate that CAIII messages can be detected in a variety of cell types but that high-level expression is limited to human fetal and adult skeletal muscle and to rodent slow skeletal muscle and liver. In addition, we have used a panel of human-mouse cell hybrids to localize the human CAIII gene to chromosome 8. Previous reports have established the CAI and CAII isozyme genes to be closely linked on chromosome 8, and the assignment of the CAIII gene to the same chromosome raises the possibility that these genes may all be linked at a single complex locus.

Nucleotide sequence and derived amino acid sequence of a cDNA encoding human muscle carbonic anhydrase

We report the nucleotide (nt) sequence of a full length cDNA clone, pCA15, which encodes the human muscle-specific carbonic anhydrase, CAIII. pCA15 identifies a 1.7-kb mRNA, which is present at high levels in skeletal muscle, at much lower levels in cardiac and smooth muscle and which appears to be developmentally regulated. The CAIII mRNA is distinguished by a 887-nt long 3'-untranslated region, containing two AAUAAA signal sequences and is longer than either of the mRNAs encoding the erythrocyte CAs, CAI and CAII, which each have relatively shorter 3'-untranslated regions, 360 and 670 nt long, respectively. The derived amino acid (aa) sequence for human CAIII shows 85% homology with ox CAIII, 62% homology with human CAII and 54% with human CAI when simple pairwise aa comparisons are made. We describe an allelic variation at a TaqI restriction site for CAIII which occurs at high frequency in the European population.

Carbonic anhydrase II deficiency in 12 families with the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification

Osteopetrosis with renal tubular acidosis and cerebral calcification was identified as a recessively inherited syndrome in 1972. In 1983, we reported a deficiency of carbonic anhydrase II, one of the isozymes of carbonic anhydrase, in three sisters with this disorder. We now describe our study of 18 similarly affected patients with this syndrome in 11 unrelated families of different geographic and ethnic origins. Virtual absence of the carbonic anhydrase II peak on high-performance liquid chromatography, of the esterase and carbon dioxide hydratase activities of carbonic anhydrase II, and of immunoprecipitable isozyme II was demonstrated on extracts of erythrocyte hemolysates from all patients studied. Reduced levels of isozyme II were found in obligate heterozygotes. These observations demonstrate the generality of the findings that we reported earlier in one family and provide further evidence that a deficiency of carbonic anhydrase II is the enzymatic basis for the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification. We also summarize the clinical findings in these families, propose mechanisms by which a deficiency of carbonic anhydrase II could produce this metabolic disorder of bone, kidney, and brain, and discuss the clinical evidence for genetic heterogeneity in patients from different kindreds with this inborn error of metabolism.

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.

Genetic variation in the carbonic anhydrase isozymes of macaque monkeys. IV. Degradation by heat and proteolysis of normal and variant carbonic anhydrase isozymes of Macaca nemestrina

Studies were undertaken on the heat denaturation and proteolytic degradation by alpha-chymotrypsin of the normal red cell carbonic anhydrase isozyme, CA II, and two electrophoretic variants of carbonic anhydrase I, CA Ia and CA Ib, of the pigtail macaque. The heat degradation results showed a difference of about 40-fold in the rate constants between CA Ia and CA Ib, which is due to the marked thermostability of CA Ib compared to CA Ia. The enthalpies and entropies of activation were calculated from the heat denaturation constants. These values were compared, on enthalpy-entropy compensation plots, with those values previously determined for the human CA I and CA II isozymes. They were highly correlated and clearly fell into two distinct clusters, separated by about 200 kJ mol-1; one group comprising the macaque and human CA I isozymes and the other the CA II isozymes. The proteolytic degradation results showed that CA Ia is degraded about 2.5 times more rapidly than CA Ib by alpha-chymotrypsin. Thus, the characteristic 3/1 ratio of CA Ib/CA Ia in mature red cells could be accounted for by the greater susceptibility of CA Ia to degradation at some stage in red cell development.

Polymorphic gene for human carbonic anhydrase II: a molecular disease marker located on chromosome 8

A panel of 28 mouse-human somatic cell hybrids of known karyotype was screened for the presence of the human carbonic anhydrase II (CA II) gene, which encodes one of the three well-characterized, genetically distinct carbonic anhydrase isozymes (carbonate dehydratase; carbonate hydro-lyase, EC 4.2.1.1). The human and mouse CA II genes can be clearly distinguished by Southern blot analysis of BamHI-digested genomic DNA with a mouse CA II cDNA hybridization probe. The two major hybridizing fragments in mouse were 15 and 6.0 kilobase pairs, and in human they were 15 and 4.3 kilobase pairs. Analysis of the somatic cell hybrids by this technique identified those containing human CA II gene sequences. Segregation analysis of the molecular marker and chromosomes in cell hybrids indicated a clear correlation between the presence of chromosome 8 and the human CA II gene (CA2). This finding provides the second polymorphic marker for human chromosome 8 and, moreover, a molecular disease marker, because human CA II deficiency has recently been linked to an autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.

Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification

The clinical, radiological, and pathological findings in three siblings affected with the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification have been reported. In an effort to explain the pleiotropic effects of the mutation producing this disorder, we postulated a defect in carbonic anhydrase II (CA II), the only one of the three soluble isozymes of carbonic anhydrase that is known to be synthesized in kidney and brain. We report here biochemical and immunological evidence for the virtual absence of CA II in erythrocytes of patients affected with this condition, whereas CA I level is not reduced. Levels of CA II in erythrocyte hemolysates from asymptomatic obligate heterozygotes are about half of normal. These findings: (i) elucidate the basic defect in one form of inherited osteopetrosis; (ii) provide genetic evidence implicating CA II in osteoclast function and bone resorption; (iii) explain previous observations that carbonic anhydrase inhibitors block the normal parathyroid hormone-induced release of calcium from bone; (iv) clarify the role of renal CA II in urinary acidification and bicarbonate reabsorption; and (v) suggest a method to identify heterozygous carriers for the gene for this recessively inherited syndrome.

Inherited variants of human red cell carbonic anhydrases

The present state of knowledge concerning the genetic control of human red cell carbonic anhydrases I and II (CA I and CA II) is reviewed. A total of 25 electrophoretic variants, and one deficiency variant of CA I, and 7 electrophoretic variants of CA II have been discovered after screening a minimum of about 50,000 (CA I) and 39,000 (CA II) individual bloods from a variety of human populations. The amino acid substitution has been determined for 8 of the CA I variants and one of the CA II variants. Three previously undescribed variants of CA I (CA I Montreal-1, CA I Montreal-2, and CA I Montreal-3) and two new variants of CA II (CA II London and CA II Detroit) are reported. Three of the CA I variants (CA I Australia-1, CA I Bombay, and CA I Mindanao), and four of the CA II variants (CA II2, CA II Australia, CA II Bombay, and CA II Baniwa) were observed to occur at frequencies of greater than 1%; however, CA I Bombay, CA I Mindanao, CA II Bombay, and CA II Baniwa appear to be private polymorphisms limited to small populations. None of the electrophoretic variants, or the CA I deficiency variant, even in the homozygous state, appear to be associated with any clinical disorder. A defective form of CA I reported as possibly responsible for an inherited type of renal tubular acidosis has not been characterized sufficiently to exclude the possibility of secondary effects.

A search for the function of human carbonic anhydrase B

Because of the very high activity and abundance of human red cell carbonic anhydrase C (carbamate hydrolase, EC 4.2.1.1), it seemed likely that the second isozyme, B, might not be essential for CO2 metabolism. It was then found that physiological concentrations of Cl- inhibited catalysis of CO2 hydration by the B enzyme (but not by type C), suggesting further that type B does not function in vivo as a carbonic anhydrase. The versatility of the catalytic activity of carbonic anhydrase for a number of 'artificial' substrates suggested that enzyme B may be utilized in reactions of intermediary metabolism. A number of hydration, dehydration, decarboxylation, kinase, and phosphatase systems were tested to determine a possible physiological function for the enzyme. Results with eighteen possible substrates were negative and the possibility is discussed that mammalian carbonic anhydrase B is an evolutionary accident.