Carbonic anhydrases (CAs, EC 4.2.1.1) are wide-spread enzymes, present in mammals in at least 14 different isoforms. Some of these isozymes are cytosolic (CA I, CA II, CA III, CA VII, CA XIII), others are membrane-bound (CA IV, CA IX, CA XII and CA XIV), CA V is mitochondrial and CA VI is secreted in the saliva and milk. Three cytosolic acatalytic forms are also known (CARP VIII, CARP X and CARP XI). The catalytically active isoforms, which play important physiological and patho-physiological functions, are strongly inhibited by aromatic and heterocyclic sulfonamides. The catalytic and inhibition mechanisms of these enzymes are understood in great detail, and this greatly helped the design of potent inhibitors, some of which possess important clinical applications. The use of such CA inhibitors (CAIs) as antiglaucoma drugs are discussed in detail, together with the recent developments that led to isozyme-specific and organ-selective inhibitors. A recent discovery is connected with the involvement of CAs and their sulfonamide inhibitors in cancer: many potent CAIs were shown to inhibit the growth of several tumor cell lines in vitro and in vivo, thus constituting interesting leads for developing novel antitumor therapies. Future prospects for drug design of inhibitors of these ubiquitous enzymes are dealt with. Although activation of CAs has been a controversial issue for some time, recent kinetic, spectroscopic and X-ray crystallographic experiments offered an explanation of this phenomenon, based on the catalytic mechanism. It has been demonstrated recently, that molecules that act as carbonic anhydrase activators (CAAs) bind at the entrance of the enzyme active site participating in facilitated proton transfer processes between the active site and the reaction medium. In addition to CA II-activator adducts, X-ray crystallographic studies have been also reported for ternary complexes of this isozyme with activators and anion (azide) inhibitors. Structure-activity correlations for diverse classes of activators is discussed for the isozymes for which the phenomenon has been studied, i.e, CA I, II, III and IV. The possible physiological relevance of CA activation/inhibition is also addressed, together with recent pharmacological/biomedical applications of such compounds in different fields of life sciences.

Carbonic anhydrase (CA) IX is a plasma membrane-associated member of the α-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 α-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 α-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.

Hereditary hemochromatosis (HH) is a common autosomal recessive disease characterized by increased iron absorption and progressive iron storage that results in damage to major organs in the body. Recently, a candidate gene for HH called HFE encoding a major histocompatibility complex class I-like protein was identified by positional cloning. Nearly 90% of Caucasian HH patients have been found to be homozygous for the same mutation (C282Y) in the HFE gene. To test the hypothesis that the HFE gene is involved in regulation of iron homeostasis, we studied the effects of a targeted disruption of the murine homologue of the HFE gene. The HFE-deficient mice showed profound differences in parameters of iron homeostasis. Even on a standard diet, by 10 weeks of age, fasting transferrin saturation was significantly elevated compared with normal littermates (96 ± 5% vs. 77 ± 3%, P < 0.007), and hepatic iron concentration was 8-fold higher than that of wild-type littermates (2,071 ± 450 vs. 255 ± 23 μg/g dry wt, P < 0.002). Stainable hepatic iron in the HFE mutant mice was predominantly in hepatocytes in a periportal distribution. Iron concentrations in spleen, heart, and kidney were not significantly different. Erythroid parameters were normal, indicating that the anemia did not contribute to the increased iron storage. This study shows that the HFE protein is involved in the regulation of iron homeostasis and that mutations in this gene are responsible for HH. The knockout mouse model of HH will facilitate investigation into the pathogenesis of increased iron accumulation in HH and provide opportunities to evaluate therapeutic strategies for prevention or correction of iron overload.

To determine if alcoholic liver fibrogenesis is exacerbated by dietary iron supplementation, carbonyl iron (0.25% wt/vol) was intragastrically infused with or without ethanol to rats for 16 wk. Carbonyl iron had no effect on blood alcohol concentration, hepatic biochemical measurements, or liver histology in control animals. In both ethanol-fed and control rats, the supplementation produced a two- to threefold increase in the mean hepatic non-heme iron concentration but it remained within or near the range found in normal human subjects. As previously shown, the concentrations of liver malondialdehyde (MDA), liver 4-hydroxynonenal (4HNE), and serum aminotransferases (ALT, AST) were significantly elevated by ethanol infusion alone. The addition of iron supplementation to ethanol resulted in a further twofold increment in mean MDA, 4HNE, ALT, and AST. On histological examination, focal fibrosis was found < 30% of the rats fed ethanol alone. In animals given both ethanol and iron, fibrosis was present in all, with a diffuse central-central bridging pattern in 60%, and two animals (17%) developed micronodular cirrhosis. The iron-potentiated alcoholic liver fibrogenesis was closely associated with intense and diffuse immunostaining for MDA and 4HNE adduct epitopes in the livers. Furthermore, in these animals, accentuated increases in procollagen alpha 1(I) and TGF beta 1 mRNA levels were found in both liver tissues and freshly isolated hepatic stellate cells, perisinusoidal cells believed to be a major source of extracellular matrices in liver fibrosis. The dietary iron supplementation to intragastric ethanol infusion exacerbates hepatocyte damage, promotes liver fibrogenesis, and produces evident cirrhosis in some animals. These results provide evidence for a critical role of iron and iron-catalyzed oxidant stress in progression of alcoholic liver disease.

We recently reported the positional cloning of a candidate gene for hereditary hemochromatosis (HH), calledHLA-H, which is a novel member of the major histocompatibility complex class I family. A mutation in this gene, cysteine 282 → tyrosine (C282Y), was found to be present in 83% of HH patient DNAs, while a second variant, histidine 63 → aspartate (H63D), was enriched in patients heterozygous for C282Y. The functional relevance of either mutation has not been described. Co-immunoprecipitation studies of cell lysates from human embryonic kidney cells transfected with wild-type or mutant HLA-H cDNA demonstrate that wild-type HLA-H binds β₂-microglobulin and that the C282Y mutation, but not the H63D mutation, completely abrogates this interaction. Immunofluorescence labeling and subcellular fractionations demonstrate that while the wild-type and H63D HLA-H proteins are expressed on the cell surface, the C282Y mutant protein is localized exclusively intracellularly. This report describes the first functional significance of the C282Y mutation by suggesting that an abnormality in protein trafficking and/or cell-surface expression of HLA-H leads to HH disease.

Hereditary hemochromatosis (HH) is the most common autosomal recessive disorder known in humans. A candidate gene for HH called HFE has recently been cloned that encodes a novel member of the major histocompatibility complex class I family. Most HH patients are homozygous for a Cys-282→Tyr (C282Y) mutation in HFE gene, which has been shown to disrupt interaction with β ₂ -microglobulin; a second mutation, His-63→Asp (H63D), is enriched in HH patients who are heterozygous for C282Y mutation. The aims of this study were to determine the effects of the C282Y and H63D mutations on the cellular trafficking and degradation of the HFE protein in transfected COS-7 cells. The results indicate that, while the wild-type and H63D HFE proteins associate with β ₂ -microglobulin and are expressed on the cell surface of COS-7 cells, these capabilities are lost by the C282Y HFE protein. We present biochemical and immunofluorescence data that indicate that the C282Y mutant protein: ( i ) is retained in the endoplasmic reticulum and middle Golgi compartment, ( ii ) fails to undergo late Golgi processing, and ( iii ) is subject to accelerated degradation. The block in intracellular transport, accelerated turnover, and failure of the C282Y protein to be presented normally on the cell surface provide a possible basis for impaired function of this mutant protein in HH.