Carbonic anhydrase inhibitors: inhibition of human and murine mitochondrial isozymes V with anions

In addition to sulfonamides, metal complexing anions represent the second class of inhibitors of the zinc enzyme carbonic anhydrase (CA, EC 4.2.1.1). The first inhibition study of the mitochondrial isozyme CA V (of murine and human origin) with anions is reported here. Inhibition data of the cytosolic isozymes CA I and CA II as well as the membrane-bound isozyme CA IV with a large number of anionic species such as halides, pseudohalides, bicarbonate, nitrate, hydrosulfide, arsenate, sulfamate, and sulfamidate and so on, are also provided for comparison. Isozyme V has an inhibition profile by anions completely different to those of CA I and IV, but similar to that of hCA II, which may have interesting physiological consequences. Similarly to hCA II, the mitochondrial isozymes show micro-nanomolar affinity for sulfonamides such as sulfanilamide and acetazolamide.     

Carbonic anhydrase inhibitors: synthesis of sulfonamides incorporating 2,4,6-trisubstituted-pyridinium-ethylcarboxamido moieties possessing membrane-impermeability and in vivo selectivity for the membrane-bound (CA IV) versus the cytosolic (CA I and CA II) isozymes

A new approach is proposed for the selective in vivo inhibition of membrane-bound versus cytosolic carbonic anhydrase (CA, EC 4.2.1.1) isozymes with a class of positively-charged, membrane-impermeant sulfonamides. Aromatic/heterocyclic sulfonamides acting as strong (but unselective) inhibitors of this zinc enzyme were derivatized by the attachment of trisubstituted-pyridinium-ethylcarboxy moieties (obtained from 2,4,6-trisubstituted-pyrylium salts and beta-alanine) to the amino, imino, hydrazino or hydroxyl groups present in their molecules. Efficient in vitro inhibition (in the nanomolar range) was observed with some of the new derivatives against three investigated CA isozymes, i.e., hCA I, hCA II (cytosolic forms) and bCA IV (membrane-bound isozyme; h = human; b = bovine isozyme). Due to their salt-like character, the new type of inhibitors reported here, unlike the classical, clinically used compounds (such as acetazolamide, methazolamide, ethoxzolamide), are unable to penetrate biological membranes, as shown by ex vivo and in vivo perfusion experiments in rats. The level of bicarbonate excreted into the urine of the experimental animals perfused with solutions of the new and classical inhibitors suggest that: (i) when using the new type of positively-charged sulfonamides, only the membrane-bound enzyme (CA IV) was inhibited, whereas the cytosolic isozymes (CA I and II) were not affected, (ii) in the experiments in which the classical compounds (acetazolamide, benzolamide, etc.) were used, unselective inhibition of all CA isozymes (I, II and IV) occurred.     

Carbonic Anhydrase Inhibitors: Synthesis of Sulfonamides Incorporating 2, 4, 6–Trisubstituted-Pyridinium-Ethylcarboxamido Moieties Possessing Membrane-Impermeability and in Vivo Selectivity for the Membrane-Bound (CA IV) Versus the Cytosolic (CA I and CA II) Isozymes

A new approach is proposed for the selective in vivo inhibition of membrane-bound versus cytosolic carbonic anhydrase (CA, EC 4.2.1.1) isozymes with a class of positively-charged, membrane-impermeant sulfonamides. Aromatic/heterocyclic sulfonamides acting as strong (but unselective) inhibitors of this zinc enzyme were derivatized by the attachment of trisub-stituted-pyridinium-ethylcarboxy moieties (obtained from 2, 4, 6–trisubstituted-pyrylium salts and β-alanine) to the amino, imino, hydrazino or hydroxyl groups present in their molecules. Efficient in vitro inhibition (in the nanomolar range) was observed with some of the new derivatives against three investigated CA isozymes, i.e., hCA I, hCA II (cytosolic forms) and bCA IV (membrane-bound isozyme; h = human; b = bovine isozyme). Due to their salt-like character, the new type of inhibitors reported here, unlike the classical, clinically used compounds (such as acetazolamide, methazolamide, ethoxzolamide), are unable to penetrate biological membranes, as shown by CJ vivo and in vivo perfusion experiments in rats. The level of bicarbonate excreted into the urine of the experimental animals perfused with solutions of the new and classical inhibitors suggest that: (i) when using the new type of positively-charged sulfonamides. only the membrane-bound enzyme (CA IV) was inhibited. whereas the cytosolic isozymes (CA I and II) were not affected, (ii) in the experiments in which the classical compounds (acetazolamide, bcn-zolamíde. etc.) were used. unselective inhibition of all CA isozymes (I. II and IV) occurred.     

Carbonic anhydrase inhibitors: sulfonamides as antitumor agents?

Novel sulfonamide inhibitors of the zinc enzyme carbonic anhydrase (CA, EC 4.2.1.1) were prepared by reaction of aromatic or heterocyclic sulfonamides containing amino, imino, or hydrazino moieties with N,N-dialkyldithiocarbamates in the presence of oxidizing agents (sodium hypochlorite or iodine). The N,N-dialkylthiocarbamylsulfenamido-sulfonamides synthesized in this way behaved as strong inhibitors of human CA I and CA II (hCA I and hCA II) and bovine CA IV (bCA IV). For the most active compounds, inhibition constants ranged from 10(-8) to 10(-9) M (for isozymes II and IV). Three of the derivatives belonging to this new class of CA inhibitors were also tested as inhibitors of tumor cell growth in vitro. These sulfonamides showed potent inhibition of growth against several leukemia, non-small cell lung, ovarian, melanoma, colon, CNS, renal, prostate and breast cancer cell lines. With several cell lines. GI50 values of 10-75 nM were observed. The mechanism of antitumor action with the new sulfonamides reported here remains obscure, but may involve inhibition of CA isozymes which predominate in tumor cell membranes (CA IX and CA XII), perhaps causing acidification of the intercellular milieu, or inhibition of intracellular isozymes which provide bicarbonate for the synthesis of nucleotides and other essential cell components (CA II and CA V). Optimization of these derivatives from the SAR point of view, might lead to the development of effective novel types of anticancer agents.     

Carbonic anhydrase inhibitors. Inhibition of transmembrane isozymes XII (cancer-associated) and XIV with anions

Metal complexing anions represent an important class of inhibitors of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1). The first inhibition study of the transmembrane isozymes CA XII (tumor-associated) and XIV with anions is reported. These isozymes showed inhibition profiles with physiologic/non-physiologic anions quite distinct from any other cytosolic (CA I and II) or transmembrane isoforms (e.g., CA IX) investigated earlier. hCA XII has a good affinity for fluoride and bicarbonate but is not inhibited by heavier halides, perchlorate, nitrate, and nitrite. The best hCA XII inhibitors were cyanide (K(I) of 1 microM) and azide (K(I) of 80 microM). hCA XIV was on the other hand weakly inhibited by fluoride and not at all inhibited by perchlorate, but showed good affinity for most other anions investigated here. Chloride and bicarbonate showed K(I)s in the range of 0.75-0.77 mM for this isoform. The best hCA XIV anion inhibitors were sulfate, phenylarsonic, and phenylboronic acid (K(I) in the range of 10-92 microM).     

Carbonic anhydrase inhibitors. Inhibition of the newly isolated murine isozyme XIII with anions

The inhibition of the newly discovered cytosolic carbonic anhydrase (CA, EC 4.2.1.1) isozyme XIII of murine origin (mCA XIII) has been investigated with a series of anions, such as the physiological ones (bicarbonate, chloride), or the metal complexing anions (cyanate, cyanide, azide, hydrogen sulfide, etc), nitrate, nitrite, sulfate, sulfamate, sulfamide as well as with phenylboronic and phenylarsonic acids. The best mCA XIII inhibitors were cyanate, thiocyanate, cyanide and sulfamide, with K(I)-s in the range of 0.25microM-0.74 mM, whereas fluoride, iodide, azide, carbonate and hydrogen sulfide were less effective (K(I)-s in the range of 3.0-5.5mM). The least effective inhibitors were sulfate, chloride and bicarbonate (K(I)-s in the range of 138-267 mM). The affinity of mCA XIII for anions is very different from that of the other cytosolic isozymes (hCA I and II) or the mitochondrial isozyme hCA V. This resistance to inhibition by the physiological anions bicarbonate and chloride suggests an evolutionary adaptation of CA XIII to the presence of high concentrations of such anions (e.g., in the reproductive tract of both female and male), and the possible participation of this isozyme (similarly to CA II, CA IV and CA V) in metabolons with proteins involved in the anion exchange and transport, such as the anion exchangers (AE1-3) or the sodium bicarbonate co-transporter (NBC1 and NBC3) proteins, which remain to be identified.     

Carbonic anhydrase inhibitors: SAR and X-ray crystallographic study for the interaction of sugar sulfamates/sulfamides with isozymes I,II and IV

A series of sugar sulfamate/sulfamide derivatives were prepared and assayed as inhibitors of three carbonic anhydrase (CA) isozymes, hCA I, hCA II and bCA IV. Best inhibitory properties were observed for the clinically used antiepileptic drug topiramate, which is a low nanomolar CA II inhibitor, and possesses good inhibitory properties against the other two isozymes investigated here, similarly with acetazolamide, methazolamide or dichlorophenamide. The X-ray structure of the complex of topiramate with hCA II has been solved and it revealed a very tight association of the inhibitor, with a network of seven strong hydrogen bonds fixing topiramate within the active site, in addition to the Zn(II) coordination through the ionized sulfamate moiety. Structural changes in this series of sugar derivatives led to compounds with diminished CA inhibitory properties as compared to topiramate.     

Carbonic anhydrase activators: amino acyl/dipeptidyl histamine derivatives bind with high affinity to isozymes I, II and IV and act as efficient activators

Reaction of histamine (Hst) with tetrabromophthalic anhydride and protection of its imidazole moiety with tritylsulfenyl chloride, followed by hydrazinolysis, afforded N-1-tritylsulfenyl-histamine, a key intermediate which was further derivatized at its aminoethyl moiety. Reaction of the key intermediate with N-Boc-amino acids/dipeptides (Boc-AA) in the presence of carbodiimides afforded, after deprotection of the imidazolic and amino moieties, a series of compounds with the general formula AA-Hst (AA=amino acyl; dipeptidyl). The new derivatives were assayed as activators of three carbonic anhydrase (CA) isozymes, hCA I, hCA II (cytosolic forms) and bCA IV (membrane-bound form). Efficient activation was observed against all three isozymes, but especially against hCA I and bCA IV, with affinities in the nanomolar range for the best compounds. hCA II was, on the other hand, activatable with affinities around 10–20 nM. This new class of CA activators might lead to the development of drugs/diagnostic agents for the CA deficiency syndrome, a genetic disease of bone, brain and kidneys.     

Simple methanesulfonates are hydrolyzed by the sulfatase carbonic anhydrase activity

The possible sulfatase activity of several carbonic anhydrase (CA, EC 4.2.1.1) isoforms have been investigated with a series of synthesized methanesulfonate derivatives of phenols. Four α-CA isozymes, i.e. hCA I, hCA II, hCA IV and hCA VI (h?=?human isoform), were included in the study. We evidenced that the original sulfonate esters are being hydrolyzed effectively to the corresponding phenols which there after act as CA inhibitors. The K_I-s of these compounds ranged from 10.24 to 4012 µM against hCA I, 0.10 to 35.42 µM against hCA II, 0.49 to 45.06 µM against hCA IV and 3.27 to 608 µM against CA VI, respectively. The relevant sulfatase activity of CA with these esters is amazing considering the fact that 4-nitrophenyl-sulfate, an activated ester, is not a substrate of these enzymes.     

Carbonic anhydrase activators: synthesis of high affinity isozymes I, II and IV activators, derivatives of 4-(4-tosylureido-amino acyl)ethyl-1H-imidazole (histamine derivatives)

Reaction of histamine (Hst) with tetrabromophthalic anhydride and protection of its imidazole moiety with tritylsulfenyl chloride, followed by hydrazinolysis, afforded N-1-tritylsulfenyl histamine, a key intermediate which was further derivatized at its aminoethyl moiety. Reaction of the key intermediate with 4-tosylureido amino acids/dipeptides (ts-AA) in the presence of carbodiimides, afforded after deprotection of the imidazole moiety, a series of compounds with the general formula ts-AA-Hst (ts=4-MeC(6) H(4) SO(2) NHCO). Some structurally related dipeptide derivatives with the general formula ts-AA1-AA2-Hst, were also prepared, by in a similar way to the amino acyl compounds mentioned above. The new derivatives were examined as activators of three carbonic anhydrase (CA) isozymes, hCA I, hCA II (cytosolic forms) and bCA IV (membrane-bound form). Efficient activation was observed against all three isozymes, but especially against hCA I and bCA IV, with affinities in the 1-10 nanomolar range for the best compounds. hCA II was on the other hand activatable with affinities around 20-50 nM. This new class of CA activators might lead to the development of drugs/diagnostic agents for the CA deficiency syndrome, a genetic disease of bone, brain and kidneys.     

α-Carbonic anhydrases are sulfatases with cyclic diol monosulfate esters

Carbonic anhydrases (CA) catalyze activated ester hydrolysis in addition to the hydration of CO(2) to bicarbonate. They also show phosphatase activity with 4-nitrophenyl phosphate as substrate but not sulfatase with the corresponding sulfate. Here we prove that the enzyme is catalyzing the synthesis of cyclic diols from sulfate esters. 5-, 6- and 8-membered ring cyclic sulfates incorporating a neighboring secondary alcohol moiety were treated with CA II and yielded the corresponding cyclic diols. Inhibitory properties of obtained cyclic and original sulfate esters were then investigated on human carbonic anhydrase I (hCA I), hCA II, hCA IV and hCA VI (h?=?human isoform). K(I)-s of these compounds ranged between 32.7-423 μM against hCA I, 2.13-32.4 μM against hCA II, 13.7-234 μM against hCA IV and 76-278 μM against CA VI, respectively. The sulfatase activity of CA with such esters is amazing considering the fact that 4-nitrophenyl-sulfate is not a substrate of these enzymes.     

Carbonic anhydrase inhibitors: inhibition of the membrane-bound human isozyme IV with anions

The membrane-associated human isozyme of carbonic anhydrase, hCA IV, has been investigated for its interaction with anion inhibitors, for the CO(2) hydration reaction catalyzed by this enzyme. Surprisingly, halides were observed to act as potent hCA IV inhibitors, with inhibition constants in the range of 70-90 microM, although most of these ions, and especially fluoride, the best hCA IV inhibitor among the halides, are weak inhibitors of other isozymes, such as hCA I, II and V. The metal poisons cyanate, cyanide and hydrogen sulfide were weaker hCA IV inhibitors (K(i)'s in the range of 0.6-3.9 mM), whereas thiocyanate, azide, nitrate and nitrite showed even weaker inhibitory properties (K(i)'s in the range of 30.8-65.1 mM). Sulfate was a good hCA IV inhibitor (K(i) of 9 mM), although it is a much weaker inhibitor of isozymes I, II, V and IX. Excellent hCA IV inhibitory properties showed sulfamic acid, sulfamide, phenylboronic acid and phenylarsonic acid, with K(i)'s in the range of 0.87-0.93 microM, whereas their affinities for the other investigated isozymes were in the millimolar range. The interaction of some anions with the mitochondrial isozyme hCA V has also been investigated for the first time here. It has been observed that among all these isozymes, hCA V has the lowest affinity for bicarbonate and carbonate (K(i)'s in the range of 82-95 mM), which may represent an evolutionary adaptation of this isozyme to the rather alkaline environment (pH 8.5) within the mitochondria, where hCA V plays important functions in some biosynthetic reactions involving carboxylating enzymes (pyruvate carboxylase and acetyl coenzyme A carboxylase). There are important differences of affinity for anions between the two membrane-associated isozymes, hCA IV and hCA IX.     

The human carbonic anhydrase isoenzymes I and II inhibitory effects of some hydroperoxides,alcohols, and acetates

The carbonic anhydrases (CAs, EC 4.2.1.1) represent a superfamily of widespread enzymes, which catalyze a crucial biochemical reaction, the reversible hydration of carbon dioxide to bicarbonate and protons. Human CA isoenzymes I and II (hCA I and hCA II) are ubiquitous cytosolic isoforms. In this study, a series of hydroperoxides, alcohols, and acetates were tested for the inhibition of the cytosolic hCA I and II isoenzymes. These compounds inhibited both hCA isozymes in the low nanomolar ranges. These compounds were good hCA I inhibitors (K_is in the range of 24.93–97.99?nM) and hCA II inhibitors (K_is in the range of 26.04–68.56?nM) compared to acetazolamide as CA inhibitor (K_i: 34.50?nM for hCA I and K_i: 28.93?nM for hCA II).     

Carbonic anhydrase inhibitors: synthesis of Schiff bases of hydroxybenzaldehydes with aromatic sulfonamides and their reactions with arylsulfonyl isocyanates

Reaction of o- or p-hydroxybenzaldehydes with sulfanilamide, homosulfanilamide and p-(2-aminoethyl)- benzene-sulfonamide afforded several new Schiff bases which were subsequently derivatized at the phenolic hydroxy moiety by reaction with arylsulfonylisocyanates. The new arylsulfonylcarbamates obtained in this way possessed interesting inhibitory properties against three carbonic anhydrase (CA) isozymes, hCA I, hCA II and bCA IV (h = human, b = bovine isozyme). All these new derivatives, the simple Schiff bases and the arylsulfonylcarbamates obtained as outlined above, were more inhibitory against all isozymes as compared to the corresponding parent sulfonamide from which they were obtained. Generally, the p-hydroxybenzaldehyde derivatives were more active than the corresponding ortho isomers. An interesting behavior was evidenced for some of the ortho-substituted arylsulfonylcarbamato-sulfonamides, which showed higher affinities for the isozyme hCA I, as compared to hCA II and bCA IV (generally hCA I is 10-1000 less sensitive to "normal" sulfonamide inhibitors, such as acetazolamide, methazolamide or dorzolamide, as compared to hCA II). This make the new derivatives attractive leads for designing isozyme I-specific inhibitors.     

Carbonic Anhydrase Inhibitors. Schiff Bases of some Aromatic Sulfonamides and Their Metal Complexes: Towards More Selective Inhibitors of Carbonic Anhydrase Isozyme IV

Abstract

Reaction of three aromatic sulfonamides possessing a primary amino group, i.e., sulfanilamide, homosulfanilamide and p-aminoethyl-benzenesulfonamide with heterocyclic and aromatic aldehydes afforded a series of Schiff bases. Metal complexes of some of these Schiff bases with divalent transition ions such as Zn(II), Cu(II), Co(II) and Ni(II) have also been obtained. The new compounds were assayed as inhibitors of three isozymes of carbonic anhydrase (CA). Several of the new compounds showed a modest selectivity for the membrane-bound (bovine) isozyme CA IV (bCA IV) as compared to the cytosolic human isozymes hCA I and II, in contrast to classical inhibitors which generally possess a 17-33 times lower affinity for bCA IV. This greater selectivity toward bCA IV is due mainly to a slightly decreased potency against hCA II relative to classical inhibitors. However, metal complexes of these Schiff bases possessed an increased affinity for hCA II, being less inhibitory against bCA IV. The first type of compounds reported here (i.e., the Schiff bases of aromatic sulfonamides with heterocyclic aldehydes) might thus lead to the development of low molecular weight isozyme specific CA IV inhibitors. The difference in affinity for the three isozymes of the inhibitors reported by us here is tentatively explained on the basis of recent X-ray crystallographic studies of these isozymes and their adducts with substratesiinhibitors     

Carbonic anhydrase inhibitors: in vitro inhibition of α isoforms (hCA I,hCA II,bCA III,hCA IV) by flavonoids

A series of flavonoids, such as quercetin, catechin, apigenin, luteolin, morin, were investigated for their inhibitory effects against the metalloenzyme carbonic anhydrase (CA). The compounds were tested against four α-CA isozymes purified from human and bovine (hCA I, hCA II, bCA III, hCA IV) tissues. The four isozymes showed quite diverse inhibition profiles with these compounds. The flavonoids inhibited hCA I with K_I-s in the range of 2.2–12.8 μM, hCA II with K_I-s in the range of 0.74–6.2 μM, bCA III with K_I-s in the range of 2.2–21.3 μM, and hCA IV with inhibition constants in the range of 4.4–15.7, with an esterase assay using 4-nitrophenyl acetate as substrate. Some simple phenols/sulfonamides were also investigated as standard inhibitors. The flavonoids incorporate phenol moieties which inhibit these CAs through a diverse, not yet determined inhibition mechanism, compared to classic inhibitors such as the sulfonamide/sulfamate ones.     

α-Carbonic anhydrases are sulfatases with cyclic diol monosulfate esters

Carbonic anhydrases (CA) catalyze activated ester hydrolysis in addition to the hydration of CO₂ to bicarbonate. They also show phosphatase activity with 4-nitrophenyl phosphate as substrate but not sulfatase with the corresponding sulfate. Here we prove that the enzyme is catalyzing the synthesis of cyclic diols from sulfate esters. 5-, 6- and 8-membered ring cyclic sulfates incorporating a neighboring secondary alcohol moiety were treated with CA II and yielded the corresponding cyclic diols. Inhibitory properties of obtained cyclic and original sulfate esters were then investigated on human carbonic anhydrase I (hCA I), hCA II, hCA IV and hCA VI (h?=?human isoform). K_I-s of these compounds ranged between 32.7–423 μM against hCA I, 2.13–32.4 μM against hCA II, 13.7–234 μM against hCA IV and 76–278 μM against CA VI, respectively. The sulfatase activity of CA with such esters is amazing considering the fact that 4-nitrophenyl-sulfate is not a substrate of these enzymes.     

Carbonic anhydrase inhibitors. Interaction of isozymes I, II, IV, V, and IX with organic phosphates and phosphonates

The interaction of five human carbonic anhydrase (hCA, EC 4.2.1.1) isozymes, that is, hCA I, II, IV, V, and IX with a small library of phosphonic acids/organic phosphates, including methylphosphonic acid, MPA; phenylphosphonic acid, PPA; N-(phosphonoacetyl)-L-aspartic acid, PALA, methylene diphosphonic acid MDPA, the O-phosphates of serine (Ser-OP) and threonine (Thr-OP) as well as the antiviral phosphonate foscarnet has been studied. hCA I was activated by all these compounds, with the best activators being MPA and PPA (K(A)s of 0.10-1.20 microM). MPA and PPA were on the other hand nanomolar inhibitors of hCA II (K(I)s of 98-99 nM). PALA showed an affinity of 7.8 microM, whereas the other compounds were weak, millimolar inhibitors of this isozyme. The best hCA IV inhibitors were PALA (79 nM) and PPA (5.4 microM), whereas the other compounds showed K(I)s in the range of 0.31-5.34 mM. The mitochondrial isozyme was weakly inhibited by all these compounds (K(I)s in the range of 0.09-41.7 mM), similarly to the transmembrane, tumor-associated isozyme (K(I)s in the range of 0.86-2.25 mM). Thus, phosphonates may lead to CA inhibitors with selectivity against two physiologically relevant isozymes, the cytosolic hCA II or the membrane-bound hCA IV.     

QSAR studies on the activation of the human carbonic anhydrase cytosolic isoforms I and II and secretory isozyme VI with amino acids and amines

The first QSAR study on the activation of the human secretory isoform of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1), CA VI, with a series of amines and amino acids is reported. A large set of topological indices have been used to obtain several tri-/tetra-parametric models. We compared the CA VI activating QSAR models with those calculated for activation of the cytosolic human isozymes hCA I and hCA II. In addition, the effect of D- and L-amino acids as activators of hCA I, hCA II and of hCA VI as compared to those of structurally related biogenic amines was investigated for obtaining statistically significant and predictive QSAR equations. The obtained models are discussed using a variety of statistical parameters. The best models were obtained for hCA II activation, followed by hCA I, whereas the QSAR models for the activation of hCA VI were statistically weaker.     

Synthesis and biological evaluation of histamine Schiff bases as carbonic anhydrase I,II, IV,VII, and IX activators

A series of 20 histamine Schiff base was synthesised by reaction of histamine, a well known carbonic anhydrase (CA, E.C 4.2.2.1.) activator pharmacophore, with substituted aldehydes. The obtained histamine Schiff bases were assayed as activators of five selected human (h) CA isozymes, the cytosolic hCA I, hCA II, and hCA VII, the membrane-anchored hCA IV and transmembrane hCA IX. Some of these compounds showed efficient activity (in the nanomolar range) against the cytosolic isoform hCA VII, which is a key CA enzyme involved in brain metabolism. Moderate activity was observed against hCA I and hCA IV (in the nanomolar to low micromolar range). The structure–activity relationship for activation of these isoforms with the new histamine Schiff bases is discussed in detail based on the nature of the aliphatic, aromatic, or heterocyclic moiety present in the aldehyde fragment of the molecule, which may participate in diverse interactions with amino acid residues at the entrance of the active site, where activators bind, and which is the most variable part among the different CA isoforms.