Activation of the hydroxamic acid

The enzyme can also catalyze the transfer of an acetyl group from an N-acetylated hydroxylamine (hydroxamic acid) to form an acetoxy product, i.e., an N to O transacetylation and this pathway does not require acetyl Co-A (12). A-hydroxy-4-acetylaminobiphenyl provides an example of this conversion as shown in Figure 7.7. The significance of this pathway is that it leads to the activation of the hydroxamic acid because acetoxy derivatives of aromatic amines are chemically reactive and many are carcinogens such as the heterocyclic amines formed when meat is heated to a high temperature, e.g., 2-amino-1-mcthyl-6-phenylirnidaz()[4,5-i ]pyri(linc. 

Recently, hydroxamic acid derivatives of common NSAIDs (meclofen-amic acid, indomethacin, sulindac, and ibuprofen) were evaluated at Warner-Lambert [293]. The order of 5-LO inhibitory potency (in RBL-1 cells) for these derivatives was CON(Me)OH > CONHOH > CONH(OMe)-> COOH. The CO potency ranking was exactly opposite, although the best 5-LO inhibitors still possessed significant CO activity. Whether the observed oral activity in CPE was due to carboxyhc acid metabolites or to the intrinsic activity of the hydroxamic acids was not clear. The more active 5-LO inhibitors were less ulcerogenic in fasted rats. 

The neutral hydroxamic acid does not show a catalytic activity as a nucleophile, so that the activation of the hydroxamic acid by imidazole is remarkable in these polymers. 

Replacement of the hydroxamic acid moiety of SAHA by an alternative chelator has been the subject of several studies. Suzuki and Miyata et al. have shown that replacement of the hydroxamic acid of SAHA with a free thiol moiety does not affect the enzymatic HDAC inhibition capability of the compound [57]. Furthermore, replacement of the hydroxamic acid of SAHA by a trifluoromethyl ketone was investigated by Frey et al. (Fig. 8) [58]. The activated ketone is readily hydrated to form the vicinal diol, a structural feature known to bind to zinc-dependent proteases [59]. The in vitro evaluation was done on a partially purified HDAC preparation consisting largely of HDAC 1 and HDAC2 [60], exhibiting an IC50 of 6.7 xM. 

Stereospecific synthesis of succinyl derivatives is now commonly carried out as shown in Scheme 7. The tert-butyl ester of the succinyl amino acid methylamides 16 was treated with TFA to deprotect the carboxy group, which was then activated by isobutyl chloroformate and reacted with H2NOTMS.[18] The OTMS protection was removed during the isolation and purification of the hydroxamic acid 17. 

Comparison of V(IV,V) hydroxamic acid complexes showed the V(V) complex induced a stronger insulin-enhancing effect than the V(IV) complex, and both complexes were better than either of the salts, vanadyl sulfate or sodium vanadate, in relieving the symptoms of mice with STZ-induced diabetes. The distribution of vanadium in tissues was the same irrespective of the complex administered however, the tissue distribution of vanadium when the salts were administered was different from that seen after vanadium complex administration [146], These results suggest that the difference in the antidiabetic activity of the hydroxaminic acid V complexes is related to the different oxidation state, although there was no difference in the final tissue distribution of these complexes. 

DuPont s hybrid structure 19 [33] bears the signature of a dipeptide-based SAR on y-secretase and the reminiscent lead, which was synthesized in a matrix metalloproteinase (MMP) program. Removal of the central amide bond of the parent dipeptide, replacement of the hydroxamic acid by an amide, and introduction of a seven-membered lactam resulted in high activity and removed some of the problems associated with dipeptide lead structures. Hot labeling by photoactivation of I125-benzophenone specifically cross-linked the inhibitor to three cell-membrane proteins. 

The N-hydroxy amino acid derivatives are likely to be applicable to other metalloproteases. Thermolysin is inhibited irreversibly at pH 7.2 by ClCH2CO-DL-HOLeu-OCH3 where HOLeu is N-hydroxyleucine (47). The inhibition reaction involves coordination of the hydroxamic acid functional group to the active-site zinc atom of the enzyme. This then places the chloroacetyl group adjacent to Glu-143, an essential catalytic residue of thermolysin (see Figure 9). An ester linkage is formed and the enzyme is inactivated irreversibly. This reagent also inactivated two neutral metalloproteases from B. subtilis, but reacted only very slowly with carboxypeptidase A (t1/2 > 3 d). 

Largely for these reasons, a range of structural analogs based on the trichostatin A structure were prepared as potential HDAc inhibitor drug candidates — the particular structural element clearly important for activity being the hydroxamic acid group. However, in terms... 

Studies on the acetate of A -hydroxy-AAF indicated that esterification of the hydroxamic acid could be the requisite transformation of an aromatic amine to an ultimate carcinogen. Bioassays indicated that A -acetoxy-AAF resembled direct-acting carcinogens in producing subcutaneous tumors at the site of injection. Unlike /V-hydroxy-AAF, this acetate ester reacted in vitro with various cellular macromolecules (339, 410). The metabolic activation of AAF and decomposition of A -acetoxy-AAF to an electrophile are illustrated in Fig. 1. The possibility that -hydroxy-AAF was esterified in vivo to sulfate or phosphate esters has been examined. Evidence that sulfate esters may be involved in the formation of the ultimate carcinogen of AAF has been supported by several in vivo bioassays. Coadministration of acetanilide, which depletes the intracellular level of sulfate, reduced the carcinogenicity of AAF (487). Coadministration of sulfate ion with AAF enhanced its activity as a liver carcinogen (97). 

Due to their chelating properties hydroxamic acids are of importance in the solubilization and uptake of iron in many microorganisms. Aerobactin participates in the uptake of iron in Escherichia coli. Sideramines, e.g., ferrichrome, are involved in the Fe + uptake in fungi (C 2.2). Some iron-free sideramine-like compounds, e.g., nocardamine, are antibiotics (E 5.2) and inhibit the activity of sideramines by competition. If the Fe +.ion is reduced to Fe + the stability of the hydroxamic acid complexes is reduced and iron may be transferred, for instance from ferrichrome, to compounds with porphyrin ring system (D 10.1). 

A seemingly complex heterocycle which on close examination is in fact a latentiated derivative of a salicylic acid shows antiinflammatory activity. It might be speculated that this compound could quite easily undergo metabolic transformation to a salicylate and that this product is in fact the active drug. Condensation of acid 134 with hydroxyl amine leads to the hydroxamic acid 135. Reaction of... 

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