Acetazolamide inhibition of renal gamma-glutamyl transpeptidase

SAR of L-ABBA analogs for GGT1 inhibitory activity and L-ABBA's effect on plasma cysteine and GSH species

Gamma-glutamyl transferase 1 (GGT1) is a critical enzyme involved in the hydrolysis and/or transfer of gamma-glutamyl groups of glutathione, which helps maintain cysteine levels in plasma. In this study, we synthesized L-ABBA analogs to investigate their inhibitory effect on GGT1 hydrolysis and transpeptidase activity, with the goal of defining the pharmacophore of L-ABBA. Our structure-activity relationship (SAR) study revealed that an α-COO^- and α-NH₃⁺ group, as well as a two-CH₂ unit distance between α-C and boronic acid, are essential for the activity. The addition of an R (alkyl) group at the α-C reduced the activity of GGT1 inhibition, with L-ABBA being the most potent inhibitor among the analogs. Next, we investigated the impact of L-ABBA on plasma levels of cysteine and GSH species, with the expectation of observing reduced cysteine levels and enhanced GSH levels due to its GGT1 inhibition. We administered L-ABBA intraperitoneally and determined the plasma levels of cysteine, cystine, GSH, and GSSG using LCMS. Our results showed time- and L-ABBA dose-dependent changes in total plasma cysteine and GSH levels. This study is the first to demonstrate the regulation of plasma thiol species upon GGT1 inhibition, with plasma cystine levels reduced by up to ∼75% with L-ABBA (0.3 mg/dose). Cancer cells are highly dependent on the uptake of cysteine from plasma for maintaining high levels of intracellular glutathione. Thus, our findings suggest that GGT1 inhibitors, such as L-ABBA, have a potential to be used for GSH reduction thereby inducing oxidative stress in cancer cells and reducing their resistance to many chemotherapeutic agents.

Design and evaluation of novel analogs of 2-amino-4-boronobutanoic acid (ABBA) as inhibitors of human gamma-glutamyl transpeptidase

Inhibitors of gamma-glutamyl transpeptidase (GGT1, aka gamma-glutamyl transferase) are needed for the treatment of cancer, cardiovascular illness and other diseases. Compounds that inhibit GGT1 have been evaluated in the clinic, but no inhibitor has successfully demonstrated specific and systemic GGT1 inhibition. All have severe side effects. L-2-amino-4‑boronobutanoic acid (l-ABBA), a glutamate analog, is the most potent GGT1 inhibitor in vitro. In this study, we have solved the crystal structure of human GGT1 (hGGT1) with ABBA bound in the active site. The structure was interrogated to identify interactions between the enzyme and the inhibitor. Based on these data, a series of novel ABBA analogs were designed and synthesized. Their inhibitory activity against the hydrolysis and transpeptidation activities of hGGT1 were determined. The lead compounds were crystalized with hGGT1 and the structures solved. The kinetic data and structures of the complexes provide new insights into the critical role of protein structure dynamics in developing compounds for inhibition of hGGT1.

Acetazolamide causes renal [Formula: see text] wasting but inhibits ammoniagenesis and prevents the correction of metabolic acidosis by the kidney

Carbonic anhydrase (CAII) binds to the electrogenic basolateral Na+-[Formula: see text] cotransporter (NBCe1) and facilitates [Formula: see text] reabsorption across the proximal tubule. However, whether the inhibition of CAII with acetazolamide (ACTZ) alters NBCe1 activity and interferes with the ammoniagenesis pathway remains elusive. To address this issue, we compared the renal adaptation of rats treated with ACTZ to NH4Cl loading for up to 2 wk. The results indicated that ACTZ-treated rats exhibited a sustained metabolic acidosis for up to 2 wk, whereas in NH4Cl-loaded rats, metabolic acidosis was corrected within 2 wk of treatment. [Formula: see text] excretion increased by 10-fold in NH4Cl-loaded rats but only slightly (1.7-fold) in ACTZ-treated rats during the first week despite a similar degree of acidosis. Immunoblot experiments showed that the protein abundance of glutaminase (4-fold), glutamate dehydrogenase (6-fold), and SN1 (8-fold) increased significantly in NH4Cl-loaded rats but remained unchanged in ACTZ-treated rats. Na+/H+ exchanger 3 and NBCe1 proteins were upregulated in response to NH4Cl loading but not ACTZ treatment and were rather sharply downregulated after 2 wk of ACTZ treatment. ACTZ causes renal [Formula: see text] wasting and induces metabolic acidosis but inhibits the upregulation of glutamine transporter and ammoniagenic enzymes and thus suppresses ammonia synthesis and secretion in the proximal tubule, which prevented the correction of acidosis. This effect is likely mediated through the inhibition of the CA-NBCe1 metabolon complex, which results in cell alkalinization. During chronic ACTZ treatment, the downregulation of both NBCe1 and Na+/H+ exchanger 3, along with the inhibition of ammoniagenesis and [Formula: see text] generation, contributes to the maintenance of metabolic acidosis.

Rat renal proximal tubular gluconeogenesis: possible involvement of nonmitochondrial carbonic anhydrase isozymes

The carbonic anhydrase (CA) inhibitor ethoxzolamide decreases the rate of glucose synthesis from 10 mM pyruvate by tubules incubating in 25 mM HCO3- but not in 50 mM HCO3-: this is evidence that rat renal cortical mitochondrial CA (CA V) provides HCO3- for pyruvate carboxylation in renal tubular gluconeogenesis at physiological total CO2 (CO2 + HCO3-). In renal proximal tubules prepared from 48-h-starved rats and incubating in 10 mM pyruvate in 25 mM HCO3- buffered saline (Krebs-Henseleit buffer) the CA inhibitors acetazolamide (AZ) and benzolamide (BZ) decreased the rate of glucose synthesis. Maximal inhibition was reached with 125 microM AZ or with 450 microM BZ. The rate of glucose synthesis increased with increasing pyruvate concentration from 3.33 to 20 mM; including 600 microM BZ or 188 microM AZ results in glucose synthesis becoming independent of increasing pyruvate concentration. Doubling the physiological concentration of bicarbonate restored the dependence of glucose synthesis on pyruvate concentration and partly, but not completely, alleviated the inhibitory effect of AZ and BZ, leading to the conclusion that AZ and BZ influence gluconeogenesis by affecting enzymes in addition to CA V. Tubules were incubated with substrates which do not require pyruvate carboxylation for synthesis of oxaloacetate. When tubules were incubated in 10 mM malate the rate of glucose synthesis was unaffected by less than 100 microM AZ or 400 microM BZ and was decreased maximally by 40 and 20%, respectively, by 125 microM AZ, 450 microM BZ, and higher concentrations of these drugs. Increasing the malate concentration from 3.33 to 20 mM increased the rate of glucose synthesis; 600 microM BZ inhibited the rate of glucose synthesis only when the malate concentration was greater than 10 mM but 188 microM AZ decreased the rate of glucose synthesis at each concentration of malate. Results were similar when tubules were incubated in glutamine with CA inhibitors. The rate of glucose synthesis differed with the substrate metabolized and the substrate concentration except when 600 microM BZ was included.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of acetazolamide on renal metabolism and ammoniagenesis in the dog

The effect of acetazolamide (ACZ) on renal metabolism and ammoniagenesis was studied in the dog in vivo and in vitro. ACZ was administered to 10 dogs with normal acid-base status and five with chronic metabolic acidosis induced by NH4Cl. In both groups of dogs, the acute administration of ACZ markedly reduced the urinary excretion of ammonium (from 33 to 10 in normal dogs and from 100 to 23 mumoles/100 ml GFR in acidotic dogs) whereas its release into the renal vein was increased in a reciprocal fashion (from 69 to 95 in normal dogs and from 91 to 152 mumoles/100 ml GFR in acidotic dogs). ACZ did not change the total ammonium production nor the renal glutamine utilization. The renal utilization or production of glutamate, alphaketoglutarate, alanine and citrate also remained unchanged. Despite a marked urinary alkalinization, citraturia remained minimal. However, the renal cortical concentrations of glutamine, glutamate, succinate, fumarate, malate, aspartate and phosphoenolpyruvate fell following ACZ administration, especially in acidotic dogs showing rapid renal utilization of glutamine. ACZ had no effect on the same metabolites in the kidney of normal dogs even when lactate utilization was enhanced by lactate infusion. This study demonstrates that an accelerated ammoniagenic flux can proceed in the dog kidney without the renal cortical changes produced by metabolic acidosis in this species. In vitro, using dog tubules, a selective effect of ACZ on glutamine metabolism as compared to lactate was observed. ACZ reduce the rate of the reactions catalyzed by alphaketoglutarate dehydrogenase and by succinyl CoA synthetase. Other enzymes of the ammoniagenic and gluconeogenic pathways (glutaminase, GLDH, malic enzyme, PEPCK) were not changed by ACZ. The metabolic effects of ACZ observed in the intact kidney in vivo or with tubules in vitro may be in part related to the effect of ACZ on these enzymes critical for the ammoniagenic process.

Studies on sound-induced epilepsy in mice

The cerebral concentrations of pyridoxal-5'-phosphate and divalent transition metal ions (Cu2+ and Zn2+) are appreciably higher in the seizure-susceptible strain of mouse (DBA/2J) than those in normal strains (CBA/Ca and Parkes ). By injecting metal ions intracranially and pyridoxal-5'-phosphate intraperitoneally, we could render the normal mouse prone to sound-induced epilepsy. The behaviour of the treated seizure-susceptible strain of mouse. The levels of glutamate and aspartate in its inferior colliculus were elevated and the concentration of gamma-aminobutyrate was lowered. Glutaminase inhibitors, 6-diazo-5-oxo-L-norleucine (DON) and 0-diazo-acetyl-L-serine (azaserine), and a transaminase inhibitor, 4-amino-3- isoxazolidone (L-cycloserine), when injected intraperitoneally, protected the seizure-susceptible mouse from undergoing convulsions, whereas pyridoxal-5'-phosphate and methionine sulphoximine, a glutamine synthetase inhibitor, exacerbated its epileptic condition. We propose a possible sequence of biochemical events associated with susceptibility to audiogenic seizures.