Design, Synthesis, and Mechanistic Studies of (R)-3-Amino-5,5-difluorocyclohex-1-ene-1-carboxylic Acid as an Inactivator of Human Ornithine Aminotransferase

Human ornithine aminotransferase (hOAT), a pyridoxal 5'-phosphate (PLP)-dependent enzyme, has been shown to play an essential role in the metabolic reprogramming and progression of hepatocellular carcinoma (HCC). HCC accounts for approximately 75% of primary liver cancers and is within the top three causes of cancer death worldwide. As a result of treatment limitations, the overall 5-year survival rate for all patients with HCC is under 20%. The prevalence of HCC necessitates continued development of novel and effective treatment methods. In recent years, the therapeutic potential of selective inactivation of hOAT has been demonstrated for the treatment of HCC. Inspired by previous increased selectivity for hOAT by the expansion of the cyclopentene ring scaffold to a cyclohexene, we designed, synthesized, and evaluated a series of novel fluorinated cyclohexene analogues and identified (R)-3-amino-5,5-difluorocyclohex-1-ene-1-carboxylic acid as a time-dependent inhibitor of hOAT. Structural and mechanistic studies have elucidated the mechanism of inactivation of hOAT by 5, resulting in a PLP-inactivator adduct tightly bound to the active site of the enzyme. Intact protein mass spectrometry, 19F NMR spectroscopy, transient state kinetic studies, and X-ray crystallography were used to determine the structure of the final adduct and elucidate the mechanisms of inactivation. Interestingly, despite the highly electrophilic intermediate species conferred by fluorine and structural evidence of solvent accessibility in the hOAT active site, Lys292 and water did not participate in nucleophilic addition during the inactivation mechanism of hOAT by 5. Instead, rapid aromatization to yield the final adduct was favored.

Pharmacologic rescue of hyperammonemia-induced toxicity in zebrafish by inhibition of ornithine aminotransferase

Hyperammonemia is the common biochemical hallmark of urea cycle disorders, activating neurotoxic pathways. If untreated, affected individuals have a high risk of irreversible brain damage and mortality. Here we show that acute hyperammonemia strongly enhances transamination-dependent formation of osmolytic glutamine and excitatory glutamate, thereby inducing neurotoxicity and death in ammoniotelic zebrafish larvae via synergistically acting overactivation of NMDA receptors and bioenergetic impairment induced by depletion of 2-oxoglutarate. Intriguingly, specific and irreversible inhibition of ornithine aminotransferase (OAT) by 5-fluoromethylornithine rescues zebrafish from lethal concentrations of ammonium acetate and corrects hyperammonemia-induced biochemical alterations. Thus, OAT inhibition is a promising and effective therapeutic approach for preventing neurotoxicity and mortality in acute hyperammonemia.

Ornithine aminotransferase versus GABA aminotransferase: implications for the design of new anticancer drugs

Ornithine aminotransferase (OAT) and γ-aminobutyric acid aminotransferase (GABA-AT) are classified under the same evolutionary subgroup and share a large portion of structural, functional, and mechanistic features. Therefore, it is not surprising that many molecules that bind to GABA-AT also bind well to OAT. Unlike GABA-AT, OAT had not been viewed as a potential therapeutic target until recently; consequently, the number of therapeutically viable molecules that target OAT is very limited. In this review the two enzymes are compared with respect to their active-site structures, catalytic and inactivation mechanisms, and selective inhibitors. Insight is offered that could aid in the design and development of new selective inhibitors of OAT for the treatment of cancer.

Biochemical characterization of pea ornithine-delta-aminotransferase: substrate specificity and inhibition by di- and polyamines

Ornithine-delta-aminotransferase (OAT, EC 2.6.1.13) catalyzes the transamination of L-ornithine to L-glutamate-gamma-semialdehyde. The physiological role of OAT in plants is not yet well understood. It is probably related to arginine catabolism resulting in glutamate but the enzyme has also been associated with stress-induced proline biosynthesis. We investigated the enzyme from pea (PsOAT) to assess whether diamines and polyamines may serve as substrates or they show inhibitory properties. First, a cDNA coding for PsOAT was cloned and expressed in Escherichia coli to obtain a recombinant protein with a C-terminal 6xHis tag. Recombinant PsOAT was purified under native conditions by immobilized metal affinity chromatography and its molecular and kinetic properties were characterized. Protein identity was confirmed by peptide mass fingerprinting after proteolytic digestion. The purified PsOAT existed as a monomer of 50 kDa and showed typical spectral properties of enzymes containing pyridoxal-5'-phosphate as a prosthetic group. The cofactor content of PsOAT was estimated to be 0.9 mol per mol of the monomer by a spectrophotometric analysis with phenylhydrazine. L-Ornithine was the best substrate (K(m)=15 mM) but PsOAT also slowly converted N(alpha)-acetyl-L-ornithine. In these reactions, 2-oxoglutarate was the exclusive amino group acceptor (K(m)=2mM). The enzyme had a basic optimal pH of 8.8 and displayed relatively high temperature optimum. Diamines and polyamines were not accepted as substrates. On the other hand, putrescine, spermidine and others represented weak non-competitive inhibitors. A model of the molecular structure of PsOAT was obtained using the crystal structure of human OAT as a template.

Secondary creatine deficiency in ornithine delta-aminotransferase deficiency

AIMS

Ornithine delta-aminotransferase (OAT) deficiency causes gyrate atrophy (GA) of the retina, as a consequence of high plasma ornithine concentrations. Because creatine synthesis requires the conversion of arginine and glycine into ornithine and guanidinoacetate, high ornithine concentration inhibits this reaction thus causing secondary creatine deficiency. The aim of this study was to evaluate the neuropsychological features and creatine metabolism in patients with GA.

METHODS

The study involved 7 GA patients, aged from 11 to 27 years who underwent neuropsychological evaluation and cerebral proton magnetic resonance spectroscopy (MRS).

RESULTS

Neurocognitive impairment was found in 5/7 patients, including mental retardation (3/7), school failure (1/7), major visuospatial dyspraxia (1/7), aggressive behavior (3/7) and epilepsy (2/7). Two patients had normal neuropsychological evaluation. Cerebral proton magnetic resonance spectroscopy revealed a profound creatine deficiency in all patients. MRS data were confirmed by decreased levels of creatine and/or guanidinoacetate in plasma and urine in all patients.

CONCLUSIONS

In our group of patients with GA, we found a high prevalence of neurological impairment, not reported so far, and possibly related to secondary creatine deficiency and hyperornithinemia. We propose to treat mentally retarded GA patients with high doses of creatine, as it may normalize brain creatine levels and help to reduce ornithine levels.

Polyamines upregulate the mRNA expression of cationic amino acid transporter-1 in human retinal pigment epithelial cells

We previously showed that ornithine was mainly transported via cationic amino acid transporter (CAT)-1 in human retinal pigment epithelial (RPE) cell line, human telomerase RT (hTERT)-RPE, and that CAT-1 was involved in ornithine cytotoxicity in ornithine-δ-aminotransferase (OAT)-deficient cell produced by a OAT specific inhibitor, 5-fluoromethylornithine (5-FMO). We showed here that CAT-1 mRNA expression was increased by ornithne in OAT-deficient RPE cells, which was reversed by an inhibitor of ornithine decarboxylase (ODC), α-difluoromethylornithine (DFMO). Polyamines, especially spermine, one of the metabolites of ODC, also enhanced the expression of CAT-1 mRNA. ODC mRNA expression was also increased by ornithine and polyamines, and gene silencing of ODC by siRNA decreased ornithine transport activity and its cytotoxicity. In addition, the mRNA of nuclear protein c-myc was also increased in 5-FMO- and ornithine-treated hTERT-RPE cells, and gene silencing of c-myc prevented the induction of CAT-1 and ODC. Increases in expression of CAT-1, ODC, and c-myc, and the inhibition of these stimulated expression by DFMO were also observed in primary porcine RPE cells. These results suggest that spermine plays an important role in stimulation of mRNA expression of CAT-1, which is a crucial role in ornithine cytotoxicity in OAT-deficient hTERT-RPE cells.

Therapeutic anticancer efficacy of a synthetic diazonamide analog in the absence of overt toxicity

Blocking cell division through the inhibition of mitosis is one of the most successful clinical strategies for the treatment of cancer. Taxanes and vinca alkaloids are in widespread use and have demonstrated substantive therapeutic efficacy. Both classes of compounds bind directly to tubulin, a structural component of the mitotic spindle. The ubiquitous utilization of tubulin in cell division in both cancerous and normal cells, however, tempers the broad spectrum of activity of currently used antimitotics by significant toxicities in normal dividing tissue. Moreover, peripheral nerve cells that rely on microtubules to shuttle cargo along axonal processes are also damaged by tubulin-binding drugs. Thus, neutropenia and peripheral neuropathy are the most frequently cited dose-limiting toxicities of this class of chemotherapeutics. Here we report the preclinical assessment of AB-5, a structural and functional analog of the natural product diazonamide A. AB-5, like taxanes and vinca alkaloids, blocks cell division during mitosis. However, AB-5 works not by binding tubulin but rather through inhibition of a newly discovered role for ornithine-delta-aminotransferase in mitosis. We hereby report that, unlike other antimitotics, AB-5 is extremely well tolerated by mice when administered under conditions where the drug cures xenografted tumors as effectively as taxanes and vinca alkaloids. AB-5-treated mice show no weight loss, no change in overall physical appearance, and no evidence of neutropenia. These observations raise the possibility that AB-5 may have clinical utility for cancer therapy under conditions largely devoid of chemotherapeutic toxicity and suggest that further preclinical evaluation of AB-5 is warranted.

Prevention of Ornithine Cytotoxicity by Nonpolar Side Chain Amino Acids in Retinal Pigment Epithelial Cells

PURPOSE

To investigate the effect of amino acids on ornithine cytotoxicity in ornithine-delta-aminotransferase (OAT)-deficient human retinal pigment epithelial (RPE) cells as an in vitro model of gyrate atrophy (GA) of the choroid and retina.

METHODS

RPE cells were treated with 0.5 mM 5-fluoromethylornithine (5-FMOrn), a specific and irreversible OAT inhibitor. OAT-deficient RPE cells were incubated with 10 mM ornithine in the presence of 20 mM of 1 of 18 amino acids or 10 mM 2-amino-2-norbornane-carboxylic acid (BCH), a conventional inhibitor of the amino acid transporter system L. Ornithine cytotoxicity and cytoprotective effects of each amino acid was evaluated with a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay 72 hours after treatment with ornithine in OAT-deficient RPE cells. Ornithine incorporation into RPE cells was evaluated using DL-[14C]ornithine.

RESULTS

An MTT colorimetric assay revealed that small and large zwitterionic amino acids, but not acidic or basic amino acids, decreased ornithine cytotoxicity in OAT-deficient RPE cells. Incorporation of DL-[14C]ornithine by RPE cells decreased to 79% of the control level after incubation for 48 hours with 20 mM leucine, the most effective cytoprotective amino acid. Further, BCH prevented ornithine cytotoxicity in a dose-dependent manner. Both light and heavy chains of L-type amino acid transporter (LAT)-1, LAT2, y+LAT1, and 4F2hc were expressed in RPE cells.

CONCLUSIONS

The present results demonstrate that L-type amino acid transporter(s) may be involved in protection against ornithine cytotoxicity in human RPE cells. Thus, amino acid transportation in RPE cells may be a good target for a new therapy for GA as well as other kinds of chorioretinal degeneration.

Is hyperprolinemia type I actually a benign trait? Report of a case with severe neurologic involvement and vigabatrin intolerance

Hyperprolinemia type I is a deficiency of proline oxidase (McKusick 23950), leading to hyperprolinemia and iminoglycinuria, usually with renal involvement. Hyperprolinemia type I is considered a benign trait. We reported a case of hyperprolinemia type I with a severe neurologic disorder and without renal involvement. The patient had marked psychomotor delay and right hemiparesis. Epilepsy was characterized by status epilepticus or a cluster of seizures. Laboratory findings revealed elevated levels of proline in the serum, urine, and cerebrospinal fluid without delta1-pyrroline 5-carboxylate dehydrogenase in the plasma or urine. Fluorescence in situ hybridization excluded a chromosome 22q11 deletion. Vigabatrin inhibits ornithine transaminase. Thus, vigabatrin could lead to a depletion of the normal pool of pyrroline 5-carboxylate dehydrogenase and could aggravate the clinical condition of the child. In this study, vigabatrin was discontinued. In the following months, the patient had marked psychomotor improvement, without modification of the epilepsy. We suggest that vigabatrin should be avoided in hyperprolinemia type I.

Ornithine aminotransferase, a potential target for the treatment of hyperammonemias

Ornithine-delta-aminotransferase (OAT) (EC 2.6.1.13) is a pyridoxal-5' phosphate dependent mitochondrial matrix enzyme. It controls the L-ornithine (Orn) level in tissues by catalysing the transfer of the delta-amino group of Orn to 2-oxoglutarate. The products of this reaction are L-glutamate-gamma-semialdehyde and L-glutamate. Among the compounds known to inhibit (or inactivate) OAT, only L-canaline and (SS)-5-(fluoromethyl)ornithine [(SS)-5FMOrn] are selective for OAT. Treatment of laboratory animals with 5FMOrn causes a dramatic accumulation of Orn in most tissues and organs, and the enhanced formation of urea due to saturation of ornithine:carbamoyltransferase with its substrate. The enhancement of urea formation by increased endogenous levels of Orn is comparable with that produced by large doses of Orn and arginine, a treatment known to enhance the detoxification of ammonia. However, protection to lethal doses of ammonium salts by exogenous Orn is rapidly fading. In contrast, inactivation of OAT by a small dose of 5FMOrn renders a long-lasting protective effect against various forms of hyperammonemic states. Among these the reduction of ammonia concentrations in blood and tissues, and the reduction of the pathologic excretion of orotic acid to normal levels in mice with hereditary defects of the urea cycle, were most impressive. In human hereditary OAT deficiency the elevated intraocular concentrations of Orn are considered to be a cause of gyrate atrophy. This is presumably the reason, why OAT has not been considered as a therapeutically useful target. Chronic inactivation of OAT by repeated administration of 5FMOrn, caused elevated intraocular Orn concentrations, but this treatment had no effect on the function and histology of the visual system, or the behaviour of adult mice. The confirmation of this and related observations in higher species will show, whether OAT inactivation has potentials in the treatment of hyperammonemic states.

Heterogeneity in ornithine cytotoxicity of bovine retinal pigment epithelial cells in primary culture

Gyrate atrophy of the choroid and retina is a chorioretinal degeneration caused by hyperornithinemia and a deficiency of ornithine-delta-aminotransferase (OAT). We recently showed that ornithine exhibits cytotoxicity to human retinal pigment epithelial (RPE) cell lines treated with the OAT inhibitor, 5-fluoromethylornithine (5-FMOrn), and suggested that this system may be an in vitro model of gyrate atrophy. In the present study, in order to apply this system to primary cultured RPE cells, we freshly prepared RPE cells from bovine eyes and studied the effect of ornithine on cell damage. Two phenotypes, epithelioid and fusiform, which coexisted in the primary culture and epithelioid phenotype cells, but not fusiform ones, were severely damaged and partially detached from the substrate by 10 m m ornithine and 0.5 m m 5-FMOrn. Neither ornithine nor 5-FMOrn alone exhibited such cytotoxicity to both phenotypes of RPE cells. Proline significantly prevented the ornithine-induced cytotoxicity. Epithelioid and fusiform phenotypes isolated from the primary culture showed different distribution of actin filaments. A combination of ornithine and 5-FMOrn time-dependently inhibited [(3)H]thymidine incorporation in the epithelioid, but not fusiform, cells. Proline prevented the inhibition of [(3)H]thymidine incorporation by ornithine in 5-FMOrn-treated epithelioid cells. Furthermore, l -azetidine-2-carboxylic acid, a collagen synthesis inhibitor, reduced [(3)H]thymidine incorporation in epithelioid, but not fusiform, cells, which was reversed by proline. These results demonstrate that the epithelioid phenotype of bovine RPE cells becomes susceptible to ornithine following inactivation of OAT. The phenotypic cells and its prevention by proline may provide insight into biochemical triggers that induce gyrate atrophy.

Crystal structure of human ornithine aminotransferase complexed with the highly specific and potent inhibitor 5-fluoromethylornithine

Ornithine aminotransferase (l-ornithine:2-oxoacid delta-aminotransferase; EC 2.6.1.13), a pyridoxal-5'-phosphate-dependent mitochondrial enzyme controls the l-ornithine level in tissues by catalyzing the transfer of the delta-amino group of l-ornithine to 2-oxoglutarate, producing l-glutamate- gamma-semialdehyde and l-glutamate. (2S, 5S)-5-Fluoromethylornithine is the only inhibitor exclusively specific for ornithine aminotransferase known to date. Both in vitro and in vivo, it blocks the enzyme by a suicide reaction leading to a covalent adduct with the cofactor. The crystal structure of the enzyme-inhibitor complex was solved at a resolution of 1.95 A. No significant conformational changes compared with the native enzyme structure were observed. The structure reveals the atomic details of the cofactor-inhibitor adduct and its interactions with the active site of the enzyme. The main residues responsible for specific binding of the inhibitor are Arg180, which forms a strong salt bridge with the alpha-carboxylate and Tyr55, which is involved in a short hydrogen bond with the alpha-amino group. The experimental observation that in the racemic mixture, (2S, 5S)-5-fluoromethylornithine is exclusively responsible for the enzyme inhibition can be explained on the basis of the active site topology. Model building studies strongly suggest that the natural substrate l-ornithine, in its external aldimine adduct with the enzyme, makes use of the same recognition site as the inhibitor. It is proposed that the neutralization of the active site Arg413 by a salt bridge with Glu235 also plays an important role in productive binding of both 5-fluoromethylornithine and l-ornithine. Arg180 and Arg413 are believed to be instrumental in recognition of l-glutamate, by binding its gamma and alpha-carboxylate groups, respectively. This requires a different side-chain conformation of Glu235. Lys292 is the only obvious candidate for catalyzing the rate-limiting proton transfer steps in the transamination reaction.

Prevention of ornithine cytotoxicity by proline in human retinal pigment epithelial cells

PURPOSE

To investigate the relationship between ornithine-delta-aminotransferase (OAT) deficiency and ornithine accumulation and the specific degeneration of retinal pigment epithelial (RPE) cells in gyrate atrophy.

METHODS

Human RPE cells, human hepatoma cells, and human fibroblast cells were treated with 5-fluoromethylornithine (5-FMOrn), a specific irreversible inhibitor of OAT. Ornithine cytotoxicity was determined by using a [3H]thymidine incorporation assay and immunohistochemical staining for cytokeratin. The effects of various metabolites of ornithine and arginine, such as creatine, creatine phosphate, I-delta 1-pyrroline-5-carboxylic acid (L-P5C), and proline, which may be deficient in gyrate atrophy on RPE cell damage by ornithine, were determined by the same procedures.

RESULTS

When the human RPE cells, HepG2 hepatoma cells, and WI-38 fibroblast cells were treated with 0.5 mM 5-FMOrn for 30 minutes, which inactivated OAT, ornithine exhibited severe time- and dose-dependent inhibition of DNA synthesis in the human RPE cells but not in the HepG2 hepatoma cells or WI-38 fibroblast cells. The inhibition of DNA synthesis was accompanied by drastic changes in morphologic appearance, disorganization of the cytoskeleton, and cell death. Ornithine or 5-FMOrn alone did not exhibit such cytotoxicity to the RPE cells. Proline prevented the cytotoxicity of ornithine.

CONCLUSIONS

These findings suggest that an elevated level of ornithine combined with an increased sensitivity to ornithine as a result of OAT deficiency may be crucial to the specific RPE degeneration in gyrate atrophy. They suggest also that abnormalities of proline metabolism may be involved in the progress of gyrate atrophy.

Endogenous synthesis of arginine plays an important role in maintaining arginine homeostasis in postweaning growing pigs

This study was conducted to determine whether endogenous synthesis of arginine plays a role in regulating arginine homeostasis in postweaning pigs. Pigs were fed a sorghum-based diet containing 0. 98% arginine and were used for studies at 75 d of age (28.4 kg body weight). Mitochondria were prepared from the jejunum and other major tissues for measuring the activities of Delta1-pyrroline-5-carboxylate (P5C) synthase and proline oxidase (enzymes catalyzing P5C synthesis from glutamate and proline, respectively) and of ornithine aminotransferase (OAT) (the enzyme catalyzing the interconversion of P5C into ornithine). For metabolic studies, jejunal enterocytes were incubated at 37 degrees C for 30 min in Krebs-Henseleit bicarbonate buffer containing 2 mmol/L L-glutamine, 2 mmol/L L-[U-14C]proline, and 0-200 micromol/L gabaculine (an inhibitor of OAT). The activities of P5C synthase, proline oxidase and OAT were greatest in enterocytes among all of the tissues studied. Incubation of enterocytes with gabaculine resulted in decreases (P < 0.05) in the synthesis of ornithine and citrulline from glutamine and proline. When gabaculine was orally administered to pigs (0.83 mg/kg body weight) to inhibit intestinal synthesis of citrulline from glutamine and proline, plasma concentrations of citrulline (-26%) and arginine (-22%) decreased (P < 0.05), whereas those of alanine (+21%), ornithine (+17%), proline (+107%), taurine (+56%) and branched-chain amino acids (+21-40%) increased (P < 0.05). On the basis of dietary arginine intake and estimated arginine utilization, the endogenous synthesis of arginine in the 28-kg pig provided >/=50.2% of total daily arginine requirement. Taken together, our results suggest an important role for endogenous synthesis of arginine in regulating arginine homeostasis in postweaning growing pigs, as previously shown in neonatal pigs.

Purification and properties of L-ornithine delta-aminotransferase from gramicidin S-producing Bacillus brevis

In gramicidin S-producing Bacillus brevis, the addition of L-ornithine to the minimal medium with L-glutamate as the sole carbon and nitrogen source caused an 8-fold induction of L-ornithine delta-aminotransferase [EC 2.6.1.13]. The enzyme was purified to homogeneity. The native enzyme had a molecular weight of about 88,000 after gel filtration and consisted of two subunits with an identical in molecular weight of about 45,000. The enzyme was specific for L-ornithine (Km = 1.05 mM) as an amino donor and for 2-oxoglutarate (Km = 6.25 mM) as an amino acceptor, and catalyzed the conversion of L-ornithine and 2-oxoglutarate, respectively, to glutamic-gamma-semialdehyde, which is spontaneously cyclized to delta 1-pyrroline-5-carboxylate and L-glutamate. The enzyme exhibits an absorption maximum at 425 nm at neutral pH, and 1 mol of pyridoxal phosphate is bound per subunit. The enzyme activity was irreversibly inhibited by gabaculine, and L-ornithine protected the enzyme from the inhibition. The N-terminal amino acid sequence revealed a noteworthy similarity between human and yeast L-ornithine delta-aminotransferases in residues 17-28 of the B. brevis enzyme.

Effects of ornithine aminotransferase inactivation by 5-fluoromethylornithine in rats following portacaval anastomosis

5-Fluoromethylornithine (5FMOrn) is a selective inactivator of ornithine aminotransferase. Administration of this compound to rodents causes a prominent increase of tissue ornithine concentrations, and prevents the neurological consequences of acute ammonia intoxication. However, long-term treatment with 5FMOrn of rats with portacaval shunts did not result in decreased circulating ammonia concentrations, nor did it prevent other pathologic manifestations of shunting. The sensitivity to ammonia intoxication of rats with portacaval shunts was also unaffected by pretreatment with 5FMOrn, although liver ornithine concentrations were significantly elevated; specific activities of urea cycle enzymes were slightly higher in portacaval shunted compared to sham-operated controls following 5-FMOrn treatment. Administration of 5FMOrn dramatically elevated urinary excretion of several amino acids in rats with portacaval shunts, but not in sham-operated animals, suggesting that the reabsorption of amino acids from the glomerular filtrate may be impaired in shunted rats. These results suggest that, in contrast to acute hyperammonemic syndromes, 5-FMOrn may be of limited therapeutic value in chronic hyperammonemia syndromes in which there is significant portal-systemic shunting.

Decreased hyperammonaemia and orotic aciduria due to inactivation of ornithine aminotransferase in mice with a hereditary abnormal ornithine carbamoyltransferase

Mice with the X-chromosomal sparse-fur (spf) mutation are an animal model of some hereditary deficiencies of ornithine carbamoyltransferase (OCT) in man. Orotic aciduria and hyperammonaemia are the most conspicuous metabolic changes in these diseases. Selective inactivation of ornithine aminotransferase (OAT) by 5-fluoromethylornithine raises endogenous ornithine concentrations so that citrulline formation is effectively catalysed by the aberrant OCT, in spite of its low affinity for ornithine. As a consequence, blood and tissue ammonia concentrations and orotic acid excretion are reduced near to normal values, and the abnormal amino acid patterns in blood, brain and liver are normalized. Selective inactivation of OAT seems a promising therapeutic approach in some hereditary OCT deficiencies, and a total that may allow us to clarify the role of ammonia and orotic acid in the development of nanism and abnormal behaviour in spf mutant mice.

Some biochemical and pathophysiological aspects of long-term elevation of brain ornithine concentrations

Mice and chicken were given 5-fluoromethylornithine (5FMOrn), a selective inactivator of ornithine aminotransferase (OAT) over extended periods of time. This treatment allowed us to maintain elevated concentrations of ornithine in all tissues. Since gyrate atrophy, an autosomal recessive human disease, is characterized by the absence of OAT, special emphasis was put on the study of the visual system. Ophthalmoscopic and histologic examinations of the eye as well as electroretinograms and locomotor behaviour demonstrated an unimpaired visual system and brain. No toxic effects were observable in the treated mice. Likewise, chick embryo development was normal in spite of highly elevated brain and tissue ornithine concentrations. A likely explanation for the absence of toxic effects of 5FMOrn treatment, disregarding the non-toxicity of ornithine, is the fact that 10-20% of tissue OAT is refractory to inactivation by 5FMOrn. This residual activity may be sufficient to maintain vital functions.

Transgenic mice over-producing putrescine in their tissues do not convert the diamine into higher polyamines

We recently described a transgenic mouse line over-expressing the human ornithine decarboxylase gene virtually in all tissues. Despite strikingly elevated tissue putrescine concentrations, no or minimal changes were found in the levels of the higher polyamines spermidine and spermine. We have now extended these studies by further increasing tissue putrescine with the aid of 5-fluoromethylornithine, a specific inhibitor of ornithine transaminase and hence the catabolism of L-ornithine. As a result of the treatment with the latter drug, the concentration of putrescine was further increased by a factor of 2-3 without any changes in the concentrations of spermidine and spermine. In the testis of transgenic mice treated with 5-fluoromethylornithine, the concentration of putrescine was nearly 60 times that in non-transgenic untreated animals, yet the concentration of spermidine was only 1.5-fold higher. A similar small increase in brain spermidine was accompanied by a 40-fold elevation in the concentration of putrescine. The apparent blockade between putrescine and spermidine was in all likelihood not attributable to an inhibition of S-adenosylmethionine decarboxylase, the rate-controlling enzyme in the biosynthesis of spermidine and spermine. Our results are more compatible with the view that in non-dividing adult tissues putrescine is sequestered through some unknown mechanisms in a way that makes it unavailable for the synthesis of the higher polyamines.

Effects of inhibition of ornithine aminotransferase on thioacetamide-induced hepatogenic encephalopathy

Repeated administration of thioacetamide (TAA) to CD1 mice produced hepatic failure and biochemical and behavioral effects characteristic of hepatogenic encephalopathy (HE). The symptoms in mice resembled those previously observed in rats after similar treatments. It is, however, obvious that both in rats and mice the severity of symptoms depends not only on dose and dosing schedule of TAA, but also on strain and body weight (age). Administration of 5-fluoromethylornithine (5FMOrn), a selective inactivator of ornithine aminotransferase (OAT), significantly reduced mortality, and it ameliorated most of the TAA-induced pathologic symptoms, such as hypothermia, decreased locomotor and exploratory behavior, pathologic liver function and amino acid patterns. The most prominent biochemical consequence of 5FMOrn administration is the elevation of ornithine concentrations in tissues, including the brain, and in body fluids. Elevated ornithine concentrations are, therefore, the most likely basis for the therapeutic effects of 5FMOrn. In agreement with this notion is the enhancement of citrulline and urea formation. These findings and the observation that administration of ornithine in combination with a branched-chain 2-oxoacid ameliorated the pathologic symptoms of portal-systemic encephalopathy suggest inhibition of OAT in the treatment of this disease. The liver protective effect of 5FMOrn is not yet understood; the enhancement of regenerative processes is a likely explanation.

Enhanced endogenous ornithine concentrations protect against tonic seizures and coma in acute ammonia intoxication

Pretreatment of mice with 5-fluoromethylornithine (5FMOrn), a selective inactivator of ornithine aminotransferase, diminishes the accumulation of ammonia in the brain after administration of ammonium acetate, and antagonizes ammonia-induced fatal tonic extensor convulsions. In about 50% of the treated animals the loss of the righting reflex and coma is prevented. Presumably these effects are based on the enhancement of urea formation by the increased liver ornithine concentrations. However, since brain ornithine concentrations are greatly enhanced by 5FMOrn, it is not excluded that ornithine has direct effects on cellular events involved in ammonia-induced seizure generation, even though 5FMOrn had no anticonvulsant properties in a series of established animal seizure models, including N-methyl-D,L-aspartate-induced convulsions. NMDA receptor antagonists are capable of preventing death, but do not protect against the generation of coma and tonic extensor convulsions in ammonium acetate intoxicated mice. Since no evidence was found for ammonia-induced glutamate release from rat hippocampus, there is no convincing evidence for the idea that the tonic convulsions are mediated by NMDA receptors. L-Methionine-D, L-sulfoximine (MSO)-induced seizures can be partially antagonized by pretreatment with 5FMOrn. However, the effect is considerably smaller than against ammonia-induced convulsions, although at the time of seizure onset brain ammonia levels of MSO-intoxicated mice were lower than in the animals receiving ammonium acetate. This suggests that MSO-convulsions are not entirely due to the elevation of brain ammonia concentrations, even though MSO administration mimics effects of ammonia on cortical inhibitory neuronal interactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of ornithine aminotransferase by 5-fluoromethylornithine: protection against acute thioacetamide intoxication by elevated tissue ornithine levels

5-Fluoromethylornithine (5FMOrn) is a selective inactivator of ornithine aminotransferase. Its administration causes a dramatic increase of ornithine concentrations in all tissues. Treatment of mice with 20 mg.kg-1 5FMOrn shortly before or after a lethal dose (600 mg.kg-1, intraperitoneally) of thioacetamide (TAA), followed by a second dose 24 hr later, prevented death of 60% of the mice. Pathologic symptoms of TAA intoxication (liver haemorrhage, elevation of amino acids in blood and tissues, diminution of liver spermidine and spermine concentrations, elevation of the activity of liver enzymes in the plasma) were significantly ameliorated by the treatment. The liver protective action of 5FMOrn is related to the elevation of ornithine concentration, as appears from the fact that other, less selective inactivators of ornithine aminotransferase, also produced some protection against acute intoxication with TAA, but not a structurally related compound with no effect on this enzyme.

DL-canaline and 5-fluoromethylornithine. Comparison of two inactivators of ornithine aminotransferase

5-Fluoromethylornithine (5FMOrn) is an enzyme-activated irreversible inhibitor or ornithine aminotransferase (L-ornithine:2-oxo-acid 5-aminotransferase, OAT). For purified rat liver OAT, Ki(app.) was found to be 30 microM. and tau 1/2 = 4 min. Of the four stereomers of 5FMOrn only one reacts with OAT. The formation of a chromophore with an absorption maximum at 458 nm after inactivation of OAT by 5FMOrn suggests the formation of an enamine intermediate, which is slowly hydrolysed to release an unsaturated ketone. L-Canaline [(S)-2-amino-4-amino-oxybutyric acid] is a well-known irreversible inhibitor of OAT. Not only the natural L-enantiomer but also the D-enantiomer reacts by oxime formation with pyridoxal 5'-phosphate in the active site of the enzyme, although considerably more slowly. This demonstrates that the stereochemistry at C-2 of ornithine is not absolutely stringent. In vitro, canaline reacted faster than 5FMOrn with OAT. In vivo, however, only incomplete OAT inhibition was observed with canaline. Whereas intraperitoneal administration of 10 mg of 5FMOrn/kg body wt. to mice was sufficient to inactivate OAT in brain and liver by 90% for 24 h, 500 mg of DL-canaline/kg body wt. only produced a transient inhibition of 65-70%. The accumulation of ornithine in these tissues was considerably slower and the maximum concentrations lower than were achieved with 5FMOrn. It appears that DL-canaline, in contrast with 5FMOrn, is not useful as a tool in studies of biological consequences of OAT inhibition.

Interaction of L-canaline with ornithine aminotransferase of the tobacco hornworm, Manduca sexta (Sphingidae)

Ornithine aminotransferase (L-ornithine:2-oxo-acid aminotransferase (EC 2.6.1.13)) has been purified to homogeneity from last instar larvae of the tobacco hornworm, Manduca sexta (Sphingidae). This enzyme is a 144,000-Da tetramer constructed from 36,000-Da protomeric units. It has a high aspartate/asparagine and glutamate/glutamine content and 2 cysteine residues/subunit. All 8 cysteine residues can react with N-ethylmaleimide to inactivate the enzyme. Maintenance of the enzyme in the presence of 2-mercaptoethanol and dithiothreitol maximizes enzymatic activity and improves storage conditions, presumably by protecting these sulfhydryl groups. The apparent Km values for L-ornithine and 2-oxoglutaric acid are 2.3 and 3.2 mM, respectively. The turnover number is 2.0 +/- 0.1 mumol min-1 mumol-1. L-Canaline (L-2-amino-4-(aminooxy)butyric acid) is a potent ornithine aminotransferase inhibitor. Reaction of the enzyme with L-[U-14C]canaline produces an enzyme-bound, covalently linked, radiolabeled canaline-pyridoxal phosphate oxime. The L-[U-14C]canaline-pyridoxal phosphate oxime has been isolated from canaline-treated enzyme. Dialysis of canaline-inactivated ornithine aminotransferase against free pyridoxal phosphate slowly reactivates the enzyme as the oxime is replaced by pyridoxal phosphate. Analysis of L-[U-14C]canaline binding to ornithine aminotransferase reveals the presence of 4 mol of pyridoxal phosphate/mol of enzyme.

Tryptic cleavage as a probe of conformational differences between active and inactive forms of ornithine aminotransferase

Treatment of ornithine aminotransferase with trypsin resulted in rapid and complete loss of enzyme activity in a process that coincided with a reduction in subunit Mr of about 3000. However, electrophoresis showed that a substantial proportion of the enzyme had not been digested. The component of the preparation of ornithine aminotransferase that was insusceptible to trypsin contained a naturally occurring but enzymically inactive form of the enzyme, and when this had been removed, the remaining fully active enzyme was completely digested. Irreversible inactivation with a substrate analogue made all of the enzyme insusceptible to trypsin. The hydrolyzed enzyme still underwent a very slow half-reaction with ornithine. Sequence analysis of the truncated protein, prepared by blotting from electrophoretic gels, showed that hydrolysis had occurred at peptide bond Lys26-Tyr27.

Participation of ornithine aminotransferase in the synthesis and catabolism of ornithine in mice. Studies using gabaculine and arginine deprivation

Gabaculine, a potent suicide inhibitor of ornithine aminotransferase (OAT), at a dose of 50 mg/kg inhibited this enzyme in mouse tissues and dramatically increased tissue ornithine concentrations, whether or not arginine was present in the diet. Thus even under arginine deprivation there is catabolism of ornithine which involves OAT. This was confirmed by administration of [14C]ornithine to arginine-deprived mice. Gabaculine (3-amino-2,3-dihydrobenzoic acid) drastically decreased the release of 14CO2 and increased the radioactivity in the basic amino acids in the tissues. When [1-14C]glutamate was injected into mice deprived of arginine, a significant amount of radioactivity was recovered in tissue ornithine and arginine, and gabaculine decreased this labelling by about two-thirds, indicating that ornithine was synthesized in vivo from glutamate via OAT. In addition, we failed to detect in liver and small intestine alpha-N-acetylornithine, N-acetylglutamate kinase or N-acetylornithine aminotransferase, which are obligatory components of a potential route of ornithine synthesis from N-acetylglutamate. Our results indicate that at least 45 mumol of ornithine was synthesized and catabolized daily via OAT in the mouse deprived of arginine.

Ornithine aminotransferase activity, liver ornithine concentration and acute ammonia intoxication

5-Fluoromethylornithine (5FMOrn) is a specific inactivator of L-ornithine:2-oxoacid aminotransferase (OAT). Inactivation of OAT causes the enhancement of L-ornithine (Orn) concentrations in all tissues. Intraperitoneal or oral administration of 10-50 mg/kg of 5FMOrn per day to albino mice rendered partial protection against lethal intoxication with 26 mmol/kg of ammonium acetate. The protective effect was maximal around 16 h after 5FMOrn administration, at the time when endogenous Orn concentrations were maximal. At this time protection by 5FMOrn against acute ammonia intoxication was comparable to that observed 1 h after the intraperitoneal administration of 10 mmol/kg of L-arginine. Pretreatment with 5FMOrn prevented the enhancement of excessive urinary excretion of orotic acid by ammonia intoxicated mice, and it enhanced urea formation in the liver. These biochemical effects demonstrate that 5FMOrn shifts Orn into the urea cycle, Orn which normally would be transaminated. Since even long-term treatment of mice with 5FMOrn did not reveal toxic effects, this compound may be considered for the treatment of certain conditional deficiencies of Orn or arginine.

Ornithine aminotransferase activity, tissue ornithine concentrations and polyamine metabolism

1. Inactivation of L-ornithine:2-oxoacid aminotransferase (OAT) by 5-fluoromethylornithine (5FMOrn), a specific inactivator of OAT, causes a great elevation of tissue ornithine (Orn) concentrations. 2. Inhibition of L-ornithine decarboxylase (ODC) by 2-difluoromethylornithine (DFMO) had no effect on Orn concentrations. 3. The combined administration of 5FMOrn and DFMO produced a 2- to 3-fold greater enhancement of tissue Orn concentrations than treatment with 5FMOrn alone. 4. The increase of tissue Orn concentrations had a long-lasting enhancing effect on polyamine metabolism. 5. In the brain this could be demonstrated by the elevation of putrescine and spermidine concentrations and the increase of spermidine turnover rate. 6. In visceral organs polyamine concentrations were not elevated because polyamines can be eliminated by transport. 7. In line with this notion is the fact that urinary polyamine excretion was increased for several days, even after a single dose of 5FMOrn. 8. Inhibitors of 4-aminobutyric acid:2-oxoglutarate aminotransferase which are also inactivators of OAT had the same effect on polyamine excretion as 5FMOrn.

5-Fluoromethylornithine, an irreversible and specific inhibitor of L-ornithine:2-oxo-acid aminotransferase

5-Fluoromethylornithine (5-FMOrn) is the first specific irreversible inhibitor of L-ornithine:2-oxoacid aminotransferase (OAT) found. Single doses (greater than 10 mg/kg) of this compound inactivate OAT to a residual OAT-like activity. This activity (10-20% of total activity) is resistant to further inactivation by higher or repeated doses of 5-FMOrn, or incubation with the inactivator in vitro. Ornithine concentrations are greatly enhanced in various tissues, and urinary ornithine is dramatically increased, but no other amino acid is affected after acute treatment with 5-FMOrn. Repeated administration decreases carnosine and homocarnosine concentrations in brain. Toxic effects were not observed. The new inactivator is considered as a tool in the establishment of functions of OAT under physiological and pathological conditions.

Effects of inhibition of ornithine aminotransferase or of general aminotransferases on urea and citrulline synthesis and on the levels of acetylglutamate in isolated rat hepatocytes

Canaline and gabaculine, inhibitors of gamma-aminotransferases and thus of ornithine aminotransferase (E.C. 2.6.1.13), decreased the flow through ornithine carbamoyl transferase (E.C. 2.1.3.3) in isolated rat hepatocytes incubated with 10 mM NH4Cl and ornithine. The levels of acetylglutamate, an essential activator of carbamoyl phosphate synthetase (ammonia) (E.C. 6.3.4.16), were also decreased, suggesting that the inhibitors had also caused a decrease in the rate of carbamoyl phosphate synthesis. Under these conditions, ornithine appears to be a precursor of acetylglutamate, via ornithine aminotransferase, possibly as a consequence of glutamate synthesis. The influence of aminooxyacetate, an aminotransferase inhibitor, has also been examined.

Interrelationships between ornithine, glutamate, and GABA. II. Consequences of inhibition of GABA-T and ornithine aminotransferase in brain

The objective of the present study was to compare the effects of elevation of GABA concentration and those of inactivation of L-ornithine: 2-oxoacid aminotransferase (OAT) on the in vivo metabolism of L-ornithine (Orn) in brain. Vigabatrin (4-aminohex-5-enoic acid) and gabaculine (5-amino-1,3-cyclohexadienyl carboxylic acid), two well known inactivators of GABA-T, were used to elevate brain GABA concentrations. The latter inactivates OAT also. Transamination of Orn is, from a quantitative point of view, a significant reaction in mouse brain. GABA is a feed-back regulator of OAT. Within GABAergic neurons Orn concentration may be regulated by endogenous GABA. Extensive inactivation of OAT causes a considerable increase of Orn concentration, both in synaptosomes and in non-synaptosomal compartments. The results are compatible with a role of Orn as precursor of glutamate and/or GABA in certain neurons.

Identification of an intermediate of delta-aminolevulinate biosynthesis in Chlamydomonas by high-performance liquid chromatography

The first committed intermediate of the chlorophyll biosynthetic pathway is delta-aminolevulinic acid (ALA). In plant cells, ALA is formed from glutamate by a pathway not yet clearly defined. One of the proposed pathways involves the reduction of glutamate to glutamate-1-semialdehyde (GSA) via a glutamyl-tRNA intermediate. GSA is then converted to ALA by an aminotransferase. We are studying this pathway using partially purified components from Chlamydomonas reinhardtii in in vitro reactions with [3H]L-glutamate as the substrate and analysis of the radioactive reaction products via HPLC. In reactions either lacking GSA-aminotransferase or containing gabaculine (an inhibitor of aminotransferase), a radioactive intermediate is formed which cochromatographs with synthetic GSA. As observed previously for ALA synthesis, the synthesis of this intermediate has an absolute requirement for RNA, ATP, and active enzymes, while the requirement for NADPH is less stringent. Both the accumulated intermediate and the synthetic GSA can be converted to ALA by the aminotransferase without any additional substrates or cofactors. These results support previous observations that GSA or a very similar compound is an intermediate of ALA synthesis.

Temperature- and time-dependent inactivation of pyrroline-5-carboxylate synthase: suggestive evidence for an allosteric regulation of the enzyme

When pyrroline-5-carboxylate (PC) synthase activity in the membrane of mitochondria of rat small intestine mucosa was assayed in the presence of 0.5 mM ornithine, the time course of inactivation showed that the activity disappeared entirely by about 8 min at 30 degrees C, whereas there was no decrease in the activity at 15 degrees C. A prior incubation of the enzyme with ornithine at 30 or 37 degrees C in the presence of 50% sorbitol as a thermal stabilizer resulted in a marked loss of the activity, while that at 0 or 15 degrees C did not lose any. This suggests that PC synthase is inactivated by ornithine regardless of the presence of substrates. The inactivation at 30 degrees C proceeded gradually for about 7 h, until an equilibrium was attained. Extensive dialysis allowed the inactivated enzyme to regain about 60% of the original activity. These results suggest that the inactivation is reversible. The concentration of ornithine and the percentage of inactivation at equilibrium was correlated by the Hill equation and displayed a sigmoidicity with n = 1.47 and [S]50 = 0.036 mM. In the presence of sorbitol, the inactivation was prevented by 0.2 mM ATP or ADP. The role of the nucleotides in PC synthase regulation is discussed.

Ornithine as a precursor of neurotransmitter glutamate: effect of canaline on ornithine aminotransferase activity and glutamate content in the septum of rat brain

Local injections of L-canaline into the septum produce a rapid and almost complete inhibition of ornithine aminotransferase activity followed by a decrease in glutamate content in this region. The time-course of canaline action shows the existence of two glutamate pools with different sizes and half-life values. Surgical lesions of the hippocampal-septal glutamatergic pathway affected the site and kinetics of the small pool of glutamate in the septum, suggesting the participation of ornithine aminotransferase in the synthesis of this pool. This indicates a possible role of ornithine as a precursor of the transmitter glutamate. The localization of ornithine aminotransferase does not seem, however, to be specific for the nerve-terminal compartment. The data obtained allow estimation of the turnover rate of the specific pool of neurotransmitter glutamate.

A kinetic model for the action of the enzyme activated irreversible inhibitor 4-amino-5-hexynoic acid in vivo

The extent of inactivation of three aminotransferases by the enzyme activated inhibitor 4-amino-hex-5-ynoate (acetylenic-GABA) increased with increasing dose in an exponential fashion. Theoretical treatment of the data allowed an estimate of the effective concentration of the drug at its site of action to be made and it was apparent that any rises in substrate concentration produced by the inactivation did not protect the enzyme significantly. Altered diet produced distinct changes in the extent of inactivation of aspartate aminotransferase, but not with ornithine aminotransferase. Cysteine sulphinate, a substrate only of aspartate aminotransferase, also affected the inactivation of ornithine aminotransferase, suggesting that secondary metabolic effects were responsible.

Purification and properties of ornithine aminotransferase from rat brain

Ornithine aminotransferase (E.C. 2.6.1.13) from rat brain was purified 100-fold by ammonium sulphate fractionation, DEAE cellulose chromatography, calcium phosphate gel and alumina C gamma gel. Pyridoxal phosphate was essential for maximum activity of the enzyme. The brain enzyme did not differ from liver and kidney enzymes in properties such as pH optimum, Km, substrate specificity and the inhibition by branched chain amino acids. Unlike rat liver enzyme, brain ornithine aminotransferase was able to catalyze the reaction between L-lysine and 2-oxoglutarate. Spermidine and spermine inhibited brain ornithine aminotransferase activity.

An investigation of the properties of ornithine aminotransferase after inactivation by the 'suicide' inhibitor aminohexynoate and use of the compound as a probe of intracellullar protein turnover

Ornithine aminotransferase is shown to bind 1 mol of amino[14C]hexynoate per mol of coenzyme in the 'suicide' inactivation process. At the same time the coenzyme pyridoxal phosphate becomes irreversibly bound to the enzyme protein. Apart from the inactivation, the labelled enzyme is indistinguishable from native ornithine aminotransferase by several separation techniques. Because the rate of degradation of the labelled enzyme is the same as that of the normal enzyme it is concluded that loss of coenzyme does not initiate turnover. Free aminohexynoate is rapidly eliminated from the liver, and 70% of the compound is excreted unchanged in 7.5 h. Inactivated ornithine aminotransferase accounts for 11% of the total labelled liver protein and significant amounts of label are found in aspartate aminotransferase which is also extensively inactivated. The rate of return of enzyme activity is determined and found to be more rapid than expected for a process in which the enzyme is synthesized at a constant rate and degraded in a single, first-order process.

Regulation of proline biosynthesis: the inhibition of pyrroline-5-carboxylate synthase activity by ornithine

Mammalian cells have the capacity for proline biosynthesis from ornithine or glutamic acid. Using a radioisotopic assay, we have studied the regulation by ornithine of delta 1-pyrroline-5-carboxylate synthase, the enzyme that catalyzes the first step of proline biosynthesis from glutamic acid. In homogenates from Chinese hamster ovary cells, ornithine was found to be a potent inhibitor of pyrroline-5-carboxylate synthase activity(50% inhibition at 0.37 mM). The effect was reversible and did not occur with amino acids other than ornithine. Preliminary findings suggest that the inhibition does not result from altered requirements for the cofactors NADPH and ATP. Significant inhibition was observed in four different Chinese hamster cell lines. Ornithine was also shown to inhibit the conversion of 3H-glutamic acid to 3H-proline in intact human skin fibroblasts. Cells from patients with a rare ocular disease, gyrate atrophy of the choroid and retina, were used for these studies since they lack interfering ornithine aminotransferase activity. We conclude that ornithine may be a physiologic regulator of the rate of proline formation from glutamic acid. This information allows us to construct an hypothetical model for the overall regulation of proline biosynthesis and also to suggest a pathophysiologic mechanism for the disease gyrate atrophy.

Enzyme-induced inactivation of transminases by acetylenic and vinyl analogues of 4-aminobutyrate

The reactions of two analogues of 4-aminobutyrate, namely 4-aminohex-5-ynoate and 4-aminohex-5-enoate, with three transaminases were studied. Three pure enzymes were used, aminobutyrate transaminase (EC 2.6.1.19), ornithine transaminase (EC 2.6.1.13) and aspartate transaminase (EC 2.6.1.1), and the course of the reactions was studied by observing changes in the absorption spectrum of the bound coenzyme and by observing loss of activity. All of the enzymes were inactivated by either inhibitor, but amino-hexenoate showed a marked specificity for aminobutyrate transaminase. Aminohexynoate was most potent towards ornithine transaminase, and with this enzyme transamination of the inhibitor is an important factor in protecting the enzyme. Most of the reactions could be analysed as first order, with the observed rate constant showing a hyperbolic dependence on inhibitor concentration.

Enzyme-activated irreversible inhibitors of L-ornithine:2-oxoacid aminotransferase. Demonstration of mechanistic features of the inhibition of ornithine aminotransferase by 4-aminohex-5-ynoic acid and gabaculine and correlation with in vivo activity

L-Ornithine:2-oxoacid aminotransferase is a specific enzyme with respect to the amino group donor. Nevertheless it was found that this enzyme is inhibited by some 4-aminobutyrate analogs, 4-aminohex-5-ynoic acid and 5-amino-1,3-cyclohexadienyl-carboxylic acid (gabaculine), which are currently considered to be enzyme-activated irreversible inhibitors of 4-aminobutyrate:2-oxoglutarate aminotransferase. The inhibitory mechanisms for the two omega-aminotransferases are identical. A close structural analog of these inhibitors, 4-aminohex-5-enoic acid, is not inhibitory for ornithine aminotransferase, whereas it effectively inhibits 4-aminobutyrate aminotransferase. The reasons for this difference are discussed. The in vitro findings are entirely transferable to the in vivo situation: 4-aminohex-5-ynoic acid and gabaculine cause a long-lasting inhibition of ornithine aminotransferase in brain and liver, and reduce significantly in vivo ornithine degradation, whereas 4-aminohex-5-enoic acid is inactive both in vivo and in vitro toward this enzyme. The enzyme-activated irreversible inhibitors allow one for the first time to study the physiological consequences of irreversible ornithine aminotransferase inhibition.

Mode of inhibition of ornithine aminotransferase by L-canaline

The mechanism of inhibition of ornithine aminotransferase [EC 2.6.1.13] by L-canaline (alpha-amino-gamma-amino-oxybutyric acid) was investigated. Spectral changes of pyridoxal 5'-phosphate in ornithine aminotransferase on addition of L-canaline showed that L-canaline formed an oxime-type compound with pyridoxal 5'-phosphate that had the same spectra as the compound formed on addition of hydroxylamine to the holoenzyme. Kinetic studies indicated that hydroxylamine was a reversible noncompetitive inhibitor, whereas L-canaline was an irreversible inhibitor of ornithine aminotransferase. Other analogs, such as delta-aminovaleric acid and alpha-N-acetyl-L-ornithine, also reacted with the pyridoxal 5'-phosphate of the enzyme, but these compounds were competitive inhibitors with respect to L-ornithine. L-Canaline and hydroxylamine also reacted with pyridoxal 5'-phosphate in pig heart aspartate aminotransferase [EC 2.6.1.1] to produce an oxime, but both of them were reversible and noncompetitive inhibitors of the enzyme. The Ki value of hydroxylamine for ornithine aminotransferase was 4.3 X 10(-7) M and those of L-canaline and hydroxylamine for aspartate aminotransferase were 1.7 X 10(-4) M and 2.2 X 10(-5) M, respectively.