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

Abstract

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.

Ornithine Aminotransferase, an Important Glutamate-Metabolizing Enzyme at the Crossroads of Multiple Metabolic Pathways

“…OAT is inhibited by aminohexanoate [35], GABA, gabaculine, 5-fluoromethylornithine (5-FMOrn) [20,36,37,38], and l -canaline [39]. A more complete list of inhibitors and their structures is given in Table 2.…”

Phenelzine Causes an Increase in Brain Ornithine that is Prevented by Prior Monoamine Oxidase Inhibition

“…This is supported by the findings that GABA-T and ORN-T are closely related in terms of structure and activity [28], and that all reported GABA-T inhibitors, except for ethanolamine-O-sulfate and vigabatrin, are also inhibitors of ORN-T [28][29][30]. Furthermore, selective inhibition of ORN-T, which is heavily concentrated in nerve terminals [31], leads to accumulation of synaptic ORN [32] and drastically elevates ORN levels in the rodent brain [33,34]. It should be noted that aside from this direct effect of PLZ, ORN-T may also be indirectly inhibited by PLZ, as GABA itself is a competitive inhibitor of ORN-T, acting as part of a negative-feedback mechanism to regulate ORN-T activity in vivo [35].…”

Heterogeneity in Ornithine Cytotoxicity of Bovine Retinal Pigment Epithelial Cells in Primary Culture

“…OAT activities in the RPE cells were assayed as described previously (Daune, Gerhart and Seiler, 1988). The standard assay mixture contained 175 m -ornithine, 35 m α-ketoglutarate, 80 µ pyridoxal phosphate, 50 m potassium phosphate buffer (pH 8n0) and 300 µl of cell extracts in a 1 ml volume.…”

“…It was demonstrated that 5-FMOrn inactivated OAT, but none of the other enzymes involved in ornithine metabolism, such as γ-aminobutyric acid-α-ketoglutaric acid transaminase, -ornithine decarboxylase or -ornithine carbamoyltransferase (Daune, Gerhart and Seiler, 1988). To confirm the inactivation of OAT in each phenotype of RPE cells, 0n5 m 5-FMOrn was added to the cells that had been plated onto 6 well plates at 5i10& cells well −" .…”

“…We have recently shown that inactivation of OAT in human RPE cell lines by 5-fluoromethylornithine (5-FMOrn), a specific irreversible inhibitor of OAT (Daune, Gerhart and Seiler, 1988), made them susceptible to ornithine, leading to cell death (Ueda et al, 1998). Furthermore, proline, one of the metabolites of ornithine, prevented ornithine-induced cell death.…”