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

Abstract

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.

Redox Regulation of Plasmodium falciparum Ornithine δ-Aminotransferase

“…To exclude unspecific side reactions of the cell extract with ninhydrin in the assay, we inhibited PfOAT in the cell extract by two known inhibitors of OATs, α-amino-γ-amino-oxobutyric acid (L-canaline) and gabaculine. 16,24,33 The IC 50 values measured for PfOAT in the cell extract were identical with those that we determined for the recombinant protein with both inhibitors (3 μM for L-canaline and 15 μM for gabaculine). Therefore, unspecific background reactions could be neglected.…”

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“…These four inhibitors and canaline, an aminooxy analogue of ornithine that has been demonstrated to be an effective aminotransferase inhibitor in other systems [16,17,26-28], were screened against M. tuberculosis and M. marinum in vitro to determine potential antimicrobial activity. M. marinum is a close relative of M. tuberculosis that causes a similar disease in fish, grows faster than M. tuberculosis in culture, and does not cause serious infections in humans [29].…”

Ornithine Aminotransferase versus GABA Aminotransferase: Implications for the Design of New Anticancer Drugs

“…Mechanism‐based inactivators have a higher chance of being selective because of the distinct mechanistic pathway that is required for covalent modification of the enzyme . 5FMOrn selectively inactivates OAT as shown in Scheme , by forming an enamine intermediate that leads to a ternary adduct in a manner similar to Class II inactivators of GABA‐AT . However, vigabatrin is categorized as both a Class I and Class II inactivator, because it displays both mechanisms .…”