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Inhibition of human ornithine decarboxylase activity by enantiomers of difluoromethylornithine.

ABSTRACT: Racemic difluoromethylornithine (D/L-DFMO) is an inhibitor of ODC (ornithine decarboxylase), the first enzyme in eukaryotic polyamine biosynthesis. D/L-DFMO is an effective anti-parasitic agent and inhibitor of mammalian cell growth and development. Purified human ODC-catalysed ornithine decarboxylation is highly stereospecific. However, both DFMO enantiomers suppressed ODC activity in a time- and concentration-dependent manner. ODC activity failed to recover after treatment with either L- or D-DFMO and dialysis to remove free inhibitor. The inhibitor dissociation constant (K(D)) values for the formation of enzyme-inhibitor complexes were 28.3+/-3.4, 1.3+/-0.3 and 2.2+/-0.4 microM respectively for D-, L- and D/L-DFMO. The differences in these K(D) values were statistically significant ( P <0.05). The inhibitor inactivation constants (K(inact)) for the irreversible step were 0.25+/-0.03, 0.15+/-0.03 and 0.15+/-0.03 min(-1) respectively for D-, L- and D/L-DFMO. These latter values were not statistically significantly different ( P >0.1). D-DFMO was a more potent inhibitor (IC50 approximately 7.5 microM) when compared with D-ornithine (IC50 approximately 1.5 mM) of ODC-catalysed L-ornithine decarboxylation. Treatment of human colon tumour-derived HCT116 cells with either L- or D-DFMO decreased the cellular polyamine contents in a concentration-dependent manner. These results show that both enantiomers of DFMO irreversibly inactivate ODC and suggest that this inactivation occurs by a common mechanism. Both enantiomers form enzyme-inhibitor complexes with ODC, but the probability of formation of these complexes is 20 times greater for L-DFMO when compared with D-DFMO. The rate of the irreversible reaction in ODC inactivation is similar for the L- and D-enantiomer. This unexpected similarity between DFMO enantiomers, in contrast with the high degree of stereospecificity of the substrate ornithine, appears to be due to the alpha-substituent of the inhibitor. The D-enantiomer may have advantages, such as decreased normal tissue toxicity, over L- or D/L-DFMO in some clinical applications.

SUBMITTER: Qu N

PROVIDER: S-EPMC1223689 | biostudies-other | 2003 Oct

REPOSITORIES: biostudies-other

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Project description:Endotoxemia impairs hypoxic pulmonary vasoconstriction which leads to systemic hypoxemia. This derogation is attributable to increased activity of nitric oxide synthase 2 and arginase metabolism. Gene expression analysis has shown increased expression of ornithine decarboxylase in lungs of endotoxemic mice, a downstream enzyme of arginase metabolism. The aim of this study was to investigate whether inhibition of ornithine decarboxylase increases hypoxic pulmonary vasoconstriction in lungs of endotoxemic mice. Mice received lipopolysaccharides or saline intraperitoneal, and hypoxic pulmonary vasoconstriction was measured using an isolated perfused mouse lung model. Additional mice with and without endotoxemia were pretreated with the ornithine decarboxylase-inhibitor difluoromethylornithine before examination of hypoxic pulmonary vasoconstriction. Hypoxic pulmonary vasoconstriction was defined as the difference of pulmonary arterial pressure between normoxic and hypoxic ventilation. In addition, lung tissue was analyzed using real-time quantitative polymerase chain reaction, Western blot and immunohistochemistry. Lipopolysaccharides caused an up-regulation of ornithine decarboxylase mRNA level (2.73 ± 0.19-fold increase, p < 0.05) as well as ornithine decarboxylase protein level (4.05 ± 0.37-fold increase, p < 0.05). Endotoxemia attenuated hypoxic pulmonary vasoconstriction when compared with untreated control mice (26.3 ± 9.7% vs. 67.0 ± 17.5%). Difluoromethylornithine (20, 100, 500 mg kg-1 body weight intraperitoneal) restored hypoxic pulmonary vasoconstriction in lungs of endotoxemic mice in a dose-dependent way (25.8 ± 9.9%, 57.3 ± 17.2%, 62.3 ± 12.4%) and decreased hypoxic pulmonary vasoconstriction in control mice (53.6 ± 13.6%, 40.0 ± 14.9%, 35.9 ± 12.4%). These results show that endotoxemia induces ornithine decarboxylase expression and suggest that ornithine decarboxylase contributes to the endotoxemia-induced impairment of hypoxic pulmonary vasoconstriction. Inhibition of ornithine decarboxylase might be a target in the therapy of diseases with inflammation impaired hypoxic pulmonary vasoconstriction, like the sepsis-associated acute respiratory distress syndrome (ARDS).

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Inhibition of human ornithine decarboxylase activity by enantiomers of difluoromethylornithine.

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