Inhibition of human ornithine decarboxylase activity by enantiomers of difluoromethylornithine

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 μM 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 ∼ 7.5 μM) when compared with D-ornithine (IC₅₀ ∼ 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 α-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.