Molecular and biochemical characterisation of ornithine decarboxylases in the sheep abomasal nematode parasites Teladorsagia circumcincta and Haemonchus contortus

摘要

Full length cDNA encoding ornithine decarboxylases (ODC; EC 4.1.1.17) were cloned from the sheep abomasal nematode parasites Teladorsagia circumcincta (TcODC) and Haemonchus contortus (HcODC). The TcODC (1272bp) and HcODC cDNA (1266bp) encoded 424 and 422 amino acid proteins respectively. The predicted TcODC amino acid sequence showed 87% identity with HcODC and 65% and 64% with Caenorhabditis elegans and Caenorhabditis briggsae ODC respectively. All binding sites and active regions were completely conserved in both proteins. Soluble N-terminal His-tagged ODC proteins were expressed in Escherichia coli strain BL21, purified and characterised. The recombinant TcODC and HcODC had very similar kinetic properties: K_m ornithine was 0.2-0.25mM, optimum [PLP] was 0.3mM and the pH optima were pH8. No enzyme activity was detected when arginine was used as substrate. One millimolar difluoromethylornithine (DFMO) completely inhibited TcODC and HcODC activity, whereas 2mM agmatine did not inhibit activity. The present study showed that ODC is a separate enzyme from arginine decarboxylase and strictly uses ornithine as substrate.