RESUMEN
While dihydrofolate reductase (DHFR) is an important drug target in mammals, bacteria and protozoa, no inhibitors of this enzyme have been developed as commercial insecticides. We therefore examined the potential of this enzyme as a drug target in an important ectoparasite of livestock, the Australian sheep blow fly, Lucilia cuprina (Diptera: Calliphoridae) (Wiedemann). The non-specific DHFR inhibitors aminopterin and methotrexate significantly inhibited the growth of L. cuprina larvae, with IC50 values at µg levels. Trimethoprim and pyrimethamine were 5-30-fold less active. Relative IC50 values for the inhibition of recombinant L. cuprina DHFR by various inhibitors were in accordance with their relative effects on larval growth. The active-site amino acid residues of L. cuprina DHFR differed by between 34% and 50% when compared with two mammalian species, as well as two bacteria and two protozoa. There were significant charge and size differences in specific residues between the blow fly and human DHFR enzymes, notably the L. cuprina Asn21, Lys31 and Lys63 residues. This study provides bioassay evidence to highlight the potential of blow fly DHFR as an insecticide target, and describes differences in active site residues between blow flies and other organisms which could be exploited in the design of blow fly control chemicals.
Asunto(s)
Dípteros/efectos de los fármacos , Dípteros/genética , Insecticidas/farmacología , Proteínas Recombinantes/genética , Tetrahidrofolato Deshidrogenasa/genética , Secuencia de Aminoácidos , Animales , Clonación Molecular , Dípteros/crecimiento & desarrollo , Dípteros/metabolismo , Activación Enzimática/efectos de los fármacos , Regulación Enzimológica de la Expresión Génica , Concentración 50 Inhibidora , Proteínas de Insectos/genética , Proteínas de Insectos/metabolismo , Larva/efectos de los fármacos , Larva/genética , Larva/metabolismo , Datos de Secuencia Molecular , Pupa/efectos de los fármacos , Pupa/genética , Pupa/metabolismo , Proteínas Recombinantes/metabolismo , Alineación de Secuencia/veterinaria , Tetrahidrofolato Deshidrogenasa/metabolismoRESUMEN
Rhagodia preissii had shown significant in vitro anthelmintic activity in a previous study, we examined the effect of including this shrub in the diet of sheep infected with Trichostrongylus colubriformis. Worm-infected merino wethers were grazed for 7 weeks on either R. preissii or annual pasture, and faecal egg counts (FECs) were conducted weekly. Plant material was collected weekly from eaten and uneaten plants, and analysed for levels of plant secondary metabolites (tannins, oxalates, saponins) and in vitro anthelmintic activity. While mean FECs were consistently lower in sheep grazing R. preissii compared to pasture (reductions of 20-74%), the differences were not significant. There was no relationship between grazing preference (eaten or uneaten) and in vitro anthelmintic activity of plant extracts. The levels of saponins and oxalates did not correlate with grazing preference or in vitro anthelmintic activity, while tannins were not responsible for the anthelmintic activity. While the identity of the grazing deterrent and in vitro anthelmintic compounds remain unknown, the presence of plants which were both highly preferred by the sheep and showed in vitro anthelmintic activity indicates a potential to develop the species as an anthelmintic shrub through selection of shrub populations dominated by such plants.