Comparative properties of schistosomal and filarial dihydrofolate reductases

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Abstract Dihydrofolate (DHF) reductase activity was detected in crude extracts of four species of adult filarial worms ( Dirofilaria immitis, Litomosoides carinii, Dipetalonema witei, and Onchocerca volvulus ) and the adult blood fluke ( Schistosoma mansoni ). The filariae were much richer sources of the enzyme than were the schistosomes. Lyophilized D. immitis and L. carinii were as rich a source of the enzyme as comparable numbers of their living counterparts subsequently subjected to acetone powdering. Furthermore, extracts of females of both these species contained more than twice as much DHF reductase activity per milligram of protein as those of the males. The apparent K m value of the schistosomal and all four filarial reductases for DHF was approximately 5 × 10 −6 M, while that of the parasites' reductases for NADPH was in the range of 1–3 × 10 −5 M. Their molecular weights were all around 20,000. Both schistosomal and filarial DHF reductases showed similar patterns of sensitivity to various drugs and generally were less sensitive than the analogous mammalian enzyme to inhibition by most of the presently available reductase inhibitors. Methotrexate and a related 2,4-diaminoquinazoline (CCNSC 529,861) were the most potent inhibitors among the 14 compounds tested, causing 50 per cent inhibition of enzyme activity in the range of 10 −8 M to 10 −10 M. However, while these two compounds were roughly equipotent against the schistosomal and mammalian enzymes, the quinazoline derivative was significantly more potent against the filarial enzymes. In terms of the ID 50 , the parasites' reductases were more sensitive than the analogous mammalian enzyme to inhibition by the 2,4-diaminopyrimidine trimethoprim. On the same basis, the parasites' reductases were more sensitive to suramin, especially that from O. volvulus , the ID 50 in this case being 2 × 10 −6 M compared with 7 × 10 −5 for the mammalian reductase.