RESUMO
Three mutations in Plasmodium falciparum yielding increased resistance to pyrimethamine were obtained following treatment with chemical mutagens and selection in presence of pyrimethamine. From parasite clone TM4/8.2 a mutant, TM4/8.2/4.1, was produced which raised pyrimethamine resistance about 500 times and was found to involve an amino acid change in the DHFR-TS enzyme molecule from Ser108 to Asn108. A clone of another isolate, T9/94, yielded a mutant, T9/94/300.300, raising pyrimethamine resistance about 10 times and involving an amino acid change from Ile164 to Met164. However, another mutant from T9/94, T9/94/M1-1(b3), although it raised the pyrimethamine resistance 100 times, did not involve any changes in the coding sequence of the DHFR-TS gene, but resulted in the production of about twice as much DHFR-TS enzyme as the original clone T9/94. No amplification of the DHFR-TS gene was detected. It is concluded that changes in pyrimethamine resistance of malaria parasites may arise in at least 2 ways: (1) by structural changes in the DHFR domain of the DHFR-TS gene (as previously found by other workers); (2) by other changes, possibly affecting the expression of the DHFR-TS gene. The relative importance of these 2 mechanisms in causing resistance in wild populations of P. falciparum is discussed.
Assuntos
Complexos Multienzimáticos/genética , Mutação , Plasmodium falciparum/efeitos dos fármacos , Pirimetamina/farmacologia , Tetra-Hidrofolato Desidrogenase/genética , Timidilato Sintase/genética , Animais , Sequência de Bases , Resistência a Medicamentos/genética , Humanos , Malária Falciparum/parasitologia , Dados de Sequência Molecular , Complexos Multienzimáticos/química , Mutagênese , Oligodesoxirribonucleotídeos/genética , Plasmodium falciparum/enzimologia , Plasmodium falciparum/genética , Reação em Cadeia da Polimerase , Tetra-Hidrofolato Desidrogenase/química , Timidilato Sintase/químicaRESUMO
One hundred and eighty nine isolates of Plasmodium falciparum collected in Thailand, and eleven originating from Cambodia, have been typed by starch-gel electrophoresis of six enzymes (GPI, LDH, GDH, PGD, ADA, PEPE). Substantial polymorphism was found only with GPI. Occasional variants occurred with ADA, while the other four enzymes appeared to be invariant by the tests used. The results are compared with those of similar studies on African isolates, and lead to the provisional conclusion that P. falciparum isolates from different different endemic areas constitute a single, world wide species, containing potentially interbreeding individual organisms.
