Pyrimethamine-resistant dihydrofolate reductase enzymes of Plasmodium falciparum are not enzymatically compromised in vitro

详细信息    查看全文

• 作者：Conner I. S ; efur ; Jason M. Wooden ; Isaac K. Quaye ; Worachart Sirawaraporn ; Carol Hopkins Sibley
• 关键词：Drug resistance ; Antifolate ; Relative fitness ; Selection
• 刊名：Molecular and Biochemical Parasitology
• 出版年：2007
• 出版时间：July 2007
• 年：2007
• 卷：154
• 期：1
• 页码：1-5
• 全文大小：190 K

文摘

Plasmodium falciparum, the protozoan that causes the most lethal form of human malaria, has been controlled principally by two safe, affordable drugs, chloroquine and sulfadoxine-pyrimethamine (SP). Studies in the laboratory and in the field have demonstrated that resistance to SP depends on non-synonymous point mutations in the dihydrofolate reductase (DHFR), and dihydropteroate synthase (DHPS) coding regions. Parasites that carry dhfr genes with 3 or 4 point mutations (51I/59R/108N triple mutation or 51I/59R/108N/164L quadruple mutation) are resistant to pyrimethamine in vitro and patients infected with these parasites respond poorly to SP treatment.

The wide spread of these pyrimethamine-resistant alleles demonstrates the increased fitness over drug-sensitive alleles in the presence of the drug. However, it is not clear whether these alleles might reduce the fitness of parasites in the absence of drug pressure. As a first step, we compared the kinetic properties of the wild type, and three mutant alleles to determine whether the native DHFR-thymidylate synthase form of the mutant proteins showed compromised activity in vitro. The mutant enzymes had K_m values for their substrate, dihydrofolate that were significantly lower than the wild type, k_cat values in the same range as the wild type enzyme, and k_cat/K_m values higher than wild type. In contrast, the K_m values for the NADPH cofactor were higher than wild type for the mutant enzymes. These observations suggest that the fitness of these parasites may not be compromised relative to those that carry the wild type allele, even without sustained SP drug pressure.