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PubMed1957

The relationship between inhibition of phosphofructokinase activity and the mode of action of trivalent organic antimonials on Schistosoma mansoni.

The addition of purified mammalian phosphofructokinase to homogenates of schistosoma mansoni increased the rate of lactic acid production from glucose and reversed the inhibition of glycolysis produced by low concentrations of trivalent organic antimonials. Neither mammalian phosphofructokinase nor trivalent antimonials affected the rate of lactic acid production from fructose-1:6-diphosphate (HDP) by schistosome homogenates. Accordingly, in the schistosome, the rate of glycolysis of glucose is determined by the activity of phosphofructokinase.The aldolase of S. mansoni has a high requirement for HDP; relatively slight reductions in the concentration of this substrate below the optimum resulted in a sharp decline of aldolase activity. Therefore, decreased formation of HDP, due to inhibition of schistosome phosphofructokinase activity by antimonials, reduced the activity of aldolase and resulted in an inhibition of glycolysis of schistosome homogenates.Kinetic data revealed differences in the nature of the phosphofructokinase of S. mansoni and that of the enzyme catalysing the same reaction in the host. Exposure of schistosomes to low concentrations of potassium antimonyl tartrate or administration of subcurative doses of stibophen to the host resulted in an accumulation of the substrate (fructose-6-phosphate), and a reduction of the product (HDP) of the phosphofructokinase reaction, indicating that the activity of this enzyme was inhibited by antimonials in the intact parasite. It is concluded that inhibition of phosphofructokinase activity can account for the mechanism of the chemotherapeutic action of trivalent organic antimonials in schistosomiasis.

Animals↗
PubMed1957

The inhibitory action of some antimalarial drugs and related compounds on the hexokinase of yeast and of Plasmodium berghei.

Of various antimalarial compounds tested, only proguanil failed to inhibit yeast hexokinase. The metabolite of proguanil, 10,580, was an effective inhibitor. Some compounds tested which were without antimalarial activity were potent inhibitors of yeast hexokinase. The degree of inhibition increased as the time during which the enzyme had been in contact with the drug increased, and the inhibitory action of mepacrine was reduced when the concentration of ATP was raised. The inhibition of yeast hexokinase by 10,732 was independent of the concentration of ATP.The hexokinase of haemolysates of the reticulocytes of mouse or rat blood was not appreciably higher than that of similar haemolysates of normal erythrocytes. Preparations of mouse or rat erythrocytes parasitized with P. berghei possessed a much higher hexokinase activity.The inhibiting action of various compounds on the hexokinase of P. berghei closely resembled those with yeast hexokinase. Again all antimalarial compounds (apart from proguanil) inhibited the enzyme, but some of the most potent inhibitors were devoid of antimalarial action. Amongst the chemotherapeutically active compounds, there appeared to be an approximate parallelism between antimalarial activity and potency as inhibitors of plasmodial hexokinase. The action of mepacrine on plasmodial hexokinase was reduced by raising the concentration of ATP, but, as with yeast hexokinase, the inhibition by 10,732 was independent of the ATP concentration.From a consideration of the results, it seems doubtful whether this type of inhibitory effect plays more than a minor part in the mechanism of antimalarial action in vivo.

Animals↗