560 2. ANALOGS OF ENZYME REACTION COMPONENTS 



slice respiration is depressed only slightly by 0.5 n\M, even after 2 hr, 

 while 1-2 mM produces a maximal depression of around 65% after 1 hr. 

 Brain respiration is more sensitive and is reduced around 75% by 0.5 mM. 

 Following inhibition by quinacrine, addition of pyruvate, lactate, citrate, 

 fumarate, or malate does not restore oxygen uptake, but addition of succi- 

 nate brings about a rapid rise in respiration, indicating only that succinate 

 oxidase is not blocked significantly at these concentrations. Because of the 

 reversal of D-amino acid oxidase inhibition by coenzyme, they suggested 

 that the block may be around the flavoenzyme locus in the respiratory 

 chain, but actually there is no evidence for this. The respiration of Plasmo- 

 dium lophurae with different substrates is depressed 15-24% by 0.1 mM 

 quinacrine and somewhat above 50% by 1 mM (Bovarnick et al., 1946). 

 Actually it is very difficult to compare in vitro and in vivo effects and con- 

 centrations because of the progressive binding and accumulation of quina- 

 crine; in other words, the free plasma concentration of quinacrine means 

 very little, nor does total tissue concentration necessarily relate to any 

 enzyme effects. 



A glycolytic inhibition by quinacrine was suggested by the early work 

 of Marshall (1948), who found a depression of glucose utilization, a decrease 

 in glucose- 1-P, an increase in glucose-6-P, some decrease in triose-P's, and 

 decreases in pyruvate and lactate in washed chick erythrocytes parasitized 

 by Plasmodium gallinaceum. It is difficult to separate the metabolic effects 

 on parasite and erythrocyte, but it is probable that the major fraction of 

 the glucose utilization was due to the parasites. The most marked effect of 

 quinacrine is an accumulation of ATP, which Marshall attributed to an in- 

 hibition of hexokinase. 



The only analysis of the effects of quinacrine on metabolism was made 

 by Bowman et al. (1961). The glucose utilization of P. bergkei free parasites, 

 parasitized reticulocytes, and reticulocytes was determined, and low con- 

 centrations of quinacrine (claimed to be near those found therapeutically) 

 exhibit a selective action on the parasites. The glucose utilization over 1 hr 

 is reduced 34% by 0.0125 mM and 94% by 0.035 mM quinacrine. There 

 is no effect on the pattern of glucose-1-C^* and glucose-6-C^^ distribution. 

 The amount of lactate formed from glucose is reduced and there is an accu- 

 mulation of hexose-6-P, so it was concluded that quinacrine inhibits some 

 enzyme which is involved in the utilization of hexose-6-P and is normally 

 rate-limiting in the free parasites; this enzyme may be phosphofructokinase. 

 Depression of respiration could thus, in part, be attributed to a glycolytic 

 inhibition and, if so, is probably not related to a flavoenzyme. 



There is probably need for more investigation of the effects of quinacrine 

 on tissues and parasites in animals given the drug for varying times, be- 

 cause of the difficulty in estimating the proper in vitro concentrations to 

 use. It may well be that some enzyme system not previously examined is 



