584 2. ANALOGS OF ENZYME REACTION COMPONENTS 



(1961) and are uncertain because it is not known whether competitive or 

 noncompetitive inhibition occurs; it may be noted that the values are one- 

 hundredth to one-tenth those given by previous workers. The pyrimidine 

 amino groups are essential for tight binding and the pyrazine ring presum- 

 ably does not participate in the binding. A binding mechanism was proposed 

 in which emphasis is placed on the tautomeric state of the analogs and the 

 formation of hydrogen bonds between the ring nitrogens and the amino 

 groups with the enzyme, the replacement of the 4-OH group of folate with 

 an amino group favoring greater hydrogen bonding. It may also be ob- 

 served that the benzoylglutamate portion of the molecule must contribute 

 around 6-7 kcal/mole binding energy. 



Another analog with less obvious structural similarity to folate is pyri- 

 methamine (Daraprim), an antimalarial drug that in chronic dosage pro- 

 duces folate deficiency in bacteria and animals (Wood and Hitchings, 1959 a; 

 Hitchings, 1960). Pyrimethamine, like the analogs previously discussed, 

 inhibits the reduction of folate, and does this at a concentration equivalent 

 to that required for growth inhibition. A 35% inhibition of folinate forma- 

 tion is caused in extracts of S. faecalis by 0.000012 mM pyrimethamine, 

 so that its potency is comparable to that of aminopterin. There are no ef- 

 fects on the biosynthesis or assimilation of folate. It is believed that the 

 antimalarial action is due to the tighter binding of the drug to the plasmo- 

 dial folate reductase than to the host enzyme. However, the uptake of 

 pyrimethamine by bacterial cells is unique inasmuch as it is inhibited 

 strongly by glucose, whereas the uptake of aminopterin is augmented by 

 glucose (see accompanying tabulation) (Wood and Hitchings, 1959 b). Fur- 



thermore, aminopterin uptake is increased by a rise in temperature, whereas 

 less pyrimethamine appears in the cells at higher temperatures. Despite 

 the apparent similarity of site of action of these two analogs, there is some 

 basic difference in the movement or disposition of the materials in the cells. 



Effects on Synthetic Processes Mediated by Tetrahydrofolate 



A block in the reduction of folate would be expected to depress the C^ 

 unit transfers and the synthesis of nucleic acids and proteins as long as 

 there is no supply of tetrahydrofolate or folinate. The analogs in addition 



