456 Physiology 



as inhibitory (83). A number of Trypanosomidae (349) also are sus- 

 ceptible to such poisons. Leptomonas ctenocephali, Strigomonas fascicu- 

 lata, and S. oncopelti show 83-95 per cent inhibition with cyanide and 

 are about as sensitive to carbon monoxide (341). Cyanide in certain con- 

 centrations inhibits respiration about 90 per cent in Trypanosoma cruzi 

 (33), 11-82 per cent in T. congolense (34), 97-98 per cent in Leishmania 

 tropica, L. brasiliensis, and L. donovani, 85-88 per cent in T. leiuisi from 

 cultures, and 66-69 per cent in T. conorhini (32). Certain other Trypano- 

 somidae are relatively insensitive to cyanide — T. equiperdum (127), 

 T. brucei, T. hippicitm, T. rhodesiense (32, 64, 126), T. evansi, and T. 

 equinum (34). Although the oxygen consumption of T. gambiense from 

 blood is not decreased by cyanide (34), flagellates from cultures are 

 moderately sensitive (32). Among the Sarcodina, Pelomyxa caroUnensis 

 is sensitive to cyanide (427), and sensitivity increases with temperature in 

 the range, 10-35° (430). Respiration of Plasmodium knoxvlesi also is 

 inhibited by cyanide (64, 371) and carbon monoxide (371). Earlier re- 

 ports (338, 364, 526) that free-living ciliates are insensitive to cyanide, are 

 contradicted by later observations. Respiration of Tetrahymena pyri- 

 formis is sensitive both to carbon monoxide (9) and to cyanide (9, 170), 

 while that of well-fed, but not of starved specimens, also is cyanide-sensi- 

 tive in Paramecium aurelia (424) and P. caudatum (66, 424). 



The results obtained with poisoning techniques indicate that in gen- 

 eral, aerobic Protozoa are cyanide-sensitive and presumably oxidize sub- 

 strates mainly through the cytochrome system. On the other hand, some 

 questions are unanswered. Why are starved ciliates, in contrast to well-fed 

 ones, relatively insensitive to cyanide? What converts insensitive T. gayn- 

 biense from the blood into cyanide-sensitive flagellates in culture media? 

 Is it necessary for these organisms to oxidize certain substrates only par- 

 tially ("anaerobically") in the blood but completely, or nearly so, in 

 culture media? And what are the biochemical differences between the 

 cyanide-sensitive "lewisi group" of trypanosomes and such insensitive spe- 

 cies as T. brucei and T. evansi? Such problems are of practical as well as 

 theoretical interest, since the response of parasites to chemotherapeutic 

 agents may depend to an important extent upon the oxidative mech- 

 anisms of particular species. 



Pyridine nucleotide enzymes. The pyridine nucleotides are coenzymes 

 for a number of important oxidative enzymes. Diphosphopyridine nucleo- 

 tide (DPN), or coenzyme I, contains nicotinamide, D-ribose, adenine, and 

 two phosphoric acid groups. Triphosphopyridine nucleotide (TPN), or 

 coenzyme II, contains a third phosphoric acid group. Both coenzymes are 

 involved in protozoan metabolism. Chilomonas Paramecium (223) and 

 Tetrahymena pyriformis S (510, 512) contain DPN, while Trypanosoma 

 hippicum requires DPN for glycolysis in vitro (194). In addition, supple- 



