Physiology 455 



involving the transfer of hydrogen step by step from one acceptor to 

 another. Each step involves an oxidation-reduction system and each dehy- 

 drogenation yields energy for anabolism. For cells in general, a number of 

 enzymes and oxidation-reduction systems are known to be involved in 

 metabolism. Gradually accumulating evidence indicates that at least some 

 of these are operative in Protozoa, as woidd be expected. 



The cytochrome system. In aerobes the final stages of the oxidative re- 

 actions — the transfer of hydrogen to oxygen (2H to Oo) — involve the 

 cytochrome system. This system includes several cytochrome pigments 

 which, in their reduced forms, show different absorption bands spectro- 

 scopically. Each cytochrome is an iron-porphyrin-protein which can exist 

 in either the oxidized or the reduced form. The oxidation-reduction po- 

 tentials (Eq') of cytochromes a and c are about 290 and 270 mv; that of 

 cytochrome h, about —40 mv. The oxidation of reduced cytochrome c, 

 catalyzed by cytochrome oxidase, involves the transfer of 2H to atmos- 

 pheric oxygen: 



cytochrome-H2 -f- cyt. oxidase — > cytochrome -\- cyt. oxidase-H2 

 cyt. oxidase-H2 -|- 1/2^2 -^ cytochrome oxidase -|- HoO 



The reduction of cytochrome c is catalyzed by dehydrogenases which 

 bring about oxidation of reduced DPN, reduced TPN, and such sub- 

 strates as succinate. Cytochrome h also may be involved in the reduction 

 of cytochrome c. Reduction of cytochrome c may be blocked by heat and 

 by such reagents as alcohol, formalin, and urethanes. The transfer of 

 hydrogen from cytochrome to cytochrome oxidase is inhibited by cyanide 

 and azide, thus maintaining cytochrome in the reduced condition. Trans- 

 fer of hydrogen from cytochrome oxidase to oxygen is inhibited by carbon 

 monoxide. 



As would be expected, aerobic Protozoa resemble other aerobic micro- 

 organisms in possessing cytochrome pigments. Cytochromes a, h, and c 

 have been reported in Astasia klebsii (83) and Tetrahymena pyrifor?nis 

 (9); cytochromes b and c, in Eiiglena gracilis, Tetrahymena (Glaucoma) 

 pyrijormis, Polytoma uvella, Strigomonas jasciculata, and 5. oncopelti 

 (341); cytochrome c, in Colpidium campylum (510, 512), and with cyto- 

 chrome oxidase, in Chilomonas Paramecium (220). Cytochrome oxidase 

 is said to occur in the pigment granules (mitochondria) of Stentor coeru- 

 leiis (569). In contrast to the typical aerobes, Trichomonas foetus ap- 

 parently contains no cytochrome (484). 



Poisoning techniques have supplied additional information. The 

 respiration of Tetrahyynena pyrijormis is decreased by 9-57 per cent in 

 different concentrations of methyl-, ethyl-, and propylurethanes (364). 

 Cyanide decreases oxygen consumption about 61-64 per cent in Astasia 

 longa and Khawkinea halli (247), about 90 per cent in Polytoma uvella 

 (364), and about 95 per cent in Astasia klebsii, for which azide is almost 



