458 Physiology 



strated in Tetrahynnena pyriformis (oil, 512) and may play a part in 

 respiration of this ciliate (364), its functions in protozoan metabolism are 

 still unknown. 



Pantothenic acid enzymes. Pantothenic acid is a component of coenzyme 

 A (319) which may be involved in acetylation reactions in general and 

 perhaps in the utilization of acetylmethylcarbinol by certain bacteria. Al- 

 though pantothenate is essential to giowth of certain ciliates and malarial 

 parasites, its possible functions in protozoan metabolism have not been 

 investigated. 



Adenosine phosphate system. The adenosine phosphate system includes 

 adenylic acid (adenosine monophosphate), adenosine diphosphate 

 (ADP), and adenosine triphosphate (ATP). Each contains adenosine (a 

 riboside of adenine) and one, two, or three phosphoric acid groups, re- 

 spectively. The system functions in phosphorylation of metabolites and 

 enzymes and especially in the transfer of high-energy phosphate bonds.^^ 

 This system apparently makes available for anabolic activities the energy 

 derived from oxidation of metabolites. Essentially, TPN serves as a par- 

 ticipant common to a variety of exergonic and endergonic reactions, mak- 

 ing it possible for reactions of the first type to drive those of the second 

 variety. Little is known about the adenosine-phosphate system in Proto- 

 zoa. However, adenylic acid, ADP, and ATP have all been demonstrated 

 in Euglena gracilis (1). In addition, Trypanosoma hippicum needs ATP 

 in the formation of hexose-phosphates (194), and Tetrahymena pyri- 

 jorynis contains adenosine triphosphatase (510). 



Tricarboxylic acid cycle.^^ This so-called cycle (Fig. 8. 2) involves the 

 oxidation of various metabolites through a common catalytic system to 

 carbon dioxide and water under aerobic conditions. The cycle is fed by 

 glycolysis, leading to pyruvate and thence to acetyl; by the breakdown of 

 fats, yielding acetyl from fatty acids; and by the breakdown of proteins, 

 through the deamination of certain amino acids to a-keto acids which 

 enter the cycle. At each turn of the oxidative cycle, COo and HoO are 

 produced as end-products in certain reactions, and energy is made avail- 

 able by the generation of high-energy phosphate bonds in several de- 

 hydrogenations. Aside from its importance in the oxidation of substrates, 

 the tricarboxylic acid cycle may also be considered a basic reservoir of 

 important materials which can be drawn upon for the synthesis of amino 

 acids, carbohydrates, and fatty acids. 



The tricarboxylic acid cycle has been studied both by the use of isotopes 

 (578) and by the control of enzyme systems ^\•ith blocking reagents. In the 

 presence of cyanide at a suitable concentration, oxalacetate is trapped; 



^^Tor a discussion of the energetics of high-energy phosphate bonds, a review by 

 Ogsdon and Smithies (419) may be consulted. 



"Representative discussions (10, 165, 416) may be consulted for details of the tri- 

 carboxylic acid cycle and its general importance in metabolism. 



