76 



wide variety of organisms (cf . Werkman and Wood, 

 '42). Parts of the system, such as the reversible 

 oxidation of succinate to fumarate, may function in 

 oxidations in other steps. The succinic fumaric sys- 

 tem (catalyzed by succinic dehydrogenase) is 

 notable in part because of the relative specificity by 

 which the competitive inhibitor, malonate, may be 

 used (cf. Krebs, '43). Green (in Lardy, '50) notes 

 that apparently there are single enzymes in cells 

 that perform individual reactions of the cycle with- 

 out, however, being necessarily a part of it. 



While the Krebs-Szent-Gyorgyi cycle may be 

 essentially reversible, it is also true that the net di- 

 rection of flow will result in decarboxylation and 

 oxidation if a source of reduced carbon chains is 

 continually being fed in (pyruvate, for example) 

 and if oxygen is available. The disposition of car- 

 bon dioxide need not be considered here, but it 

 should be noted that a supply of oxidized coenzyme 

 must be present in order for the cycle to keep going. 

 Since the total absolute amount of coenzyme per 

 unit volume of cell is small, a regenerating system 

 to provide the oxidized coenzyme must be present. 

 Otherwise the over-all reaction would fail even in 

 the presence of carbon chain substrate, oxygen, and 

 enzyme protein. By links that are not completely 

 known at present (cf. Potter, in Lardy, '50), the 

 major regeneration of reduced coenzyme is accom- 

 plished in aerobic cells by the cytochrome system, 

 together with the large nimiber of other enzyme 

 systems that involve the reversible oxidation of 

 DPNH2 or TPNH2. With the cytochrome system, 

 an elegant method is provided for the excretion of 

 the hydrogen of the original carbon chain food by 

 transferring electrons to the final acceptor oxygen, 

 which then is free to imite with the protons left by 

 electron removal from hydrogen. The review by 

 LaValle and Goddard ('48) gives an excellent back- 

 ground for understanding the oxidative steps medi- 

 ated by the cytochrome system. The diagram of 

 Figure 6, taken from Ochoa ('47), illustrates one 

 way in which linkage of the DPN system with the 

 cytochrome system may take place. 



With heart muscle systems it has been reported 

 (Ochoa, '47) that the over-all reaction 



pyruvate + 2.5 O2 >- 3 CO2 + 2 H2O 



yields fifteen labile phosphate bonds. While not all 

 of the steps are known it is reasonable to assiune 

 that throughout the oxidative cycles the two coen- 

 zyme systems, pyridine nucleotide and adenylic 

 acid, ai-e so linked that the energy packets released 

 in each oxidative step are stored for further use by 

 the phosphorylating coenzyme system or its equiv- 

 alent. 



A general summary of the fate of pyruvate in 

 cellular metabolism is given by Barron ('52) and is 

 schematized in Figure 5, which is taken from his 

 paper. 



Carbohydrate metabolism, as a model sys- 

 tem, may now be summarized even more 

 briefly than has been done in the description 

 of Figiwe 3. Sugar may possibly be oxidized 

 directly, but the main pathway known in- 

 volves a preliminary phosphorylation to a 



Cellular Structure and Activity 



diphosphate form. The breakdown of this to 

 pyruvate (three-carbon) involves an oxida- 

 tive step via DPN;^DPNH2 and an acidic 

 enol formation, both of which, in the pres- 

 ence of the adenylic acid coenzyme system 

 for phosphorylation, result in the formation 

 of energy-rich phosphate bonds. Pyruvate, 

 the common focal point, may be reduced with 

 DPNH2 to lactic acid or may enter the oxi- 

 dative cycles of the six-, live-, and four- 

 carbon acid systems and the cytochrome sys- 

 tem, again using DPN and the adenylic acid 

 systems. 



The ubiquity of the two coenzyme systems 

 has already been remarked. Their importance 

 as possible controlling factors needs further 

 emphasis. Each system is typically present 

 in more or less constant amount. That is, 

 there is little net change during short time 

 intervals, the systems acting in catalytic con- 

 centrations. So long as it is possible for the 

 DPN system to be reversibly reduced, or for 

 the adenylic acid system to be reversibly 

 phosphorylated, orderly reactions can pro- 

 ceed. Should either system be fixed in one 

 state, however, far-reaching changes might, 

 and do, take place. For example, in the 

 Pasteur reaction, where fermentation is in- 

 hibited in the presence of oxygen, the DPN 

 system has been implicated (cf. Lardy, '50). 

 Under aerobic conditions where the revers- 

 ible system DPN;=±DPNH2 might be far to 

 the left, there would be insufficient reduced 

 coenzyme to form lactate (or alcohol) from 

 pyruvate, and the rate of fermentation would 

 be correspondingly slowed down. Meyerhof 

 ('49) has shown that the stabilizing of the 

 adenylic acid system as ATP can result in 

 decreased fermentation of yeast extracts, a 

 condition that can be explained by the failure 

 to provide ADP as necessary phosphate ac- 

 ceptor for some of the early stages of glycol- 

 ysis. Addition of an enzyme converting ATP 

 to ADP allows the reaction to proceed. 



These considerations should be kept in 

 mind in experiments where metabolism is 

 tampered with, by adding either poisons or 

 excess substrates. For example, it might be 

 tempting to think of adding the universal 

 substrate, pyruvate, to a cellular system. 

 Under aerobic conditions, however, pyruvate 

 might be oxidized rapidly with the resultant 

 formation of excess ATP and oxidized DPN, 

 thus throwing out of gear all of the pre- 

 pyruvate steps of the metabolic flow line. An 

 effect possibly like this has been noted by 

 Goldinger and Barron ('46) with sea urchin 

 eggs, where addition of pyruvate has little 

 effect on the Q02 even though pyruvate is 



