214 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



The physicochemical nature of the electron trans- 

 port enzymes in the mosaic of the sarcosome has 

 been studied by Green (88) and Stotz et al. (228). 

 Througli various devices, such as sonication and 

 treatment of the insoluble sarcosomes with various 

 lipid-emulsifying and splitting reagents such as the 

 Tweens and bile salts, they have been able to obtain 

 soluble lipoproteins which are enzymatically active 

 and contain a variety of complex lipids. The lipo- 

 protein character of these enzymes is essential for 

 their biological activity, particularly that of oxidative 

 phosphorylation. The lipid medium may be essential 

 for the formation of highly labile phosphate esters 

 during electron transport intermediate in the forma- 

 tion of ATP. 



As the electrons traverse the chain they transfer 

 energy at three stages by phosphorylation of ADP 

 to ATP viz. : 



AF -I- P, 4- .\DP ^ .ATP 



(16) 



The over-all P-'O ratio of 3.0 for the oxidation of 

 fatty acids and carbohydrate signifies that about 65 

 per cent of the chemical bond energy of the sub- 

 strates is converted to high-energy bonds of ATP. 

 Only about 10 per cent of the total energy conserved 

 in ATP is deri\ed from substrate le\el phosphoryla- 

 tions. The remainder occurs in the electron transport 

 chain. The mechanisms of this process are still not 

 completely delineated, but some progress has been 

 made clue largely to the studies of Lehninger el al. 

 (133) and Chance et al. (39, 41, 44), who have studied 

 this problem extensively. The mechanism by which 

 the energy of the electron is transferred to a chemical 

 linkage of ATP is unknown. Studies of inorganic P'*'- 

 uptake into ATP and of the ADP-ATP exchange 

 reaction by mitochondrial fragments have suggested, 

 however, that one can formulate this process in three 

 steps as follows : 



carrier,i.j -|- X (coupling molecule) — ► carriefoj 



(■7) 



^. X -I- ac 



- X -I- P, — carrier,,,, -|- P 

 X -I- .\DP ,-=± .•\TP + X 



X 



(18) 



(19) 



The enzyme catalyzing the third step, a phos- 

 photransferase, has been isolated and purified by 

 Lehninger (133). This formulation postulates that 

 the energy contained in the oxidized form of the 

 carrier is transferred to a high-energy bond between 

 unknown intermediate (possiblv an enzyme) and 

 the oxidized form of the hydrogen carrier. This inter- 

 mediate then reacts with inorganic phosphate to 



form another high-energy bond. The energy P ^ X 

 is subsequently transferred to ATP by reaction with 

 ADP as shown in equation 19. The over-all reaction is: 



2e -f ADP -t- P, — 2e (lower energy) + .\T? (20) 



Dinitrophenol, an agent known to uncouple oxida- 

 tion and phosphorylation, appears to act on the 

 carrierox.-X complex and hence blocks ATP forma- 

 tion by inhibiting the reaction shown in equation 18. 

 Packer (186) has shown that heart sarcosomes ordi- 

 narily are tightly coupled, i.e., that in the absence 

 of the phosphate acceptor they will not transfer 

 hydrogen and, conversely, when hydrogen is trans- 

 ferred, phosphorylation occurs. 



The substrate succinate and fatty acid CoA de- 

 rivatives transfer hydrogen directly to fiavoprotein 

 as shown in figure 1 5, so that these dehydrogenations 

 result in only two high-energy phosphate bonds in- 

 stead of three, since the DPN-flavoprotein transfer 

 is missing from these two systems. The energy release 

 in transferring two electrons from DPNH to 0> is 

 about 50 kcal per mole whereas the energy released 

 from transferring two electrons from FAD from 

 succinate to oxygen is about 35 cal per mole, the 

 difference being essentially the value of one high- 

 energv phosphate bond. 



ROLE OF CREATINE PHOSPHATE. In musclc tissUC 



creatine phosphate (71) serves as a high-energy 

 phosphate buffer. A specific enzyme, creatine ATP- 

 transphosphorylase catalyzes (138) the reaction: 



C -I- ATP ^ CP -1- ADP 



(2l) 



The structures of creatine phosphate and ATP are 

 shown in figure 16. 



The classical experiments of Lundsgaard (143) 

 showed that muscle poisoned with iodoacetate could 

 contract anaerobically in the absence of glycolysis, 

 and that this action was accompanied by the dis- 

 appearance of creatine phosphate and the appearance 

 of free creatine and inorganic phosphate in equivalent 

 amounts. He also showed that the amount of creatine 

 phosphate consumed was proportional to the amount 

 of muscular work done and that when the creatine 

 phosphate was gone the muscle could no longer 

 respond to stimulation. In cardiac muscle the creatine 

 phosphate is present in relatively low amounts of the 

 order of 15 to 20 mg per cent of inorganic phosphate 

 compared to 100 mg per cent in skeletal muscle, and 

 pro\ides a relatively small reserve of high-energy 

 phosphate bonds to the heart. The difference in con- 

 centration of creatine phosphate between skeletal 



