86 



The Nature of Biological Diversity 



Recent studies by Dempsey and Boyer (37) have shown that in the 

 presence of ATP, myosin and actomyosin catalyze an exchange of 

 oxygens of inorganic phosphate with those of water. One experimental 

 result which led to these findings is depicted in Fig. 8. This figure 



Table 5. Exchange reactions catalyzed by mitochondria 

 capable of oxidative phosphorylation 



nil hstritf* 



(Overall: ADP + Pi L-» ATP + HOH) 



oxidation 



32 



ATP exchange 



Op, 18 — HOH exchange 



13 jumoles 



280 /xmoles 



Mitochondria with ATP, P ( , Mg ++ , K + , at 23°, 15 minutes. No 

 substrate and no oxygen uptake. 



source: Ref. 36. 



shows the time course of appearance of O 18 from water into phosphate 

 cleaved from ATP by actomyosin. The phosphate appearing in the ini- 

 tial part of the reaction contains only the one water oxygen expected 

 from the hydrolytic reaction. With continued incubation, more than 

 one water oxygen is found in the phosphate, suggesting a concentration 

 dependent on exchange of phosphate oxygens of the medium with 

 water oxygens. The experiments reported in Table 6 provide a con- 



Table 6. P, oxygen exchange during hydrolysis of ATP 



elusive demonstration that most, if not all, of the oxygen exchange 

 accompanying ATP cleavage results from exchange of phosphate 

 oxygens of the medium. In these studies, the exchange was measured 

 both by incorporation of oxygen from HoO 18 into phosphate as well 



