Control of Rate of Intracellular Respiration 67 



of inorganic phosphate, either A ---^ I or X '^ I must be 

 hydrolysed (unless the hydrogen transfer in reaction (1) can 

 occur without the participation of I). The resemblances 

 between the ATPase reaction in these preparations and the 

 DNP-stimulated ATPase of liver mitochondria (Myers and 

 Slater, 1957a) suggest that the same hydrolysis of X /^ I 

 occurs. However, there are sufficient differences to suggest 

 that X /^ P (or one of the I ^-^ P's) may also be hydrolysed, 

 by a Mg 2+ -requiring system, so that, in addition to the 

 ATPases described by reactions (4)-(6), one described by 

 reactions (7)-(8) is possible (Myers and Slater, 19576; Purvis 

 and Slater, 1959) : 



Sum reaction ATP + HgO -> ADP + Pj 



Indeed, this reaction sequence may constitute the DNP- 

 insensitive ATPase which is found in freshly prepared liver 

 mitochondria at high pH's (pH optimum, 9 •4). It may also 

 account for the high ATPase activity, measured in the pre- 

 sence of added Mg2+, found with isolated sarcosomes. It 

 remains an open question whether this activity in isolated 

 sarcosomes is due to structural damage to the sarcosomes, 

 during isolation, or is related to an energy-utilization reaction 

 in the cell. It is possible that, in vivo, one or more of the 

 energy-rich intermediates which are formed prior to ATP in 

 reactions (l)-{Sb) [i.e. A ^ I, X ^ I or X ^ P (I ^ P)] are 

 utilized directly for some of the energy-requiring reactions in 

 the cell (cf. Slater, 1953). In fact, it is possible that reaction 

 (Sb) is, in vivo, more important when it runs from right to left 

 (thereby utilizing the ATP synthesized in glycolysis) than 

 from left to right. In other words, the biochemist when 

 measuring oxidative phosphorylation in isolated sarcosomes, 

 might be measuring to some extent the reverse of an ATP- 

 utilizing reaction, rather than an ATP-synthesizing reaction. 

 Some differences between the DNP-stimulated ATPase of 



