308 A. W. FRENKEIi 



It may be of interest that the washed preparations cannot utilize 

 AMP as acceptor of high-energy phosphate (Table I, 2) although the 

 crude preparations can use it quite effectively. Upon addition of a 

 trace of ATP and of a partially purified adenylate kinase preparation 

 obtained from the same organisms according to the method of Colo- 

 wick and Kalckar (6), the particles sedimenting at 60,000 X g can 

 again utilize AMP as the acceptor of high-energy phosphate. In 

 contrast to animal mitochondria, where adenylate kinase usually is 

 associated closely with these structural units (7), the photophos- 

 phorylating particles described here can be obtained free of such 

 enzymatic activity. There is no evidence for the presence of an 

 enzyme transforming IDP into IMP and ITP, as only IDP is active as 

 acceptor of high-energy phosphate, and IMP is inactive even in the 

 presence of traces of ITP in crude or washed preparations. Other 

 nucleotides besides those listed in Table I may well be able to act as 

 acceptors of the high-energy phosphate produced by the photolytic 

 system. 



The reaction described above very likely represents a net conver- 

 sion of light energy into chemical energy by a cell-free bacterial sys- 

 tem, but this assumption will have to be tested, for example, by the 

 calorimetric method described by Arnold (8), 



PHOTOPHOSPHORYLATION AND OXIDATIVE PHOSPHORYLATION 



When the cells of Rhodospirilliim ruhrwn are disintegrated in a 

 sugar solution, cell-free preparations are obtained which will esterify 

 orthophosphate aerobically in the dark. With substrate amounts of 

 alpha ketoglutarate (KGA) one can obtain P/0 ratios as high as 1.5, 

 and this phosphorylation is partially sensitive to 10"^ M dinitro- 

 phenol (at pH 7.2) which will lower the P/0 ratio to about 0.8. The 

 rate of aerobic dark phosphorylation of these preparations in the 

 presence of KGA is approximately the same as the rate of anaerobic 

 light phosphorylation. The washed photophosphorylating system, 

 on the other hand, shows little or no oxygen uptake aerobically with 

 or without KGA, and phosphorylation cannot be observed in the 

 dark even when hexokinase and mannose are used as trapping agents 

 for ATP. Oxidative phosphorylation by cell-free preparations from 

 R. ruhrum also has been observed by L. Smith (10) who has evidence 

 that this phosphorylation is associated with particles of a different 

 nature than the particles which carry out light phosphorylation. 



