229 



C. C. Black and A. San Pietro 



pmoles of ATP produced. Phosphodoxin appears to be ubiquitous among 

 photosynthetic organisms. 



Table 1 



Photosynthetic Organisms Containing Phosphodoxin 



jjmoles of ATP/mg 

 Source chlorophyll/hr. 



Spinach, chloroplasts 196 



Spinach, leaves 220 



Phormidium luridum , whole cells 49 



Tribonema aequale , whole cells 27 



Euglena gracilis , whole cells 25 



Chlorella pyrenoidosa , whole cells 210 



Rhodospirillum rubrum , chromatophores 54 



Rhodospirillum rubrum , whole cells 50 



Chromatium , strain D, chromatophores 56 



Chromatium , strain D, whole cells 115 



All preparations assayed with spinach chloroplasts 



Spinach phosphodoxin was purified by acetone fractionation and paper 

 chromatography ^^^>. Absorption spectra of spinach phosphodoxin is given 

 in Fig. 1. The absorption spectrum shifts with pH, in contrast to the 

 fluorescence activation spectra, which does not show a pronounced shift 

 with pH (Fig. 2). Since the intensity of the fluorescence spectrum is pH- 

 dependent but the activation spectrum is not (Fig. 2), it appears that the 

 alkaline form of spinach phosphodoxin may be the fluorescent type. Alkaline 

 solutions of spinach phosphodoxin are yellow, while acid solutions are 

 nearly colorless. The fluorescent maximum was at 440 m^ and the activa- 

 tion maximum was at 358 mp. 



Aqueous solutions of spinach phosphodoxin are stable at 4° C when 

 stored near neutrality. Boiling phosphodoxin for 10 minutes in N HCl does 

 not affect its activity, whereas boiling in N NaOH inactivates the phospho- 

 doxin. Irradiation by ultraviolet light does not alter its activity. Spinach 

 phosphodoxin does not contain a functional metal (23). At the present stage 

 of our research, we do not know the structure of phosphodoxin, nor are we 

 certain that each photosynthetic organism contains the same compound. 



