PHOTOOXIDATION AND PHOTO REDUCTION REACTIONS 255 



TABLE 5. 



Rates of DPIPH9 photooxidatiou. The values given for R. rubrum are average 

 values obtained from 50 experiments, with the exception of the aerobic reaction. 

 The values given for Chromatium are average values taken from 5 experiments . 



^molesAr/mg Chlorophyll 

 R. rubrum Chromatium 



Fast Reaction with NAD 

 Fast Reaction with Fumarate 

 Slow Reaction with NAD 

 Slow Reaction with Fumarate 

 Aerobic Reaction 



piled in Table 6, Perusal of this table shows that the fastest rate re- 

 ported is that by Amesz for the photo reduction of NAD by whole cells 

 (21). This rate of 360 ^moles per hour per mg of BChl places this 

 photo reduction within the same range asphotophosphorylation, and just 

 below the rate of the fast DPIPH2photooxidation reported in this paper. 

 The rate reported by Amesz was the initial rate obtained when the cells 

 went from a nonilluminated to an illuminated condition, and represents 

 the maximal rate at which electrons could be transferred from cyto- 

 chrome through chlorophyll to NAD. 



The rate of 145 reported by Ash et al. for the photo reduction of 

 methyl red in the presence of quinacrin is higher than the usual re- 

 action reported. It appears from the evidence at hand that methyl red 

 photo reduction (in the presence of ascorbate and DPIP as electron 

 donor) is one of the more stable reactions catalyzed by chromato- 

 phores and involves only a portion of the chromatophore system. None 

 of the ordinary enzymatic components involved in NAD photo reduction 

 and fumarate photo reduction are required in the case of methyl red 

 (17). The remainder of the photoreactions have rates between the range 

 of 4 to 45 /^moles per hour per mg of BChl. The slow photooxidation 

 reactions coupled to NAD and fumarate reported in this investigation 

 fall in this range also. These apparently, then, are the photooxidation 

 reactions which are coupled to the enzymatic components within the 

 electron transfer system. 



The information available on rates of the various photoreactions is 

 consistent with the idea that an initial fast photooxidation of DPIPH2 is 

 representative of the fast photochemical reactions induced following 

 the absorption of light quanta by the chromatophore. It is generally 

 thought that the bacterial chromatophore contains a closed electron 



