EFFECT OF HYDROSULFITE 191 



DISCUSSION 



Hydrosiilfite readily reduces the ferric forms of hemoglobin, 

 myoglobin, cytochromes, and peroxidases. The only well-documented 

 exception is intact catalase (2). Pigments isolated from R. rubrum 

 are reduced (3). Thus we must seek an explanation for the apparent 

 insensitivity of the light effect in R. rubrum. There are significant 

 differences in the kinetics of reduction of the hemoprotein in the 

 two records of Fig. 1. On the left, the reduction starts slowly and 

 accelerates to a rapid rate. On the right, the reaction starts at the 

 maximum rate, which is more rapid than in the figure on the left. 

 Thereafter a fairly slow and prolonged reaction ensues which pro- 

 ceeds 18% farther than in the left figure. We attribute the first 

 rapid phase to a combination of a direct reaction of hemoprotein 

 on the cell surface with the reducing agent and to the spontaneous 

 reduction caused by exhaustion of oxygen. The slower phase of the 

 reduction reaction is attributed to the slower penetration of hy- 

 drosulfite to more remote parts of the bacterial cell. It is significant 

 that appreciably more hemoprotein is reduced on hydrosulfite 

 addition and that slightly more is available for oxidation by light 

 than in the absence of hydrosulfite. 



The result that the light-mediated oxidation of hemoprotein 

 proceeds even in the presence of hydrosulfite gives strong support 

 to the idea that oxygen is not evolved in this photosynthetic process 

 (4,5) and further indicates that transfer of oxygen in a bound form 

 from chlorophyll to hemoprotein is unhkely. A hypothesis in accord 

 with these data is that the oxidant is produced within a structure 

 of chlorophyll and hemoprotein that is inaccessible to the concentra- 

 tion of reductant used here. 



[See Discussion following paper by Britton Chance, Margareta 

 Baltscheffsky, and Lucile Smith, pp. 197-202.] 



References 



1. Chance, B., and Smith, L., Nature, 175, 803 (1955). 



2. KeUin, D., and Hartree, E. F., Proc. Roy. Soc. (London), B, 119, 141 (1936). 



3. Vernon, L. P., and Kamen, M., /. Biol. Chem., 211, 643 (1954). 



4. Van Niel, C. B., Advances in EnzymoL, 1, 236 (1941). 



5. Johnston, J. A., and Brown, A. H., Plant Physiol, 29, 177 (1954). 



