ANTHONY SAN PIETRO 641 



Moreover, this factor is active in catalyzing the reduction of TPN 

 by illiuninated chloroplasts (6, 7) . 



It is of interest, therefore, to compare the properties of PPNR 

 with those of the purified factor which have been reported by Daven- 

 port and Hill (9). These are: "(1) the active fraction from all 

 plants examined A\as pinkish orange in color. In every case absorp- 

 tion in the visible range increased towards the shorter wavelengths 

 with two diffuse absorption bands at 465 and 422 m^x. (2) The molecu- 

 lar weight of the protein, calculated from sedimentation and diffusion 

 measurements, is 19,000. (3) The chemical nature of the group con- 

 ferring oxidation-reduction properties to the protein has not yet 

 been determined but neither flavin nor haem could be detected." 

 It should be noted that the factor does not stimulate the reduction of 

 ferricyanide or 2,6-dichlorophenol-indophenol by illuminated chloro- 

 plasts. In addition, the minimum molecular weight from amino acid 

 analysis is 17,400 and there is no methionine in it (8) . 



At the suggestion of Dr. H. E. Davenport, the ability of PPNR to 

 catalyze the reduction of cytochrome c by illuminated chloroplasts 

 was examined, and it was found to catalyze this reduction. 



In view of the similarities between PPNR and the "methaemo- 

 globin-reducing factor," it is tempting to speculate that these two 

 activities are associated with the same protein. The validity of this 

 speculation must await the results of further experimentation. 



In conclusion, the enzyme photosynthetic pyridine nucleotide re- 

 ductase is required in addition to chloroplasts for the photochemical 

 reduction of TPN. The evidence available to date supports the 

 hypothesis that this enzyme catalyzes the transfer of hydrogen (or 

 electrons) from the photolytic system to TPN. It seems reasonable, 

 therefore, that the study of this catalytic activity Avill further the 

 understanding of the mechanism for converting light energy into 

 chemical energy. 



REFERENCES 



1. Amesz, J., and Diiysens, L. N. M., Discussions Faraday Sac, 27, 173 (IOjO). 



2. Arnon, D. I., Nature, 167, 1008 (1951). 



3. Arnon, D. I., ^Vhatlcv, F. R., and Allen, M. B., Nature, 180, 182 (1957). 



4. Arnon, D. I., Whatley, F. R., and Allen, M. B., Science, 127, 1026 (1958). 



5. Colowick, S. P., Kaplan, N. O., Neufeld, E. P., and Ciotti, M. M., ./. Biol. 



Chem., 195, 95 (1952). 



6. Davenport, H. E., Biochem. /., 73, 45P (1959). 



7. Davenport, H. E., Nature, 184, .524 (1959). 



8. Davenport, H. E., pers. commun. 



9. Davenport, H. E., and Hill, R., Bioc//em. /., 74, 493 (1960). 



