282 



H. E. Davenport 



The results described appear to be consistent with the reduction sequence 

 shown diagrammatically as Fig. 4. The electron transport pathway leading to 

 the reduction of NADP is that proposed by Tagawa and Arnon(5) and the diagram 

 shows also the position of metmyoglobin in the reduction sequence. This inter- 

 pretation is supported by the further observation that in a soluble system both 

 ferredoxin and flavoprotein are required for the dark reduction of metmyoglobin 

 by NADPH. Lazzarini and San PietroC^) have described a similar reaction where 

 cytochrome c replaced metmyoglobin and where the flavoprotein was identified 

 as transhydrogenase. Attempts to detect transhydrogenase activity in the pea 

 chloroplast flavoprotein by the method of Keister and San Pietro(8) have been 

 unsuccessful and this negative finding would appear to preclude the reaction 

 sequence postulated by Lazzarini and San Pietro for the photochemical reaction. 



Fe 



+++ 



RED 



Ferredoxin 



ox 



Myoglobin 



NADP 



NADPH 



Fig. 4. Diagrammatic representation 

 of metmyoglobin and NADP reduc- 

 tion by illuminated chloroplasts 



The pea leaf flavoprotein has 

 properties similar to those of the 

 NADPH specific diaphorase ex- 

 tracted from spinach chloroplasts 

 by Avron and Jagendorf(9). 



In the original isolation of ferredoxin from plant material^ ^' '^' the 

 metmyoglobin reducing activity was used as a rapid and convenient assay 

 method for the protein without reference to the nature of the natural hydrogen 

 acceptor in the plant cell. The results recorded above suggest that the method 

 may still be of value as a general assay method uncomplicated by a require- 

 ment for a second catalytic factor. Some experiments with a partially purified 

 preparation of Clostridium ferredoxin (a gift of Dr. J. E. Carnahan) support 

 this suggestion. 



