The Relationship between "Methaemoglobin 



Reducing Factor" and " Photosynthetic Pyridine 



Nucleotide Reductase" 



H. E. Davenport 



University of Bristol, Research Station, 

 Long Ashton, Bristol, England 



In his earliest experiments with isolated chloroplasts Hill [i] found that 

 extracts of acetone-dried leaf contain material capable of accepting hydro- 

 gen from illuminated chloroplasts with the concomitant evolution of 

 oxygen. Although the pathway of hydrogen transport in this reaction has 

 not been determined, investigations on the leaf extracts led to the first 

 demonstration of the need for a naturally occurring catalyst of reduction 

 in the Hill reaction. Davenport ct al. [2] showed that neither methaemo- 

 globin nor metmyoglobin was reduced directly by illuminated chloroplasts, 

 but reduction of either was initiated by the addition to the system of an 

 extract of acetone-dried leaf. The methaemoglobin reducing factor (MRF) 

 was found to be associated with a protein fraction in the extracts. 



More recently [3] the active protein has been obtained in a state where 

 it is homogeneous both electrophoretically and in the ultracentrifuge. 

 Purification was achieved by electrophoresis on paper after a preliminary 

 fractionation with ammonium sulphate. The product is a protein of small 

 molecular weight {c. 19 000), reddish-brown in colour, and it is active in 

 catalyzing the photochemical reduction of a number of haem-protein 

 compounds including cytochromes b-^ and c. The specificity of the catalyst 

 towards illuminated chloroplasts as a source of reducing power, its high 

 catalytic activity and its localization in the chlorophvll-containing cells of 

 higher plants and algae [4] suggested that it plays a part in the transport of 

 hydrogen in photosynthesis. Howe\'er, the pattern of specificity towards 

 haem-compounds of comparatively oxidizing potential did not appear to 

 be relevant to the energetic requirements of carbon dioxide reduction. 



]\Iore significant in this connexion was the isolation and partial purifica- 

 tion by San Pietro and Lang [5] of a protein factor active in catalyzing the 

 photochemical reduction of pyridine nucleotides. This "photochemical 

 pyridine nucleotide reductase" (PPNR) was prepared by acetone frac- 

 tionation, isolation of the active material as a protamine sulphate complex 

 with subsequent recovery of the protein from this complex. 



VOL. n. — 2 G 



