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H. E. DAVENPORT 



States. From the data shown in Fig. 3 it would appear Hkely that these 

 failures can be attributed to the use of rate-limiting amounts of the 

 catalytic protein. For maximum stimulation all the ingredients of the 

 phosphate-accepting system were found to be required and adenosine 

 5'-phosphate did not replace ADP unless it was supplemented by catalytic 

 amounts of either ADP or ATP. 



In contrast with these observations the rate of reduction of metmyo- 

 globin was found to be unaffected by the presence of the phosphate 

 accepting ingredients and no evidence for photophosphorylation has been 



01 0-2 



mg protein added 



Fig. 3. Effect of phosphate acceptor system on TPN reduction catalyzed by 

 pea leaf protein (PPNR preparation further purified by electrophoresis). Reaction 

 mixtvires contained (in 3 ml.) leaf protein as indicated ; spinach chloroplasts 

 (chlorophyll o-oi6 mg.), and (in /xmoles) NaCl, 40; TPN, 0-4; tris HCl buffer, 

 pH 70, 150. Reaction mixtures for points • contained in addition: ADP, 0-5; 

 Na2HP04, 15; MgClo, 20. Leaf protein was omitted from the blank cells. 



obtained with this hydrogen acceptor. Evidence that TPN is able to 

 compete with metmyoglobin for hydrogen produced in the photochemical 

 reaction was obtained by measuring metmyoglobin reduction in the 

 presence and absence of substrate amounts of TPN. The presence of TPN 

 was found to inhibit metmyoglobin reduction and this inhibition was 

 further enhanced when the phosphate accepting system was also present. 

 At the present state of this work it appears that the haem-protein and 

 TPN reducing activities are common properties of a homogeneous protein. 

 The absence of detectable activity towards diphosphopyridine nucleotide 

 emphasizes a very unusual pattern of specificity towards hydrogen 

 acceptors. 



