ANTHONY SAN PIETRO 



635 



15 30 45 60 



MICROGRAMS PROTEIN 



Fig. 1. Enzyme concentration (photosynthetic pyridine nucleotide reductase, 

 PPNR) versus rate of TPN reduction in the presence of illuminated chloroplasts. 

 Each reaction mixture contained 135 /xmoles of Tris buffer, pH 7.2, 0.5 jumole of 

 TPN, enzyme, and chloioplasts equivalent to 88 micrograms of chlorophyll. Final 

 volume, 3 ml; time of illumination, 2 min; temperature, 24°C. 



for quantities of enzyme which reduce up to 25 per cent of the TPN 

 initially present. 



It should be noted that some six years ago Vishniac and Ochoa 

 (28) explained the low rate of pyridine nucleotide reduction ob- 

 served in their experiments by the statement that "the grana may be 

 deficient in one or more of the factors required to transfer hydrogen 

 from the photolytic system to the pyridine nucleotides." The results 

 of our studies support their explanation. It is clear from the data 

 presented in Fig. 1 that the rate of pyridine nucleotide reduction 

 is proportional to the enzyme concentration. In the absence of enzyme, 

 there is no measurable reduction of pyridine nucleotides. 



Although this is not indicated in Fig. 1, it has been possible, under 

 the proper conditions, to observe reduction of TPN by illuminated 

 chloroplasts at a rate of about 180 /xmoles per hr per mg chlorophyll. 

 This rate is about 50 per cent of the maximum rate of carbon assimi- 

 lation by intact leaves (18) . 



Nucleotide Specificity 



It has been shown previously (21, 22) that the enzymatic reaction 

 requires the intact dinucleotide structure. 



