598 



LIGHT AND LIFE 



MOLARITY X 10 OF NUCLEOTIDES 



Fig. 3. Effect of mitleotide concentration on rate of Pi^- incorporation. Each 

 tube contained: AMP, ADP or 0.4 ^itmoles AMP -f ATP as indicated; K„HPOi, 

 l.fiS /imoles; PMS, 0.03 /^nioles; MgCl,,, 3.38 Mmoles; NaCii, 17.0 /umolcs; tris-HCl. 

 pH 8.0, 19.0 /imoles; chloroplast suspension, 0.15 ml containing 91 /xg chlorophyll; 

 and ?,=•% 2.91 X 10'^ counts/min. Final volume. I.O ml. Incubation, 20 min at 

 l.')°C. Ordinate values are counts/min ad.sorbed on charcoal from aliquots corres- 

 ponding to 0.306 ml of reaction mixture. 



incorporation of Pj^-' into ATP. If there is in addition a photochemi- 

 cal dephosphorylation of ATP, it would appear to be a small effect. 

 The data, while thus revealing a significant ATPase activity in chloro- 

 jjlasts, do not support the thesis that ATP is utilized in the photo- 

 reduction of TPN. 



Of interest here is the light-dependent dephosphorylation of ATP 

 recently reported by Petrack (14) in chloroplasts in which photo- 

 phosphorylation was inhil)ited by high concentrations of sulfhydryl 

 compounds. It remains to be seen whether this represents a reversal 

 of one of the steps of photojihosphorylation as suggested by the 

 author, or perhaps the exposure of some other ATP-utilizing reaction 

 somehow missed by the Pj^^ incorporation experiments. 



