258 H. M. HABKRMANN AM) A. H. BROWN 



In the absence of an ethanol-catalase trap for the hydrogen per- 

 oxide, a negligibly small oxygen metabolism by chloroplast prepara- 

 tions is observed manometrically either in light or dark. With tracer 

 oxygen, however, an exchange of oxygen is found to occur in light 

 in 1 : 1 ratio. There is some evidence that this production balanced 

 by consumption is the consequence of catalytic dismutation of the 

 hydrogen peroxide by endogenous catalase ; in other words reaction 

 (E) 2H2O2 -^ 2H2O + O2 replaces (D) in the previous scheme to ac- 

 count for the light dependent oxygen exchange. We have found that 

 this exchange reaction, detectable only with isotopically enriched 

 oxygen, also is subject to stimulation by a previous quinone reduc- 

 tion in the light. Therefore we can study the properties of either the 

 stimulated or the unstimulated exchange reaction. 



Among the substances which Mehler tested for their possible 

 effect on the quinone-stimulated Mehler reaction, ascorbic acid was 

 especially interesting in that its enhancement of the reaction rate 

 was not influenced by the presence of metabolic poisons which have 

 been considered relatively specific for the photosynthetic partial 

 reaction of oxygen liberation, Mehler suggested that one or both of 

 the steps of ascorbic acid oxidation in the light might utilize an oxy- 

 gen precursor from the chloroplasts rather than molecular oxygen — 

 the stimulation of gas uptake being therefore due partly or entirely 

 to decreased production instead of increased consumption of oxygen. 

 It is of some interest to investigate further this suggestion and to de- 

 termine w^hether ascorbic acid is indeed a reagent which can attack a 

 reactant in that part of the photosynthetic reaction sequence lying 

 between the oxidized photolysis product and molecular oxygen. 



To test Mehler's hypothesis we have employed tracer oxygen and a 

 recording mass spectrometer in a fashion described in previous com- 

 munications. The rates of simultaneous production and consumption 

 of oxygen were computed from the rates of metabolism of the oxygen 

 isotopes when the oxygen of the gas phase in the experimental vessel 

 was enriched with tracer of mass 34. Chloroplasts of "Pokeweed" 

 {Phytolacca decandra L.) have been used throughout. The experi- 

 ments are still in progress but enough has been learned to make 

 worth-while the presentation of some of our results which pertain to 

 the manner in which ascorbic acid affects the oxygen metabolism 

 of illuminated chloroplast preparations. 



