Certain Effects of Ascorbic Acid on the Reduction 

 of Oxygen in Chloroplast Preparations* 



HELEN M. HABERMANNf and ALLAN H. BROWN, Botany Department, 

 University of Minnesota, Minneapolis, Minnesota 



Some years ago Alan Mehler discovered that oxygen should be 

 added to the growing list of Hill oxidants which are effective in 

 promoting photochemical oxygen production by chloroplast prepa- 

 rations. Mehler observed a light-dependent oxygen consumption in 

 the presence of a hydrogen peroxide trap (ethanol plus a large ex- 

 cess of catalase). He reasoned that oxygen was being reduced to 

 peroxide by a photochemically generated reductant. This reaction 

 sequence accounts for oxygen consumption in accordance with the 

 stoichiometry of the following scheme: 



(A) 4H20^4[H] +4[0H] 



(B) 4[OH]^2H20 + O2 



(C) 2O2 + 4[H]-*2H202 



(D) 2C2H5OH + 2H2O2 -* 2CH3CHO + 4H2O 



O2 + 2C2H5OH -^ 2CH3CHO + 2H2O 



The net uptake of oxygen is thus the consequence of two molecules 

 consumed for every one produced. Mehler was able to confirm this 

 two-way traffic in oxygen metabolism by the use of tracer oxygen 

 monitored with a mass spectrometer. 



When IVIehler described the properties of this reaction, he noted 

 that the rate was considerably enhanced if the chloroplast prepara- 

 tion had previously been allowed to reduce quinone. One can speak 

 of a stimulated as well as an unstimulated Mehler reaction and, since 

 a number of substances are capable of effecting such a stimulation, 

 it is meaningful to distinguish between, say, a quinone-stimulated and 

 a manganese-stimulated Mehler reaction. 



* This work was aided b}^ a contract between the Office of Naval Research, 

 Department of the Navy, and the University of Minnesota (NR160-030) and was 

 also supported by the Graduate School. 



t Present address: Research Institute (Fels Fund), University of Chicago, 

 Chicago, 111. 



257 



