1568 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



enough the latter was now 2-3 times faster than in a qiiinone-free system. 

 (In the latter, the rate of oxygen consumption in Hght is only V2 or Vs of 

 that of oxygen liberation with quinone; while in the mixed system, after 

 quinone had been nearly exhausted, the rate of oxygen consumption is 

 about equal to that of initial oxygen liberation.) This stimulation of the 

 photochemical alcohol oxidation by preceding quinone reduction was sus- 

 tained even w^hen an equivalent of five times the total amount of ciuinone 

 present had been oxidized. Checks showed that this catalytic effect is not 

 due to the presence of hydroquinone. The stimulation is still present if 

 alcohol and catalase are added 10-20 minutes after the quinone reduction 

 had been completed. No similar stimulation follows the Hill reaction with 

 f erri cyanide ; but 0- or p-naphthaciuinone have the same effect as benzo- 

 quinone. No stimulation is brought about by simply incubating chloro- 

 plasts in quinone solution in the dark. Reducing quinone in the dark by 

 ascorbic acid also caused no stimulation if the two reagents were mixed 

 before adding the chloroplasts, but it produced the same stimulation as 

 photochemical reduction if ethanol and catalase were first mixed with the 

 chloroplasts and quinone and ascorl)ic acid added afterward. The rate of 

 oxygen uptake was enhanced by still another factor of two if more than the 

 stoichiometric quantity of ascorbic acid was used. (It is uncertain whether 

 the thus quadrupled rate of oxygen consumption equalled, or exceeded, the 

 rate of photosynthesis by the same amount of chlorophyll, cf. section 5(6) 

 below.) Only a slow photoxidation of ascorbic could be observed in il- 

 luminated chloroplast suspensions in the absence of quinone. In the sys- 

 tem (chloroplasts + quinone + ascorbic acid), one mole of oxygen was 

 consumed in light. Adding only catalase caused a 50% inhibition of this 

 reaction, but adding both catalase and ethanol lead to the above-described 

 maximum stimulation of oxygen consumption. It continued until two 

 moles of O2 had been used up per mole ascorbic acid, after which the reac- 

 tion rate dropped to the level characteristic of the (quinone + ethanol + 

 catalase) system without ascorbate. Similar effects were observed with 

 glutathione; it, too, was photoxidized by chloroplasts slowly in the absence 

 of quinone, and rapidly in its presence. Because of Gerretsen's observa- 

 tions (cf. below), Mehler tried adding Mn++ salt to the above-described 

 systems, and found that the only one on which it had an effect was (ox3'gen 

 -f- ethanol -|- catalyse + chloroplasts) ; the rate of oxygen consumption by 

 this mixture in light was doubled by the addition of 10~* mole/1. MnCla. 

 The enhancing effects of quinone reduction and of Mn++ addition could not 

 be superimposed upon each other. The effect of manganous ions was 

 catalytic; ferrous ions did not produce it; instead, they themselves were 

 rapidly photoxidized by chloroplasts in the presence of ethanol and cata- 

 lase. 



