1532 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



chloroplast-impregnated filter paper strips: their capacity to catalyze carbon dioxide 

 reduction by hydrogen in the dark. She found that these chloroplast preparations con- 

 tain a "hydrogenase" capable of taking up molecular hydrogen and using it for the reduc- 

 tion of various substrates (O2, CO2, etc. — cf. the reactions of the hydrogen bacteria, Vol. 

 I, page 116, and of "adapted" algae, Vol. I, page 130). The tests were made by observ- 

 ing the decoloration of methylene blue (r/. Vol. I, page 131) by the chloroplast-covered 

 paper strip in an atmosphere of hydrogen; the uptake of hydrogen was confirmed by 

 manometric measurements. The activity was highest with those species (Trifulium 

 repens, Chenopodiurn album) whose chloroplasts did not easily disintegrate into grana. 



Instead of methylene blue, oxygen could be used as hydrogen acceptor: with 0.0- 

 0.67% O2 in the air, from 75 to 288 mm.' of oxygen were taken up l)y 25 mg. of chloro- 

 plasts, together with from to 200 mm.' hydrogen. At 0.6-1.5% O2 in the atmosphere, 

 the ratio AH2/AO2 was 1.06-1.17; at [O2] > 2%, the uptake of hydrogen declined 

 rapidly. Boichenko interpreted this as evidence of deactivation of the hydrogenase by 

 oxygen (cf. Gaffron's observations, Vol. I, page 132). When [O2] approached 10%, 

 the uptake of oxygen also ceased. 



The gas exchanges shown in Table 35. V were observed when carbon dioxide and 

 hydrogen were offered to the chloroplast films in the dark. According to this table the 

 uptake of CO2 occurred without (or with only a slight) uptake of oxygen (without which 

 no chemosyn thesis had been observed in bacteria and algae!). Since the ratio AHj/ 

 ACO2 is close to 1.0 (rather than 2.0, as in Gaffron's adapted algae), the reaction reminds 

 one not so much of "chemosynthesis" of carbohydrates, as of the synthesis of formic acid 

 from H2 and CO2 by E. coli (Woods, cf. Volume I, pages 185, 208). The uptake of 

 CO2 and H2 was notetl even in the absence of oxygen, but was stimulated by its presence, 

 even though only little oxygen was taken up. (It thus looks as if, when both CO2 and 

 O2 are present, the former is used as oxidant in preference to the latter; a remarkable 

 selection, considering the difference of oxidizing potential. However, a similar situa- 

 tion exists in photosynthesis, where, too, photoxidation gets under way only when the 

 cells are deprived of carbon dioxide.) 



Table 35.V 

 Gas Uptake by 25 Mg. Chloroplasts on Filter Paper in Darkness, in Hj -|- CO2 



(after Boichenko 1947, 1948) 



Boichenko measured the effect of CO2 concentration on the rate of gas uptake 

 ( ACO2 + AH2) and found that the uptake (by 25 mg. chloroplasts) increased from 12.5 

 mm.3 in 5 min. at 0.6% CO2 to 375 mm.' at 6% CO2, but declined rapidly at the higher 

 carbon dioxide pressures (e. g., to 75 mm..» at 10% CO2). The ratio AH2/ ACO2 remained 

 constant (1.0 to 1.1) up to 6% CO2, but only carbon dioxide was taken up at 10% CO2. 

 The temperature coefficient of the gas uptake ( ACO2 + AH2) was Qio ^ 2 between 20 

 and 35° C.; above 35° C., the hydrogenase was destroyed. The rate of the chloroplast- 

 catalyzed reduction of methylene blue by hydrogen appeared to be independent of tem- 

 perature. (Table 35.VI.) 



