61 



John M. Olson and Robert M. Smlllie 



maximum net rate of oxidation during the "light -on" phase is much 

 less than the initial rate of reduction observed in the "light- 

 off" phase. A simple explanation of the "sluggish" cytochrome ox- 

 idation is that the rate of cyclic electron flow is rapid enough 

 at room temperature to compete vith the oxidation reaction at low 

 light intensity. The maximimi net rate of oxidation upon illumin- 

 ation is not proportional to intensity at these very low intensi- 

 ties; the rate vs. intensity curve is sigmoid as shown in Figure 

 5, As the temperature is dropped, however, the rate cxurve more 

 nearly approaches a straight line. 



Quantum Requirements ; Preliminary estimates of the quantum 

 requirement for cytochrome -5 52 oxidation in Euglena range from 2 

 to 8 quanta per electron based on rates of absorbancy change upon 

 illxomination. In Figure 5 the slope of the rate curve at 2* indi- 

 cates a quantum requirement of 2 if ^6552 " ^®5U0 ^^ assumed to 

 be 2 X 10^ M-1 cm"^. The lowest observed values are substantial- 

 ly lower than the estimate of 7-IO for Anacystis (^) . 



Cytochrome b Reactions ; Light-induced reactions of cytochrome 

 b in whole cells are ctoservable only under special circxjmstances. 

 Under physiological conditions cytochrome f-555 is the main pig- 

 ment to respond in Anacystis ; sometimes a slight response of the 

 low potential cytochrome C-550 is also obseirved. In Euglena , 

 only the high potential cytochrome -5 52 is observed. Cytochrome b 

 oxidation in Anacystis caused by far-red light (O.7O n) can be 

 observed in addition to cytochrome f oxidation when cells are 

 cooled to 2" C or \^en cells are permitted to become anaerobic at 

 room temperature. In both Anacystis and Euglena , the addition of 

 5 X 10-5 M CCCP permits the light-induced oxidation of cytochrome 

 b with far-red light (R^ only) either with or without the oxida- 

 tion of f-type cytochrome (Fig. 6). In Anacystis the cytochrome 

 b oxidation is superimposed on the usual cytochrome f + c oxida- 

 tion. In Euglena , the cytochrome b oxidation appears to replace 

 cytochrOTie-552 oxidation initially, but the cytochrome -5 52 light 

 reaction reappears almost completely after ^4-0 min without appre- 

 ciable change in the cytochrome b reaction. The mechanism of 

 CCCP action on photosynthetic electron transfer is not known. 



An attempt to demonstrate the light-induced reduction of cyto- 

 chrome b by R2 in whole cells ( Euglena ) indicated a possible 

 transient small increase in reduced cytochrome upon illimiination 

 with high intensity red light (O.65 u) . The steady-state change 

 was, however, either zero or a slight net oxidation. 



