38 



Bessel Kok 



III . Photo-oxidation of cytochromes and plastocyanin by- 

 detergent treated chloroplasts . 



Photosystem II is quite sensitive to ageing, heating or 

 detergent, Photosystem I, hov/ever, survives such treatments 

 to a considerable extent. Possibly by opening up the chloro- 

 plast structure, detergent makes P700 accessible for large 

 molecules such as cytochrome c, f, or plastocyanin so that 

 they can be photo-oxidized (like reduced indophenol dye). 

 The photo-oxidation of cytochrome c by this material was 

 first observed by Niemann and Vennesland (8), We may briefly 

 summarize the results of our own recent studies of such photo- 

 oxidations (9)« The process is sensitized quite efficiently 

 by long wave light (the yield approaches 1 eq./hv) involving 

 photosystem I — P700* being the photo-oxidant. Two types of 

 catalysts accelerate the photo-oxidation of f errocytochrome cl 



Viologen, flavin and other auto-oxidizable, single electron 

 transfer agents stimulate by mediating between primary re- 

 ductant X~ and oxygen. 



Cytochrome f (Cyt. 552 from Euglena) and plastocyanin (10) 

 stimulate by mediating between P?©©"*" and ferrocyt. c. Figure 

 2 illustrates the effect of various concentrations of plasto- 

 cyanin upon the rate of cyt. c photo-oxidation in the presence 

 and absence of viologen. Rates approaching 5000 eqs./chl. 

 hour have been observed, severalfold higher than found in 



10 12 14 16 18 20 22 24 



CONCENTRATION OF PLASTOCYANIN COPPER 



28«10"*M 



Figure 2 



photosynthesis or chloroplast reductions. The rates proved 

 remarkably independent of temperature. (Q 10-^1. 3i+ 30"- 

 -5*0) Cyt. f acts very similar to plastocyanin but never 



