L.K.M. Buysens 



tuBj which has peaks at 420 and 555 Ba|i,is similar but not iden- 

 TTcal to the difference of thi'^ab sorption spectra of oxidized 

 and reduced cytochrome f or £. We call this cytochrome C 420. 

 The maximum of the /difference spectrum of cytochrome £ in vitro 

 occurs at a 424 mn\ 21 123)5 the band shift may have been caused 

 by the extraction procedure* slnv contradistinction to the action 

 spectrum of photosynthesis^-''^ , which shows much stronger 

 photo synthetic activity of phycoerythrin thaja^of chlorophyll a, 

 the action spectrum for cytochrome oxidation (2)' shows strong 

 activity for chlorophyll a but little activity for phycoery- 

 thrin; furthermore, the kinetics of cytochrome oxidation were 

 qualitatively different at 680 and 56O mii, which indicated that 

 at least two photochemical reactions participated in the cyto- 

 chrome reactions. If the cytochrome is brought in the oxidized 

 state by illumination with actinic light of 68O m^, addition 

 of ^strong light of 5^0 m|i caused a reduction of the cytochrome 

 U5»o7^ As discussed in the introduction, oxidation by light 1 

 and reduction by light 2 can be explained by means of scheme 1, 

 and, in fact, a scheme like this was first proposed on basis of 

 the cytochrome reactions. The light-driven reduction of cyto- 

 chrome was inhibited by DGMU, hydroxylamine or M-ethylure thane. 

 This suggests that these inhibitors inhibit one or more reac- 

 tions between photoreaction 2 and C 420, Evidence will be gi- 

 ven below that DCMU inhibits between ^ and quinone. Prom scheme 

 1 it follows that in the presence of an inhibitor like DCMU the 

 action spectrum for the initial rate of C 420 oxidation is pro- 

 portional to the action spectrum of system 1. The action spec- 

 trum of system 1 of Porphyridium between 540 and 710 mix shown 

 in the insert of the scheme was determined in this way. The ac- 

 tion spectrum of photosynthesis, measured against a background 

 of relatively strong light of 56O m^ is, according to the dis- 

 cussion in the introduction, also proportional to the action 

 spectrum of system 1, and was found to be similar to the action 

 spectrum for cytochrome oxidation, albeit not identical. So far 

 only ad hoc explanations have been given of the deviation be- 

 tween the two spectra . Similar observations were made for 

 the blue-green alga Anacystis nidulans ^^4;, 



Phospho pyridine nucleotide reduction . Upon illtimination of 

 photosynthetic organisms an increase in fluorescence in general 

 occurs in the blue region. The fluorescence difference spectrum 

 and that for the excitation of this fluorescence are similar 

 to the corresponding spectra of PNH bound to an enzyme. Since 

 DPN and TPN do not fluoresce, these experiments indicate that 

 upon illumination of photosynthetic organisms reduction of py- 



