THE NATURE AND POSSIBLE FUNCTION 

 OF CHLOROPLAST CYTOCHROMES 



Robert Hill and Walter D. Bonner, Jr. 



Department of Biochemistry, University of Cambridge, 



England, and Johnson Research Foundation, 

 University of Pennsylvania, Philadelphia, Pennsylvania 



This discussion will be limited exclusively to the cytochromes of 

 higher plant chloroplasts. It was in higher plants that a cytochrome, 

 localized in the photosynthetic apparatus, was first discovered. 



Hill and Scarisbrick (9) showed that a cytochrome, named by them 

 cytochrome /, coidd be observed in acetone powders of green leaves. 

 This cytochrome was readily extractable from green tissues and was 

 obtained in a highly purified form. Later work by Davenport and 

 Hill (4) localized cytochrome / to green leaves and described an im- 

 proved method for extraction and purification of cytochrome /. In 

 his work relating to the localization of cytochrome / within the chloro- 

 plast, Davenport (5) observed a second cytochrome. The spectro- 

 scopic properties of this second chloroplast cytochrome were studied by 

 Hill (7) , who named this component cytochrome ^g. Some of the 

 characteristics of cytochromes / and b^ are given in Table I. 



Cytochrome / is the only chloroplast cytochrome which is readily 

 brought into solution and to a high state of purity. It is non- 

 autoxidizable, is reduced by ascorbate, DPNH, and TPNH, and is 

 oxidized by ferricyanide. 



The absorption maxima of cytochrome / are slightly shifted toward 

 the red, when compared to cytochrome c\ and the former has a very 

 asymmetric a-band which at low temperature is resolved into very 

 sharp «! and a^ peaks. Fig. 1 illustrates the low temperature difference 

 spectrum (oxidized-reduced) of highly purified cytochrome / com- 

 pared to its room temperature absolute spectrum. 



The spectroscopic observation of cytochrome of freshly prepared 

 chloroplasts is complicated, because of the large concentration of 

 chlorophyll which is present. Through the use of a low dispersion 

 spectroscope it is possible, however, to observe the appearance of a 

 broad absorption band, in the region 550-560 m^i, following hydro- 



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