150 M. D. KAMEN 



dwarf changes in other components expected to participate in energy 

 transfer during the early phase of photosynthesis. 



In elaboration of these remarks this report will be concerned, first, 

 with the data relating to distribution and nature of hematin com- 

 pounds (21) in photosynthetic tissue, and, second, with the observa- 

 tions which have been made on the steady state of oxidation of cell 

 components during photosynthesis. 



OCCURRENCE AND NATURE OF HEMATIN COMPOUNDS IN GREEN 



PLANT CHLOROPLASTS 



Two hematin compounds appear to be unique to the photosyn- 

 thetic apparatus in green plants. One of these, cytochrome /, was the 

 first chloroplast hematin compoimd to be detected (30). Its 

 isolation and properties have been described exhaustively by Hill 

 and his collaborators (8,15,16). 



Cytochrome / is widelj'' distributed in the higher plants and algae. 

 It appears to be an integral part of the chloroplast structure inas- 

 much as it cannot be separated from the chloroplast unless organic 

 solvents are used to split off lipid. Cytochrome / is estimated to 

 account for at least one-third of the hematin of the chloroplast and is 

 present in the ratio of 1 mole for approximately 400 moles chloro- 

 phyll (8,16,24). It has been possible to obtain a preparation purified 

 some 300-fold over the state in which cytochrome / is initially ex- 

 tracted from the leaf, using ammoniacal ethanol. The yield of puri- 

 fied material is about 13% of the hematin originally present in the 

 crude extract. 



Cytochrome / is classified as a "modified" cytochrome of the c 

 type because of its spectrochemical characteristics, the spectra of 

 the hemochromogens derived from it by reaction with alkaline 

 cyanide or pyridine, and its relatively high potential. However, it will 

 not function as a substrate for the classical cytochrome c oxidase. 

 In fact, no system spectrochemically analogous to cytochrome a or as 

 appears to be present in the chloroplast, although it exists in the cyto- 

 plasm outside the chloroplast along with normal cytochrome c. 

 Oxidase activity responding to respiratory inhibitors has been re- 

 ported in chloroplasts (29) but such activity is not proof for the 

 presence of cytochrome a. Cytochrome / will couple with cytochrome 

 c nonenzymatically so that it can be oxidized slowly when incubated 



I 



