Hematin Compounds in the Metabolism of 

 Photosynthetic Tissues 



MARTIN D. KAMEN, Edward Mallinckrodt Institute of Radiology, 

 Washington Medical School, St. Louis, Missouri 



Two salient facts, established only recently, provide the basis for 

 this discussion. One is that all photosynthetic tissues contain hematin 

 compounds. The second concerns the effect of the light energy utilized 

 in photosynthesis. This energy sets up a steady state in which the 

 major change relative to the dark steady state is a shift in the direc- 

 tion of greater oxidation of one or more hematin compounds. 



In the case of green plants and algae, it is not difficult to limit the 

 data to be considered to those known to be unique to photosynthetic 

 tissue. In these organisms, the sites of photometabolism are well- 

 defined subcellular particles ("chloroplasts") about which a large body 

 of knowledge is steadily accumulating. Much less well defined are the 

 corresponding entities in the bacteria. This uncertainty is not serious 

 in the case of the obligate photoanaerobes, because in these systems 

 growth is possible only by photosynthesis under strictly anaerobic 

 conditions. In the facultative bacteria, there is evidence that cell 

 particles much like chloroplasts exist in at least two species, Rhodo- 

 spirillum ruhrurn and Rhodopseudomonas spheroides (31,33,37). 



There is a great deal of significance in the fact that hematin com- 

 pounds are found both in green plant chloroplasts and in large 

 amounts in all varieties of the photosynthetic bacteria. This signifi- 

 cance lies in the fact that, whereas the green plants, algae, and the 

 photosynthetic bacteria cover practically the whole range of living 

 metabolic patterns, they share no property except that of using 

 light energy for growth. Furthermore, until now only two classes of 

 compounds have been found in all photosjmthetic systems. These are 

 the photoactive pigment complexes — the chlorophylls, carotenoids, 

 etc. — and the hematin compounds. 



The significance of the observation that absorption of light energy 

 results in oxidation of component hematins lies in the apparent 

 magnitude of these changes, in both amount and duration, which 



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