434 RADIATION BIOLOGY 



to protochlorophyll in the dark by thermochemical reactions. This 

 mechanism is suggested by the formation of magnesium-containing com- 

 pounds through irradiation of etiolated leaves at low temperature (cf. 

 Fig. 7-8). 



We are indebted to Granick for much of whatever detailed knowledge 

 we have of the precursors of protochlorophyll. His isolation of proto- 

 porphyrin-9 from X-ray-induced mutants of Chlorella has given evidence 

 for the similarity of the biosynthesis of porphyrins in animals and plants 

 (Granick, 1951). Since the biosynthesis of protoporphyrin-9 in animals 

 has been shown to come entirely from glycine and acetate molecules 

 "through a compound arising from the tricarboxylic acid cycle" (Shemin 

 and Wittenberg, 1951), it may be presumed that the porphyrins in plants 

 are derived by the same metabolic path. This hypothesis has been made 

 more reasonable by the observation of Salomon et al. (1950), who showed 

 by the use of tracers that C. vulgaris "is able to utilize directly the alpha 

 carbon atoms of glycine and acetate for the biosynthesis of chlorophyll." 



On the basis of the tracer studies and the compounds isolated from 

 Chlorella mutants — protoporphyrin-9, magnesium protoporphyrin, and 

 magnesium vinylpheoporphyrin-as — Granick (1948-1949, 1951) has pro- 

 posed a rational sequence for the biosynthesis of protochlorophyll and 

 chlorophyll a (Table 7-7). 



Table 7-7. A Scheme for the Biosynthesis of Chlorophyll 



(Granick, 1951.) 

 Glycine -f- acetate 



I n steps 



Hypothetical pyrrole 



I n steps 

 Uroporphyrin type III (hypothetical first tetrapyrrole) 



I 4 steps? 

 Coproporphyrin type III 



I n steps 



Protoporphyrin 9 

 ^ n steps 

 Mg protoporphyrin 

 I 4 or 5 steps 

 Mg vinylpheoporphyrin-a6 



i 

 Mg vinylpheoporphyrin-a6 phytyl ester (protochlorophyll) 



i ? 



Chlorophyll a -^ Chlorophyll b 



From the authors' own experience it is proposed that, in those plants 

 which form chlorophyll only in the light, some of the intermediate reac- 

 tions prior to protochlorophyll formation are light-induced, particularly 

 those associated with the incorporation of magnesium into organic combi- 

 nation; also that chlorophyll b does not arise from chlorophyll a but 

 comes from a precursor, probably preceding protochlorophyll, which is 



