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Articles

Reversal of ,'-Dipyridyl-induced Porphyrin Synthesis in Etiolated and Greening Red Kidney Bean Leaves ¹

^a Department of Biological Sciences, University of Illinois at Chicago Circle, Box 4348, Chicago, Illinois 60680

The chemical induction of porphyrin synthesis has been investigated in etiolated and greening leaves of Phaseolus vulgaris L. var. Red Kidney. When these leaves are incubated in darkness with solutions of transition metal ion chelators such as ,'-dipyridyl, 1,10-phenanthroline, pyridine-2-aldoxime, or other related aromatic heterocyclic nitrogenous bases, they synthesize large amounts of protochlorophyllide and Mg protoporphyrins. Greening leaves produce more porphyrin than do etiolated leaves under such conditions. If the leaves are then transferred to 1 millimolar solutions of various transition metal salts such as Fe²⁺, Zn²⁺, or Co²⁺ (but not Mn²⁺ or Mg²⁺), Mg protoporphyrin (monomethyl ester) synthesis immediately ceases and the pigment(s) rapidly disappear(s); protochlorophyllide synthesis gradually diminishes during 4 to 8 hours of treatment. The loss in Mg protoporphyrin(s) can be accounted for by a simultaneous increase in protochlorophyllide in partially greened leaves but not in etiolated leaves. In the latter, the decline in Mg protoporphyrin(s) initiated by the application of Zn²⁺ is retarded by low temperature and anaerobiosis but not by respiratory inhibitors. Cycloheximide inhibits the loss of Mg protoporphyrin(s) but does not affect their conversion to protochlorophyllide.

These results indicate that: (a) greening leaves have a greater capacity to synthesize -aminolevulinic acid than do etiolated leaves; (b) ,'-dipyridyl induction of porphyrin synthesis in etiolated and greening leaves can be blocked by application of certain transition metal salts; (c) in greening leaves the accumulated Mg protoporphyrin(s) are stoichiometrically converted to protochlorophyllide upon treatment with these salts whereas in etiolated leaves the accumulated Mg protoporphyrin(s) are labile and are not quantitatively converted to protochlorophyllide upon such treatment; (d) in etiolated leaves the accumulated Mg protoporphyrin(s) are destroyed via a light-independent, probably enzymic process which requires cytoplasmic protein synthesis.

¹ This work was supported by National Science Foundation Grant PCM 75-07681 A01 to MLG. These studies formed part of a thesis submitted by LMV to the Graduate College of the University of Illinois at Chicago Circle in partial fulfillment of the requirements for the M.S. degree.