PROTOCHLOROPHYLLIDE PHOTOREDUCTION IN PLANTS: SOME COMMENTS ON RECENT DATA

ABSTRACT When photoreducible protochlorophyllide is illuminated in etiolated, fresh or freeze- dried leaves at a very low temperature non-fluorescent, long-wavelength-absorbing pigments are accumulated. These pigments transform in the dark into chlorophyllide as soon as the leaves are warmed up. For this reason, they appear to be intermediates between protochlorophyllide and chlorophyllide. A stable configuration of a ternary complex between protochlorophyllide, a reductant (NADPH) and a photoreductase, where all the reactants are ready to perform the photoreduction, explains the reactivity of the system which forms the intermediates at a low temperature and in the dry state. The spectroscopic properties of the ternary complex suggest a charge-transfer interaction between NADPH and protochlorophyllide. Light should trigger the addition of two electrons and one proton on the 7,8 carbon-carbon double bond of cycle IV of the porphyrin ring; the origin of the second proton is still unknown. The stability of t...