Response of magnesium chelatase activity in green pea (Pisum sativum L) leaves to light, 5-aminolevulinic acid and dipyridyl supply

The role of extra-porphyrin production in the magnesium branch of the porphyrin pathway was investigated in green pea (Pisum sativum L.) leaves treated with 5-aminolevulinic acid (ALA) and 2,2'-dipyridyl (DP). The magnesium chelatase activity was estimated during protoporphyrin IX, Mg-protoporphyrin IX (monomethyl ester) (MgProto(E)) and protochlorophyllide accumulation in the presence of exogenous ALA and/or DP. The results showed a close correlation only between the enzyme activity and MgProto(E) levels. The inverse dependence of the magnesium chelatase activity on the amount of MgProto(E) accumulated supports the hypothesis that a mechanism is involved which controls the magnesium branch of chlorophyll biosynthesis in vivo through inhibition of the enzymes by their products. Illumination of intact pea leaves predarkened for 17 h resulted in a 1-2 h lag phase of magnesium chelatase activity with consequent strong stimulation. After 3 h of illumination it reached 200% compared with the enzyme activity immediately before illumination. Cycloheximide, if applied during the entire period of dark or light treatment, decreased the magnesium chelatase activity by 19% in the dark and by 39% in light, in comparison with the corresponding control variant on H2O. The ADP and especially the ATP content increased in the leaves treated with cycloheximide both in darkness and in light. The photosynthetic activity, measured as O-2 evolution by leaf segments, did not change in the presence of cycloheximide. The results are discussed as an additional influence of light on the magnesium chelatase activity not only via photosynthetic supply with ATP but also through light induced synthesis of the enzyme molecules de novo.