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.) lea ves treated with 5-aminolevulinic acid (ALA) and 2,2'-dipyridyl (DP). The magnesium chelatase activity was estimated during protoporphyrin I X, Mg-protoporphyrin IX (monomethyl ester) (MgProto(E)) and protochlor ophyllide accumulation in the presence of exogenous ALA and/or DP. The results showed a close correlation only between the enzyme activity a nd MgProto(E) levels. The inverse dependence of the magnesium chelatas e activity on the amount of MgProto(E) accumulated supports the hypoth esis 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 1 7 h resulted in a 1-2 h lag phase of magnesium chelatase activity with consequent strong stimulation. After 3 h of illumination it reached 2 00% compared with the enzyme activity immediately before illumination. Cycloheximide, if applied during the entire period of dark or light t reatment, decreased the magnesium chelatase activity by 19% in the dar k and by 39% in light, in comparison with the corresponding control va riant on H2O. The ADP and especially the ATP content increased in the leaves treated with cycloheximide both in darkness and in light. The p hotosynthetic activity, measured as O-2 evolution by leaf segments, di d not change in the presence of cycloheximide. The results are discuss ed as an additional influence of light on the magnesium chelatase acti vity not only via photosynthetic supply with ATP but also through ligh t induced synthesis of the enzyme molecules de novo.