MAGNESIUM CHELATASE OF HORDEUM-VULGARE L IS NOT ACTIVATED BY LIGHT BUT INHIBITED BY PHEOPHORBIDE

The enzyme activity of magnesium chelatase was determined in intact et ioplasts of barley (Hordeum vulgare L.) seedlings. Irradiation of isol ated plastids with white light for 15 min does not lead to any activat ion of the enzyme but to a decrease in activity, especially in etiopla sts. The enzyme was inhibited by chlorophyllide and zinc pheophorbide only to a certain extent. Strong inhibition was observed with the meta l-free pheophorbide (K-i = 0.92 mu M) but not with pheophytin or chlor ophyll. Penetration of chlorophyllide through the envelope membrane wa s confirmed by the chlorophyll synthase reaction that occurs in the in ner membranes of etioplasts and chloroplasts. The possible role of inh ibition of magnesium chelatase by pheophorbide in senescent leaves and tetrapyrrole transport across the plastid envelope are discussed.