The magnesium-chelatase in developing cucumber cotyledons
- Clemson Univ., SC (USA)
Mg-chelatase in chloroplasts catalyzes the insertion of Mg into protoporphyrin. This step is the first in the tetrapyrrole pathway unique to chlorophyll biosynthesis. Mg-chelatase was assayed in semi-green cucumber cotyledons by an adaptation of the method of Fuesler at al, (1984) Plant Physiol. 74, 928-933. Plastid intactness was essential for Mg-chelatase activity. The estimated Km for the protoporphyrin substrate was between 0.5 and 1.0 {mu}M. In the presence of 4 mM ATP and an ATP regenerating system, activities of 500 pmol Mg-Protoporphyrin/mg protein/20 min were routinely recorded. In greening cotyledons, the specific activity of the Mg-chelatase increased steadily until the chlorophyll content of the plastids reached about 30 nmol/mg plastid protein, at which point further greening caused a decrease in Mg-chelatase activity. The total heme content of the plastids also rose with greening and paralleled the increase in Mg-chelatase activity. The regulation of heme levels and Mg-chelatase activity in greening cotyledons will be discussed. Substrate specificity studies showed that mesoporphyrin and deuteroporphyrin were active substrates for Mg chelation. The Mg-chelatase was not inhibited by its product, Mg-Protoporphyrin, but was inhibited by N-methyl mesoporphyrin (I{sub 50} = 2.5 {mu}M). Mg-chelatase activity was also measured in fully mature pea and corn chloroplasts (grown under diurnal light); the specific activities were comparable with those in cucumber.
- OSTI ID:
- 5628968
- Report Number(s):
- CONF-9007196--
- Journal Information:
- Plant Physiology, Supplement; (USA), Journal Name: Plant Physiology, Supplement; (USA) Vol. 93:1; ISSN PPYSA; ISSN 0079-2241
- Country of Publication:
- United States
- Language:
- English
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