Properties of Magnesium Chelatase in Greening Etioplasts: METAL ION SPECIFICITY AND EFFECT OF SUBSTRATE CONCENTRATIONS.

Evidence was obtained by means of low temperature spectrofluorimetry and high pressure liquid chromatography for the exclusive chelation of Mg(2+) into protoporphyrin IX by a plastid preparation extracted from greening cucumber cotyledons. Under the experimental conditions which were optimized for Mg(2+) chelation, Zn(2+) chelation was not detected. However, Zn(2+) chelation was observed when ATP was omitted and ZnCl(2) was added to the incubation mixture. The observed Zn(2+) chelation was partially nonenzymic. The requirements for the enzyme, Mg chelatase, are discussed in terms of the regulatory function of this enzyme.