Properties and Regulation of Adenylate Kinase From Zea mays Leaf Operating in C4 Pathway Photosynthesis

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Abstract
Adenylate kinase from the mesophyll cells of Z. mays was partially purified and its kinetic and regulatory properties determined. Fractionation of mesophyll cell extracts by (NH4)2SO4 precipitation, filtration on Sephacryl S-200 and DEAE chromatography gave a fraction purified .apprx. 90-fold. For the reaction in the direction of ADP synthesis (forward reaction), activity was influenced by a complex interaction between Mg2+, ATP and pH. Optimal activity occurred with a Mg2+ to ATP ratio of about unity and the pH optimum under these conditions was .apprx. 8.0. With an excess of Mg2+, activity was reduced and the pH optimum was shifted to lower values. Varying ATP or AMP in a lower range of concentrations gave simple Michaelis-Menten responses. At higher concentrations there was a complex response to increasing ATP and AMP became inhibitory. For the reverse reaction the Km (ADP) was .apprx. 70 .mu.M and the pH optimum was between 8.0 and 8.5. AMP was a strong inhibitor of the reverse reaction (competitive with respect to ADP), the Ki being .apprx. 40 .mu.M. Several other metabolites tested had no effect on adenylate kinase activity. Adenylate kinase extracted from washed mesophyll chloroplasts had essentially identical properties.

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