CO2 is the inorganic carbon substrate of NADP malic enzymes from Zea mays and from wheat germ

Abstract

NADP malic enzyme (EC 1.1.1.40) was extracted and partially purified from the green leaves of Zea mays var. Felix and from wheat germ. The active inorganic carbon species for both enzymes was, in contrast to an earlier report, CO₂not HCO⁻₃. The apparentK_m, co₂, for the maize enzyme was 1.2 mM and the apparent K_m, co₂, for the wheat germ preparation was 4.2 mM under conditions of substrate saturation, pH 7.3 and 17°C. These observations support the views that CO₂, rather than HCO₃⁻ as has been suggested, is produced in bundlesheath chloroplasts and that the reaction mechanism catalysed by plant cytosolic and chloroplastic NADP malic enzymes is similar to that proposed for avian NADP malic enzymes.