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

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, CO2 not HCO3-. The apparent Km, CO2 for the maize enzyme was 1.2 mM and the apparent Km, CO2 for the wheat germ preparation was 4.2 mM under conditions of substrate saturation, pH 7.3 and 17 degrees C. These observations support the views that CO2, rather than HCO3- as has been suggested, is produced in bundle-sheath 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.