Activity, Location and Role of NAD Malic Enzyme in Leaves With C4-Pathway Photosynthesis

Abstract

C4 acid decarboxylation in many C4-pathway species is accounted for either by an NADP-specific malic enzyme or phosphoenolpyruvate carboxykinase but a major group lack these enzymes. The present paper provides evidence for the mediation of C4 acid decarboxylation in this group by an NAD malic enzyme located in bundle sheath mitochondria. This enzyme was most active with NAD and Mn2+ and, depending upon its source, activity was stimulated 5- to 15-fold by low con- centrations of CoA or acetyl-CoA. The activity in leaf extracts was 20-50 times that found in other groups of C4 species or in C3 species and was commensurate with the enzyme having an integral function in photosynthesis. For most species showing high NAD malic enzyme activity there was little activity when Mg2+ replaced Mn2+ and the low activity recorded with NADP was not activated by CoA or acetyl-CoA. In others there was an activator-dependent rate with NADP equivalent to 25-30% of the rate with NAD. Evidence for the location of the NAD malic enzyme in bundle sheath mitochondria is provided. On the basis of these and earlier studies a detailed scheme is proposed to account for decarboxylation of aspartate derived from mesophyll cells.