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.