Characterization of NADP-dependent Malic Enzyme from Developing Castor Oil Seed Endosperm

Metabolic pathways sequestered within the leucoplast of developing oilseeds ensure a balanced supply of substrates and cofactors for fatty acid biosynthesis. NADP-dependent malic enzyme (NADP-ME) may be important in supplying both carbon and NADPH for fatty acid biosynthesis in the developing endosperm of the oilseed Ricinus communis. NADP-ME was purified 5160-fold to a specific activity of 18.2 U/mg protein. NADP-ME is a homotetramer with a native mass of 254 kDa and a subunit size of approximately 63 kDa. Effectors of castor NADP-ME are typical of the NADP-malic enzymes, with the exception of acetyl-CoA and its derivatives, which were found to act as activators. This is consistent with a regulatory role for these molecules during fatty acid biosynthesis in vivo. NADP-ME was found to have maximal activity at stage 7 of endosperm development, coincident with maximal lipid accumulation.