Mitochondrial NAD‐dependent malic enzyme: A new regulatory enzyme

Introduction Electrophoretic evidence for multimolecular forms of malic enzyme (EC 1.1.1.40, NADP-malic dehydro- genase, decarboxylating) was reported by Henderson in 1966 [ 11. Subsequent purification and assay by spectrophotometric and other methods revealed that malic enzyme isoenzymes were present in cytosol and mitochondrial fractions derived from several different animal tissues [2-61. Of particular importance were the recent findings of Frenkel [7] that the mitochon- drial isoenzyme has allosteric properties, and that succinate was a positive modulator. Here, we demon- strate that calf adrenal cortex mitochondria contain an NAD-dependent malic enzyme (EC 1.1.1.39) with regulatory properties. Fumarate was a positive and ATP a negative modulator. The enzyme also reduced NADP but only at about one-quarter of the rate of NAD reduction. These kinetic properties suggest that this enzyme is an important regulator of the rate of malate oxidation. 2. Materials and methods Calf adrenal glands were obtained from the local