Abstract 1. 1. NADP-malic enzyme was purified from ribbed mussel gill tissue mitochondria and found to be a tetramer composed of subunits with M r = 65,000 . 2. 2. At pH 7, with 1 mM MnCl 2, Michaelis-Menten kinetics were observed. 3. 3. The apparent K M values for NADPH, pyruvate, sodium bicarbonate malate and NADP were 129 μM, 8.5 mM, 19 mM, 43 μM and 4 μM, respectively. 4. 4. NADPH and pyruvate exhibited substrate inhibition at high concentrations ( NADPH > 100 μM ; pyruvate > 20 mM ). 5. 5. ATP was a mixed inhibitor of NADP utilization ( K i = 0.5 mM ). 6. 6. While the enzyme reaction is freely reversible when the substrate concentrations are optimized, reversibility (malate synthesis) in vivo is probably limited by low tissue bicarbonate (CO 2) and pyruvate concentrations.