Pigeon liver malic enzyme (EC 1.1.1.40) has a double dimer quaternary structure. The NADP + analogs, aminopyridine adenine dinucleotide phosphate and nicotinamide-1.N 6 -ethenoadenosine dinucleotide phosphate, bind to the enzyme anti-cooperatively. In the presence of non-cooperative competing ligand NADP + the binding parameter Hill coefficients of these analogues changed very little. Binding of L-malate with enzyme-AADP + complex first enhanced then reduced the nucleotide fluorescence. Two L-malate binding sites, with K d values of 23-30 and 270-400 μM, respectively, for the tight and weak binding sites were postulated. A hybrid model between the sequential and pre-existing asymmetrical models was proposed for the pigeon liver malic enzyme.