1.Alanopine dehydrogenase from the foot muscle of the common periwinkle,Littorina littoreawas purified to homogeneity using a combination of ammonium sulphate fractionation, gel filtration and chromatofocusing.2.The enzyme has a molecular weight of 42,200±500 and is a monomer.3.l-Alanine and pyruvate are the preferred substrates. Alternate amino acids (glycine>l-α-aminobutyrate>l-serine>l-cysteine) are used at rates less than 37% of enzyme activity withl-alanine. Alternate keto acids used include oxaloacetate, α-ketobutyrate and glyoxylate. The enzyme is specific for meso-alanopine in the reverse direction;d-strombine is not oxidized.4.ApparentKm's for both pyruvate andl-alanine decrease with increasing co-substrate (l-alanine or pyruvate) concentration or with decresing pH.5.AbsoluteKm's for pyruvate andl-alanine are 0.17±0.02 and 14.9±0.85 mM at pH 6.5 rising to 0.26±0.01 and 23.8±0.52 mM at pH 7.5, respectively. ApparentKm's for meso-alanopine are 6.5 mM at pH 6.5 and 50 mM at pH 8.5 while apparentKm's for NADH (9±0.1 μM) and NAD+(0.18±0.03 mM) are pH independent.6.Substrate inhibition by pyruvate (I50=8 mM) andl-alanine (I50=450–550 mM) occurs at saturating co-substrate levels while NAD+(Ki=0.16±0.012 mM) and meso-alanopine (Ki=35±0.4 mM) are product inhibitors of the forward reaction. ATP and ADP are competitive inhibitors with respect to NADH whilel-lactate,d-strombine and succinate inhibit with respect to pyruvate andl-alanine.7.The kinetic properties of alanopine dehydrogenase favour enzyme function in cytoplasmic redox balance during anoxia stress in this intertidal gastropod. In particular the effects of rising cosubstrate levels (pyruvate and alanine are products of glycolysis) and decreasing pH (occurring during anoxia) on enzyme apparentKm's for substrates would favour alanopine accumulation as a product of anaerob