Purification and properties of NAD+-dependent glyceraldehyde-3-phosphate dehydrogenase from spinach leaves

An NAD⁺-dependent glyceraldehyde-3-phosphate dehydrogenase (d-glyceraldehyde-3-phosphate:NAD⁺ oxidoreductase (phosphorylating), EC. 1.2.1.12) has been purified from spinach leaves as a homogeneous protein of 150 000 daltons.Kinetic constants of 2.5·10⁻⁴ M and 4 · 10⁻⁴ M have been calculated for NAD⁺ and glyceraldehyde 3-phosphate, respectively.The amino acid composition is characterized by a cysteine content higher than that found in analogous enzymes.On sodium dodecyl sulphate gel electrophoresis, the native enzyme dissociates into two subunits of 37 000 and 14 000 daltons. The two subunits have been isolated in equimolar amounts by gel filtration; end-group analysis shows that alanine is the N-terminal residue of the large subunit, while serine is found at the N-terminus of the small subunit.Comparison of amino acid analyses and peptide maps shows that the two subunits have a different amino acid sequence. These results indicate that the NAD⁺-dependent glyceraldehyde-3-phosphate dehydrogenase, isolated from spinach leaves has an atypical oligomeric structure, the protomer being formed by two different subunits.