Action of sulphite on plant malate dehydrogenase

SO₃^2? acts on NAD- and NADP-dependent malate dehydrogenase in several ways. Firstly, SO₃^2? favours the appearance of low MW species (65 000 and 39 000 daltons) in Sephadex gel chromatography. Secondly, the enzyme form which is obtained by gel chromatography with dithioerythritol plus nucleotide cofactor is changed in the presence of SO₃^2?. This is indicated by the appearance of a linear reaction (instead of curvilinear), and by the abolition of the biphasic sigmoidal kinetics on varying substrate and cofactor concentrations. SO₃^2? causes the loss of negative cooperativity at low substrate or cofactor concentrations. Thus the inhibition of initial velocity at high substrate or cofactor concentrations is even more marked than at lower ones. Thirdly, SO₃^2? strongly reduces the activity in substrate saturating conditions.