L-Threonine dehydrogenase of chicken liver. Purification, characterization, and physiological significance.

L-Threonine dehydrogenase (L-threonine:NAD+ oxidoreductase (EC 1.1.1.103) has been purified to apparent homogeneity from chicken liver mitochondria. The presence of 2-mercaptoethanol and glycerol is necessary for stabilizing the enzyme during purification and storage. The enzyme is a monomer and has Mr approximately equal to 88,000. The pH optimum is 8.6 to 8.7, and the isoelectric point of the enzyme is 5.9. The enzyme is specific for L-threonine and NAD+. The Km values for L-threonine and NAD+ are 8.4 and 0.98 mM, respectively. Kinetic studies indicate that the reaction proceeds through an Ordered Bi Bi mechanism where NAD+ is added first, followed by L-threonine. Chicken liver mitochondria contain aminoacetone synthase (acetyl-CoA:glycine C-acetyltransferase (EC 2.3.1.29)). The enzyme has been partially purified. In the presence of L-threonine dehydrogenase and aminoacetone synthase, L-threonine is cleaved to glycine and acetyl-CoA via 2-amino-3-oxobutyrate. The result suggests that L-threonine dehydrogenase and aminoacetone synthase have a physiological role in L-threonine metabolism in vertebrates.