Identification and Characterization of a Biodegradative Form of Threonine Dehydratase in Senescing Tomato (Lycopersicon esculentum) Leaf.

Threonine dehydratase (TD; EC.4.2.1.16) is a key enzyme involved in the biosynthesis of isoleucine. Inhibition of TD by isoleucine regulates the flow of carbon to isoleucine. We have identified two different forms of TD in tomato (Lycopersicon esculentum) leaves. One form, present predominantly in younger leaves, is inhibited by isoleucine. The other form of TD, present primarily in older leaves, is insensitive to inhibition by isoleucine. Expression of the latter enzyme increases as the leaf ages and the highest enzyme activity is present in the old, chlorotic leaves. The specific activity of the enzyme present in older leaves is much higher than the one present in younger leaves. Both forms can use threonine and serine as substrates. Whereas TD from the older leaves had the same Km (0.25 mM) for both substrates, the enzyme from the young leaves preferred threonine (Km = 0.25 mM) over serine (Km = 1.7 mM). The molecular masses of TD from the young and the old leaves were 370,000 and 200,000 D, respectively. High levels of the isoleucine-insensitive form of threonine dehydratase in the older leaves suggests an important role of threonine dehydratase in nitrogen remobilization in senescing leaves.