PLANT PHYSIOLOGY , Vol 101, Issue 3 999-1004, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Identification and Characterization of a Biodegradative Form of Threonine Dehydratase in Senescing Tomato (Lycopersicon esculentum) Leaf
I. Szamosi, D. L. Shaner and B. K. Singh
American Cyanamid Company, P.O. Box 400, Princeton, New Jersey 08543-0400
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.
