Reason for indispensability of threonine in humans and other mammals in comparative aspect

Abstract

The essential amino acid threonine is not synthesized in vertebrates, so it must be obtained from food. During evolution, the decomposition of threonine has changed. Because the decomposition of threonine catalyzed by threonine dehydratase is irreversible, in the present work attention is focused on threonine dehydrogenase to show the inability of this enzyme to synthesize threonine in a reaction that would be the reverse of the reaction of threonine decomposition. The reason why threonine dehydrogenase cannot be used for the biosynthesis of threonine in mammalian tissues is discussed. It is concluded that some quantity of threonine is involved in transamination.