Threonine dehydratase degradative enzyme

Tryptophan administration to rats results in the stabilization of a number of enzymes for which tryptophan is not a substrate. These include tyrosine aminotransferase (Cihak et al., 1973), phosphoenolpy-ruvate carboxykinase (Ballard and Hopgood, 1973), threonine dehydratase (Peraino et al., 1965), and ornithine aminotransferase (Chee and Swick, 1976). With these enzymes, the decreased lability cannot be shown in vitro, so it is doubtful whether tryptophan itself is the active compound. Perhaps a tryptophan metabolite either binds to the enzymes or affects the dissociation of substrates or cofactors. This latter possibility could be important in the case of the aminotransferases, which have pyridoxal phosphate cofactors, since Litwack and Rosen-field (1973) demonstrated a correlation between the degradation rate constants of a number of enzymes and the rate of coenz)nne dissociation from the molecules. 

The initial reaction associated with isoleucine biosynthesis is catalyzed by threonine dehydratase (18). In microorganisms and a few plants, both degra-dative and biosynthetic enzymes have been identified. The former are not regulated by amino acids, but some are activated by AMP or ADP. An apparently unregulated threonine dehydratase has been purified from seeds of Cus-cuta campestris (Madan and Nath, 1983). The lack of identifiable regulatory phenomena precludes identification of the function of this enzyme as either biosynthetic or degradative in nature. 

---