Properties of Rat-Liver L-Threonine Deaminase

We have studied several properties of rat liver L-threonine deaminase: (1) the affinity for the two substrates, L-serine and L-threonine; (2) the threonine/serine activity ratio which changes with increasing pH; (3) the activation, by pyridoxal 5'-phosphate which is linked to the nonprotonated form of the coenzyme and to at least an -SH group of the enzyme, and (4) the reactivation by pyridoxal 5'-phosphate and pyridoxamine 5'-phosphate after dissociation of the coenzyme. The mechanism of the reactivation by pyridoxamine 5'-phosphate is the most interesting problem opened by the present research.