Purification and properties of rat liver L-threonine deaminase

Rat liver threonine deaminase has been partially purified. The enzyme deaminates L-threonine and L-serine, is not affected by isoleucine, nor by AMP and ADP. L-cysteine and analogues are inhibitors of threonine deaminase and it is very likely that the inhibition is due to the formation of a thiazolidine ring with PLP bound to the enzyme. However, the simple formation of this ring does seem to explain completely the different degree and type of inhibition shown by L-cysteine and analogues. The hypothesis that the different behaviour of L- and D-cysteine is due also to interactions independent of the formation of thiazolidinic ring is discussed.