In vitro regulation of rat liver L-threonine deaminase by different effectors

The regulation of liver L-threonine deaminase by different effectors--bile acids, bile pigments and monocarbon molecules--was investigated. Total inhibition of the enzyme was observed with physiological concentrations of bile acids and biliverdin. Purely competitive inhibition of the holoenzyme by several monocarbon molecules was demonstrated; the mechanism was partially competitive for bicarbonate. Inhibition was more pronounced in the case of the dialyzed enzyme. From the higher Km values for pyridoxal-5'-phosphate (PLP), obtained in the presence of the inhibitors, the results are explained on the basis of interference in the association reaction: apoprotein + PLP----holoenzyme. The various effects determined by bicarbonate may play a specific role in vivo since they occur at physiological concentrations of this compound.