Imidazolylacetolphosphate Aminotransferase

Abstract The biosynthesis of the pyridoxal phosphate-dependent enzyme imidazolylacetolphosphate aminotransferase (EC 2.6.1.9) was investigated in a pyridoxine auxotroph of Salmonella typhimurium grown under conditions in which the intracellular pool size of pyridoxal phosphate would be expected to be low. In the absence of cofactor the apoprotein is synthesized at a normal rate and is stable in vivo. The protein assumes a tertiary and quaternary structure very similar to that of holoenzyme since apoenzyme sediments through sucrose gradients at the same rate as the holoenzyme and apoenzyme can be constituted in vitro with pyridoxal phosphate. We conclude that there is no stoichiometric cofactor involvement during the biosynthesis of this protein. The cofactor does play a positive role in maintaining the structural integrity of the protein since apoprotein loses its ability to be constituted after short incubations at 45° and after chromatography through Sephadex G-200 gels. Binding of cofactor to apoprotein protects against both perturbations.