Affinity labeling of the pyridoxal phosphate binding site of the beta2 subunit of Escherichia coli tryptophan synthase

We have synthesized bromoacetylpyridoxamine phosphate and bromoacetylpyridoxamine and have shown that they meet three criteria for affinity labels of the beta2 subunit of tryptophan synthase: (i) the kinetic data of inactivation indicate that a binary complex is formed prior to covalent attachment; (ii) inactivation is largely prevented by the presence of pyridoxal phosphate; and (iii) inactivation is stoichiometric with incorporation of 0.7 to 0.8 mol of chromophore/mol of beta monomer. Our conclusion that inactivation of the apo beta2 subunit by bromoacetylpyridoxamine phosphate is due to the modification of cysteine is based on the disappearance of 1 mol of -SH/beta monomer and on the finding that [14C]carboxymethyl derivative in the acid hydrolysate of the protein modified by bromo[14C]acetylpyridixamine phosphate. A 39-residue tryptic peptide containing this essential cysteine has been isolated and purified from the bromo[14C]acetylpyridoxamine phosphate-labeled beta2 subunit.

Mode of binding of pyridoxal 5'-phosphate in rat liver ornithine aminotransferase

1. Pyridoxal 5'-phosphate and pyridoxamine 5'-phosphate were effective for the association of apo-form II of ornithine aminotransferase [EC 2.6.1.13]; whereas other B6 derivatives, such as pyridoxal, pyridoxamine, pyridoxine and pyridoxine 5'-phosphate, had no effect on form II of this apoenzyme. 2. The pyridoxal 5'-phosphate contents of the native enzyme, and reconstituted forms I and II were determined by two different methods to be 1.5 moles, 2.5 moles and 3.3 moles of pyridoxal 5'-phosphate/mole of enzyme, respectively.

Uptake and utilization of vitamin B6 and its phosphate esters by Escherichia coli

Escherichia coli KG980, a vitamin B6 auxotroph derived from E. coli K12, utilized the three unphosphorylated forms of vitamin B6, more or less effectively, for growth, but did not utilize the three phosphate forms at concentrations ranging from 10(-7) to 10(-5) M in the minimum medium of Davis and Mingioli. The bacterium, however, utilized the phosphate forms, although less effectively than the unphosphorylated forms, in the phosphate starving medium of Garen and Levinthal which is known to derepress alkaline phosphatase. Correspondingly, the phosphate forms of 3H-labeled vitamin B6 in the minimum medium were not taken up by the bacterial cells grown in the same medium, but were taken up when the phosphate starving medium was used for growth and uptake experiments; the unphosphorylated forms were taken up with either of the media used. After 30-min incubation of the cells grown in the phosphate starving medium with 3H-pyridoxine phosphate added to the same medium, the main extracellular 3H-vitamin B6 was found to be pyridoxine, evidently indicating an involvement of alkaline phosphatase action. It is concluded from these results that the phosphate forms of vitamin B6 can be taken up and utilized only after dephosphorylation but not taken up in their intact form. The initial rate of 3H-pyridoxal and 3H-pyridoxamine uptake in the minimum medium showed saturation kinetics. The Km and Vmax values were 1.2 X 10(-6) M and 62 pmoles/min/mg (dry cell weight) for pyridoxal; 11 X 10(-6) M and 65 pmoles/min/mg for pyridoxamine. 3H-Pyridoxine uptake apparently consisted of a high-affinity saturable component, whose Km value was tentatively estimated to be 2.2 X 10(-6) M, and an unsaturable component. The uptake rates of these three unphosphorylated vitamin B6 compounds compared at limiting concentrations for the growth of E. coli KG980 appear to be essentially in good agreement with the response pattern of this bacterium to the three compounds.

[The interaction of pyridoxamine-5'-thiophosphate with aspartate apo-transaminase]

Interaction of 5'-bromopyridoxamine with thiophosphate yeld 5'-phosphate thio ester of pyridoxamine (I). Apotransaminase and I reacted in equimolar amounts; a stable complex with 10--15 per cent of activity formed exhibiting a positive CD at 335 nm. Incubation of the complex with alpha-ketoglutarate led to formation of corresponding aldimine form and to partial splitting of the S--P bond. The same splitting take place during incubation of the I--apo-enzyme complex with substrate mixture.

31P nuclear-magnetic-resonance studies of pyridoxal and pyridoxamine phosphates. Interaction with cytoplasmic aspartate transaminase

The 31P nuclear magnetic resonance (NMR) spectrum of the phosphate in free pyridoxal or pyridoxamine phosphate reveals a resonance signal that is coupled to the methylene protons of the 5-CH2 with JHP of 6.0 +/- 0.3 Hz. Proton noise decoupling results in a single signal with a pH-dependent chemical shift with deprotonation of the phosphate resulting in a shift of the 31P resonance to lower fields. A single 31P NMR signal at a frequency corresponding to fully ionized phosphate monoesters is observed in aspartate-transaminase-bound pyridoxal or pyridoxamine phosphate. The 31P resonance in the holotransaminase is pH-independent and is unaffected by saturating concentrations of substrates or inhibitors. Only denaturation with 6 M guanidine with HCl results in changes in the 31P of the holoenzyme. It appears that the phosphate group of pyridoxal phosphate is bound to a positive pocket in the holoenzyme and remains fully ionized in the pH range of 5.6 to 9.2. The phosphate-binding properties are present even in the apoenzyme which is able to bind inorganic phosphate which then can be displaced by pyridoxal or pyridoxamine phosphate in the process of holoenzyme formation.