Fluorescence Changes Induced by Binding of 4-Pyridoxic Acid 5′-Phosphate to Proteins

Quality and stability of extemporaneous pyridoxal phosphate preparations used in the treatment of paediatric epilepsy

Objectives

To assess the pyridoxal 5′-phosphate (PLP) content and stability of extemporaneous PLP liquids prepared from dietary supplements used for the treatment of vitamin B6-dependent epilepsy.

Methods

Pyridoxal 5′-phosphate liquids were prepared in accordance with the guidelines given to patients from marketed 50 mg PLP dietary capsules and tablets. The PLP content and its stability were evaluated under conditions resembling the clinical setting using reverse phase HPLC and mass spectrometry.

Key findings

Pyridoxal 5′-phosphate content in most of the extemporaneously prepared liquids from dietary supplements was found to be different from the expected amount (~16–60 mg). Most of these PLP extemporaneous liquids were stable at room temperature (protected from light) after 24 h but unstable after 4 h when exposed to light. A key photodegradation product of PLP in water was confirmed as 4-pyridoxic acid 5′-phosphate (PAP).

Conclusion

Pyridoxal 5′-phosphate tablets from Solgar® were found to be the most reliable product for the preparation of extemporaneous PLP liquids. This work highlighted the difference between the marketed PLP dietary supplements quality and the importance of proper storage of aqueous PLP. There is a need to develop pharmaceutical forms of PLP that ensure dose accuracy and avoid potentially unsafe impurities with the aim of enhancing safety and compliance.

Photophysical study of the Schiff bases of 5'-deoxypyridoxal and n-hexylamine in cationic micelles

The absorption and fluorescence spectra of the Schiff bases formed between 5'-deoxypyridoxal and n-hexylamine in aqueous media containing different concentrations of the cationic surfactant hexadecyltrimethylammonium bromide were recorded at 25 degrees C. The quantum yields of fluorescence of the different zwitterionic and enol forms of the chemical species of the Schiff bases occurring in media of pH 4.5-8.5 were determined. Also, the fluorescence quenching resulting from the presence of the surfactant and that of iodide ion were analyzed. From the results obtained it follows that the zwitterionic forms do not interact with the cationic surfactant, whereas the enol forms do interact with it.

Spectroscopic properties of the photoproducts of pyridoxal-5'-P irradiation: Catalytic site recognition of ribonuclease A

Photoproducts of pyridoxal-5'-P, i.e., 4-pyridoxic-5'-P and bis-pyridoxal-5'-P, have been studied by spectroscopic methods. The spectroscopic properties of bis-pyridoxal-5'-P (bis-PLP) resemble those of pyridoxal-5'-P (PLP) under similar experimental conditions. The coupling of methylen hydrogens to the phosphorus atom has been shown by NMR spectroscopy. The singlet in the(31)P-NMR spectra and the triplet in(1)H-undecoupled experiments confirm the presence of the phosphate group in the 5' position of the structure of the vitamin. The effect of pH and solvent composition on the relative distribution of species of bis-pyridoxine-5'-P (bis-PNP) has been investigated by absorption and fluorescence spectroscopy. The acid-base dissociation of the phosphate group is easily detected by emission spectroscopy. Bis-PNP and bis-PLP bind to the enzyme RNase A and they behave as competitive inhibitors with respect to the substrate cytidine-2'-3'-cyclic phosphate. The natural forms of vitamin B6, pyridoxine, and pyridoxine-5'-P have no effect on the catalytic activity of the protein. Experimental evidence derived from fluorescence and inhibition experiments is consistent with the hypothesis that bis-PNP recognizes the catalytic site of RNase A.

Luminescence spectroscopy of pyridoxic acid and pyridoxic acid bound to proteins

Luminescence techniques, i.e. fluorescence and phosphorescence, have been employed to study pyridoxic acid bound to proteins through a stable amide linkage. Proteins tagged with 4-pyridoxic acid display the following fluorescence properties: (a) emission and excitation spectra centered at around 430 and 320 nm, respectively; (b) fluorescence quantum yields of 0.3-0.4 and (c) average decay times covering the range 8-9.6 ns. The fluorescence properties of the probe have been used to study the dynamics of the protein in the nanosecond time scale. In the absence of molecular oxygen, free and bound 4-pyridoxic acid exhibit long-lived emission at room temperature. The long-lived emission is red-shifted when compared to fluorescence and decays with average life times ranging over 2.2-0.6 ms depending on the nature of the protein. The fluorophore pyridoxic acid covalently linked to proteins is suitable to study the dynamics of proteins, i.e. fast and slow motions of the macromolecule in the nanosecond and millisecond time scales, respectively.

Buffers for the reconstitution of aspartate aminotransferase

The cofactor activation of the apoenzyme of pig heart cytosolic aspartate aminotransferase was studied in various buffers. Cationic buffers are shown to allow maximal reconstitution in the pH range of 5.0 to 9.0. Anionic buffers made up of mono- and dicarboxylates are found to affect reconstitution in a pH-dependent manner. At low pH, the carboxylates strongly inhibit reconstitution, but at high pH, they show less effect. In contrast, the more potent inhibitor Pi shows the opposite pH profile. Dicarboxylates are considerably more inhibitory than monocarboxylates. Substantial protection against inhibition by a number of carboxylates may be achieved by the addition of sodium chloride.

The interaction of pyridoxal phosphate with aspartate apoaminotransferase

The rate of biniding of pyridoxal phosphate to the apoenzyme of pig heart cytoplasmic aspartate aminotransferase (L-aspartate: 2-oxoglutarate aminotransferase, EC 2.6.1.1) was measured by adsorption spectroscopy and by formation of active enzyme. At pH 5.1 and 8.3 the binding of coenzyme follows saturation kinetics. The binding process thus involves at least two steps. The rate of pyridoxal phosphate binding to the apoenzyme is dependent on the anion present in the pH 8.3 triethanolamine buffer. Chloride activates somewhat at very low concentrations. Phosphate and its methyl, ethyl, and phenyl esters are very effective inhibitors of the recombination in that 0.2--0.4 mM inhibit the rate of coenzyme binding by 50%. This is below the physiological concentration of phosphate. Sulfate also inhibits the rate of binding, but nitrate and acetate have little effect.