Description
Pyridoxal phosphate is an organic compound formed by the combination of vitamin B6 and phosphoric acid, including pyridoxal, pyridoxamine, and pyridoxine, which exist in the form of phosphate esters in the body. Among them, pyridoxal phosphate and pyridoxamine phosphate can be converted into each other, and they are all active types. Pyridoxal phosphate is the active form of vitamin B6. Several reactions catalyzed by pyridoxal phosphate are: transamination, α-decarboxylation, β-decarboxylation, β-elimination, γ-elimination, racemization and aldol reaction.
Chemical Properties
Pyridoxal phosphate is a white, odourless crystalline powder that turns bright yellow in alkaline solutions. Its aqueous solution remains stable at low temperatures and in the absence of light but decomposes gradually when exposed to light. It is soluble in formic acid, aqueous pyridine, and dilute alkaline solutions, with limited solubility in organic solvents such as water, ethanol, acetone, chloroform, and ether. The oral LD50 in rats is 5900 mg/kg.
Uses
Vitamin (enzyme co-factor).
Definition
Pyridoxal Phosphate also known as PLP, pyridoxal 5'-phosphate or P5P, is the active form of vitamin B6 and a coenzyme for many pyridoxal phosphate (PLP)-dependent enzymes. PLP is involved in numerous enzymatic transamination, decarboxylation and deamination reactions; it is necessary for the synthesis of amino acids and amino acid metabolites, and for the synthesis and/or catabolism of certain neurotransmitters, including the conversion of glutamate into gamma-aminobutyric acid (GABA) and levodopa into dopamine. PLP can be used as a dietary supplement in cases of vitamin B6 deficiency. Reduced levels of PLP in the brain can cause neurological dysfunction.
Preparation
Synthesis of Pyridoxal Phosphate: Pyridoxal phosphate is synthesized from pyridoxine. Initially, pyridoxamine dihydrochloride is reacted with anhydrous phosphoric acid to produce pyridoxamine phosphate. This intermediate is then oxidized with manganese dioxide to yield pyridoxal phosphate.
Biological Functions
Pyridoxal phosphate is the coenzyme form of vitamin B6. In the catalytic reaction, the intramolecular aldehyde group of pyridoxal phosphate is combined with the amino group of α-amino acid to form an aldimine, also known as Schiffbase, and then the amino acid undergoes transamination, decarboxylation or racemization according to the characteristics of different enzymes and proteins. . As a coenzyme of amino acid transaminase, decarboxylase and racemase, it plays a very important role in amino acid metabolism.
Synthesis Reference(s)
Journal of the American Chemical Society, 73, p. 4693, 1951
DOI: 10.1021/ja01154a062
Mechanism of action
Pyridoxal phosphate(PLP) acts as a coenzyme in all transamination reactions, and in certain decarboxylation, deamination, and racemization reactions of amino acids. The aldehyde group of PLP forms a Schiff-base linkage (internal aldimine) with the ε-amino group of a specific lysine group of the aminotransferase enzyme. The α-amino group of the amino acid substrate displaces the ε-amino group of the active-site lysine residue in a process known as transaldimination. The resulting external aldimine can lose a proton, carbon dioxide, or an amino acid sidechain to become a quinoid intermediate, which in turn can act as a nucleophile in several reaction pathways.
Purification Methods
PLP has been purified by dissolving 2g in H2O (10-15mL, in a dialysis bag a third full) and dialysing with gentle stirring against 1L of H2O (+ two drops of toluene) for 15hours in a cold room. The dialysate is evaporated to 80-100mL, then lyophilised. Lemon yellow microscopic needles of the monohydrate remain when all the ice crystals have been removed. The purity is checked by paper chromatography (in EtOH or n-PrOH/NH3) and the spot(s) visualised under UV light after reaction with a spray of p-phenylene diamine, NH3 and molybdate. Solutions stored in a freezer are 2-3% hydrolysed in 3weeks. At 25o, only 4-6% hydrolysis occurs even in N NaOH or HCl, and 2% is hydrolysed at 37o in 1day-but is complete at 100o in 4hours. It is best stored as a dry solid at -20o. In aqueous acid the solution is colourless but is yellow in alkaline solutions. It has UV max at 305nm ( 1100) and 380nm ( 6550) in 0.1 N NaOH; 330nm ( 2450) and 388nm ( 4,900) in 0.05M phosphate buffer pH 7.0 and 295nm ( 6700) in 0.1N HCl. [Peterson et al. Biochemical Preparations 3 34, 119 1953.] The oxime decomposes at 229-230o and is practically insoluble in H2O, EtOH and Et2O. The O-methyloxime decomposes at 212-213o. [Heyl et al. J Am Chem Soc 73 3430 1951.] It has also been purified by column chromatography through Amberlite IRC-50 (H+) [Peterson & Sober J Am Chem Soc 76 169 1954]. [Beilstein 21/13 V 46.]
