Pyridoxal 5’-Phosphate: Introduction , Uses and Diagnosis

Introduction of Pyridoxal 5'-Phosphate

Pyridoxal 5'-Phosphate(PLP) is the metabolically active form of vitamin B6, which is also a cofactor used as a coenzyme in the synthesis of amino acids, neurotransmitters (serotonin, norepinephrine), sphingolipids, and aminolevulinic acid. During amino acid transamination, pyridoxal phosphate is transiently converted to pyridoxamine phosphate (pyridoxamine). It has been used to study the pyridoxal 5′-phosphate-dependent active site of these enzymes. It can be used in neonatal antiepileptic drug therapy.

Pyridoxal 5'-phosphate is an organic cofactor employed by ~ 4% of enzymes. The structure of the PLP cofactor allows for the stabilization of carbanions through resonance. A small number of PLP-dependent enzymes employ molecular oxygen as a cosubstrate. The structure of PLP dictates several features about how it is used in enzymes. For example, its aldehyde functional group allows it to form a Schiff base with lysine in the active site of PLP-dependent enzymes, giving the internal aldimine, which is the shared feature of most PLP-dependent enzymes and the starting point for catalysis . Furthermore, the pyridine ring enables PLP to stabilize carbanions through resonance. Using PLP, enzymes catalyze diverse reactions, ranging from transaminations to Claisen condensations; α-, β-, and γ-eliminations and replacements to transaldolations. The type of reaction catalyzed is dependent on the stereochemistry of the external aldimine intermediate and on which of the three a-carbon groups is aligned perpendicular with the conjugated ring system, as is described in Dunathan's hypothesis.

Uses of Pyridoxal 5'-Phosphate

PLP-dependent seizures are caused by a deficiency of pyridoxine 5ʹ-phosphate oxidase (PNPO) encoded by the PNPO gene. Similar to PDE, PNPO deficiency should be suspected in neonates with intractable seizures that do not respond to AED therapy, and empirical PLP therapy should be attempted. Because PLP is not as readily available as pyridoxine, in practice many neonates have completed a pyridoxine trial before starting a PLP trial. However, if PLP is available, it may be used as empirical treatment for PDE and PLP-dependent seizures.

Diagnosis of Pyridoxal 5'-Phosphate

The diagnosis of PLP is confirmed by serum, urine and cerebrospinal fluid metabolic tests. Elevated cerebrospinal fluid levodopa, 3-methoxytyrosine, threonine, and glycine, decreased cerebrospinal fluid homovanillic acid and 5-hydroxyindoleacetic acid, elevated urinary vanillic acid, and elevated plasma threonine and glycine levels are characteristic of PNPO deficiency. This can be tested more directly by reduced PLP levels in CSF or PNPO gene sequencing.

References:

[1] ELESHA R HOFFARTH; Katherine S R; Kristina W Rothchild. Emergence of oxygen- and pyridoxal phosphate-dependent reactions.[J]. FEBS Journal, 2020, 287 7: 1403-1428. DOI:10.1111/febs.15277.

[2] JOLITA CIAPAITE. Pyridox(am)ine 5′-phosphate oxidase (PNPO) deficiency in zebrafish results in fatal seizures and metabolic aberrations[J]. Biochimica et biophysica acta. Molecular basis of disease, 2020, 1866 3: Article 165607. DOI:10.1016/j.bbadis.2019.165607.