Pyridoxamine

Pyridoxamine is an amine form of Vitamin B6. Its substituted pyridine ring carries a methyl group at position 2, a hydroxyl group at position 3, an aminomethyl group at position 4 and a hydroxymethyl group at position 5. Its molecular formula is ${\mathrm{C}}_8{\mathrm{H}}_{12}{\mathrm{N}}_2{\mathrm{O}}_2$.^[ref]

The position-4 group makes the B6 family a useful example of how a small structural change distinguishes related molecules. Pyridoxine has $-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ there, pyridoxal has $-\mathrm{CHO}$, and pyridoxamine has $-\mathrm{C}{\mathrm{H}}_2\mathrm{N}{\mathrm{H}}_2$. These are respectively alcohol, aldehyde and amine forms, rather than three interchangeable names for one structure.^[ref]

Adding a phosphate at the position-5 hydroxymethyl group produces pyridoxamine phosphate, an active coenzyme form. Pyridoxal phosphate is the other principal active B6 coenzyme form. The free vitamin and its phosphorylated derivative must therefore be distinguished when describing an enzyme assay: the name of the vitamin family alone does not identify the molecular species supplied to the enzyme.^[ref]

Pyridoxamine phosphate usually denotes pyridoxamine 5'-phosphate, the phosphate ester of the position-5 hydroxymethyl group of pyridoxamine. Its neutral-form molecular formula is ${\mathrm{C}}_8{\mathrm{H}}_{13}{\mathrm{N}}_2{\mathrm{O}}_5\mathrm{P}$. It is an active Vitamin B6 coenzyme form, chemically distinct from both free pyridoxamine and pyridoxal phosphate.^[ref][ref]

A concrete reaction distinguishes the two phosphorylated forms. Pyridoxal 5'-phosphate synthase can oxidatively convert pyridoxamine phosphate into pyridoxal phosphate, producing Ammonia and hydrogen peroxide while consuming water and oxygen.^[ref] The transformation changes the aminomethyl group into an aldehyde group while retaining the phosphate ester.

For an ideal $1\mu \mathrm{mol}$ conversion by this reaction, one micromole each of Ammonia and hydrogen peroxide is produced. A peroxide assay can consequently report a coupled reaction signal, but identifying the B6 product remains a separate analytical question. A peroxide-producing impurity could mimic that signal without making the intended coenzyme.