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Pyridoxal 5'-phosphate synthase, EC 1.4.3.5, is an enzyme that oxidizes pyridoxamine phosphate or pyridoxine phosphate to pyridoxal phosphate. Despite the accepted synthase name, these reactions start with an already phosphorylated vitamin. The enzyme uses flavin mononucleotide as a cofactor.^[ref]

Both reactions consume oxygen and produce hydrogen peroxide. The pyridoxamine-phosphate reaction additionally consumes water and releases Ammonia, because its aminomethyl group becomes an aldehyde. The pyridoxine-phosphate reaction changes a hydroxymethyl group into an aldehyde without releasing that nitrogen-containing coproduct.^[ref]

This difference supplies an analytical example. If an ideal mixture undergoes complete conversion of $2\mu \mathrm{mol}$ of each substrate, it produces $4\mu \mathrm{mol}$ pyridoxal phosphate and $4\mu \mathrm{mol}$ peroxide, but only $2\mu \mathrm{mol}$ ammonia. A peroxide-only readout combines contributions from both substrates; the coproduct balance contains additional information.

Those values assume only the stated coupled reactions. In an actual enzyme assay, other peroxide- or ammonia-producing activities can interfere, so chemical product identification and appropriate controls remain important. Enzyme identity, substrate identity and the chosen reporter reaction each determine what the signal means.

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.

Pyridoxine phosphate is the 5′-phosphate derivative of pyridoxine, one of the forms of Vitamin B6. Attaching a phosphate group and changing an alcohol group into an aldehyde are different chemical modifications. Pyridoxine phosphate still differs from pyridoxal phosphate, the form used as a coenzyme in many reactions.^[ref]

An important connection is the oxidation of pyridoxine phosphate by pyridoxal 5'-phosphate synthase. The official reaction isHere oxygen accepts the reducing equivalents, producing hydrogen peroxide; the phosphate group is retained. The enzyme contains flavin mononucleotide.^[ref]

For a constructed material-balance example, complete conversion of 4 micromoles of pyridoxine phosphate through this reaction consumes 4 micromoles of oxygen and forms 4 micromoles each of pyridoxal phosphate and hydrogen peroxide. There is no net release of free phosphate in that reaction. Measuring only total phosphorus would therefore miss the conversion: the substrate and product each retain one phosphorus atom. A measurement that distinguishes their functional groups can answer a question that elemental accounting cannot.