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Pyridoxal is an aldehyde-containing member of the Vitamin B6 family, with formula ${\mathrm{C}}_8{\mathrm{H}}_9\mathrm{N}{\mathrm{O}}_3$. Its substituted pyridine ring carries methyl, hydroxy and hydroxymethyl groups as well as the aldehyde group. It differs chemically from the phosphorylated coenzyme pyridoxal phosphate.^[ref]

The distinction can be located in the structure. Formation of pyridoxal phosphate modifies the primary alcohol group through a phosphate ester; it does not mean that the two names are interchangeable descriptions of one unchanged compound.^[ref] The broader Vitamin B6 family also includes other chemical forms.^[ref]

A simple assay example illustrates the implication. Suppose one sample contains a measured amount of free pyridoxal and another the same molar amount of pyridoxal phosphate. The molecular counts can match while charge, binding and catalytic availability differ. Interpreting a coenzyme-dependent reaction requires identifying the actual form supplied and any conversion steps, rather than inferring functional equivalence from the shared vitamin-family name.

Pyridoxal phosphate is the 5′-phosphate derivative of pyridoxal, a Vitamin B6 form whose ring carries a reactive aldehyde group. It serves as a coenzyme in amino acid chemistry. The phosphate-bearing substituent and the aldehyde have different roles: identifying one does not explain the reactivity of the other.^[ref]

An example is 1-aminocyclopropane-1-carboxylate synthase. In a 1990 experiment, chemical reduction with tritium-labelled reagent trapped the coenzyme attached to the enzyme. Subsequent protein digestion and sequencing localized the labelled active-site segment; acid hydrolysis released a modified lysine bearing the pyridoxyl group. This provided chemical evidence for the linkage, beyond observing that adding pyridoxal phosphate improves activity.^[ref]