Pyridoxine is the alcohol form of Vitamin B6, with molecular formula . Its pyridine ring has a methyl group at position 2, a hydroxyl group at position 3 and hydroxymethyl groups at positions 4 and 5.[ref]
At position 4, pyridoxine differs from the aldehyde pyridoxal and the amine pyridoxamine. Replacing by changes the formula by two hydrogen atoms; replacing it by changes both the oxygen and nitrogen counts. This provides a concrete structural meaning for the three names, beyond their membership of the same vitamin family.
The distinction also matters for biochemistry. Pyridoxal phosphate and pyridoxamine phosphate are active coenzyme forms; free pyridoxine is not simply another name for either phosphorylated molecule.[ref]
Vitamin B6 Fact Sheet for Health Professionals by the Office of Dietary Supplements. Introduction, chemical forms and active coenzymes
For example, an experiment comparing equal molar amounts of two B6 forms holds the number of molecules constant, while their functional groups and ability to enter a particular reaction can still differ. Equal mass would additionally fail to hold molecular amount constant because the forms have different molar mass values. New to topics? Read the docs here!