= Vitamin B6
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<Vitamin B6> is the collective name for six related compounds with vitamin-B6 activity. They comprise <pyridoxine>, <pyridoxal> and <pyridoxamine>, together with their corresponding 5′-phosphate forms. The phosphorylated forms include <pyridoxal phosphate>, an active <coenzyme>. A family name and a particular molecular species therefore describe different levels of chemical identity.<Vitamin B6 Fact Sheet for Health Professionals by the Office of Dietary Supplements>[Introduction, six vitamers and active coenzyme forms]{ref}
An enzymatic example makes the distinction concrete. <1-aminocyclopropane-1-carboxylate synthase> requires <pyridoxal phosphate> when converting <S-adenosylmethionine> into the precursor of the plant signal <ethene>. Naming the broader <Vitamin B6> family does not establish that the required coenzyme form is present at an enzyme's active site.<Characterization and sequencing of the active site of 1-aminocyclopropane-1-carboxylate synthase by W. K. Yip and others>[Abstract, PLP-dependent catalytic activity]{ref}
For an analytical comparison, measuring total members of the family and measuring only <pyridoxal phosphate> answer different questions. Equal total chemical amounts can conceal different distributions among forms, and equal coenzyme concentration does not establish equal enzyme abundance. Separating those quantities helps interpret a biochemical assay without reducing the vitamin's role to the amount of one named precursor.
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