PABA

Para-aminobenzoic acid (pABA) is an aromatic carboxylic acid bearing an amino group in the para position. The chemical formula is C7H7NO2, and it appears as a white to off-white crystalline solid. It is sparingly soluble in water and more soluble in basic solutions. pABA has historically been discussed as a vitamin, but it is not considered an essential nutrient for humans.

In biochemistry, pABA is a key intermediate in the bacterial synthesis of folate. It serves as a precursor to dihydropteroic acid, which is formed via dihydropteroate synthase and leads to dihydrofolate and ultimately tetrahydrofolate. Humans do not synthesize folate de novo and rely on dietary folate, so pABA is not required as a vitamin for human metabolism. The bacterial folate pathway is a common target for sulfonamide antibiotics, which inhibit dihydropteroate synthase and disrupt folate production in bacteria.

Uses and regulatory status have changed over time. pABA was once marketed as vitamin Bx, but this is no longer recognized in human nutrition. It has had limited medical utility beyond research contexts. In cosmetics, pABA has been used as a UV filter in some sunscreens, but many formulations have moved away from pABA due to concerns about allergic reactions and staining of fabrics; non-pABA UV filters are now more common.

Safety considerations include the potential for allergic skin reactions in sensitive individuals and possible interactions with certain medications. As a nutrient, pABA is not required for humans, and its primary significance lies in its role in bacterial folate biosynthesis and its historical and regulatory context.