Occurs naturally combined in many glycosides. Constit. of vanillin. Isol. in free state from opium poppy (Papaver somniferum) 4-Hydroxybenzaldehyde reacts with NAD+ and H2O to produce 4-hydroxybenzoate, NADH, and 2 protons. The reaction is catalyzed by 4-hydroxybenzaldehyde dehydrogenase.; ; 4-Hydroxybenzaldehyde is found in the benzoate degradation via hydroxylation, bisphenol A degradation, toluene and xylene degradation, and biosynthesis of phenylpropanoids pathways.