4-Pyridoxic acid is an inactive metabolite of pyridoxine .1 It is formed from pyridoxine via pyridoxal 5'-phosphate and pyridoxal intermediates by pyridoxine kinase or pyridoxamine phosphate oxidase, as well as pyridoxine-5'-phosphate oxidase, aldehyde dehydrogenase, or aldehyde oxidase.2
1.Sharma, S.K., and Dakshinamurti, K.Determination of vitamin B6 vitamers and pyridoxic acid in biological samplesJ. Chromatogr.578(1)45-51(1992) 2.Kohlmeier, M.Water-soluble vitamins and nonnutrientsNutrient metabolism567-671(2015)