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. 2013 May 21;4:143. doi: 10.3389/fpls.2013.00143

FIGURE 1.

FIGURE 1

Plants as a source of vitamin B6 and its uptake by humans. Plants can biosynthesize vitamin B6 de novo. It is predominantly produced in the metabolically active parts of the plant like the leaves but can be found in all organs. The de novo pathway (upper left corner) is operating in the cytosol involving only two enzymes: a synthase, PDX1, and a glutaminase, PDX2. The abbreviations used are Glu (glutamate), Gln (glutamine), G3P (glyceraldehyde 3-phosphate), R5P (ribose 5-phosphate), PA (pyridoxic acid), PN (pyridoxine), PN-Glu (PN-glycoside), PL (pyridoxal), PM (pyridoxamine), PNP (pyridoxine 5′-phosphate), PLP (pyridoxal 5′-phosphate), PMP (pyridoxamine 5′-phosphate). The cofactor form of the vitamin, PLP, is produced directly from R5P, G3P, and Gln. A salvage pathway also operates in plants (upper right corner) through which the vitamers can be interconverted. PDX3 is a PNP/PMP oxidase (only the reaction for PNP is depicted), SOS4 is a PN/PL/PM kinase and PLR is a PL reductase. Plants can also convert PN to PN-Glu, a form that is only partially bioavailable to humans, using a glycosyl transferase. In the human body, vitamin B6 is taken up in the jejunum (the second part of the small intestine; lower panel) where the free forms PN, PL, and PM can be taken up directly. The phosphorylated vitamers are dephosphorylated prior to uptake by a membrane bound alkaline phosphatase. In both cases uptake is driven by diffusion, which necessitates the phosphorylation of the vitamers immediately after their internalization to keep a low concentration of the non-phosphorylated forms inside the cells. The PN-Glu is taken up in its glycosylated form and processed by a cytosolic β-glucosidase to release PN. Its low bioavailability is not determined by the uptake itself but by the limiting activity of a β-glucosidase. The uptake is followed by further conversion inside the cells (salvage pathway) yielding the active cofactor form PLP involved in numerous biochemical and physiological processes in the human body (right lower panel). The catabolism of the vitamin involves conversion to PA (as well as more oxidized derivatives as depicted). The latter are excreted in the urine.