If I am reading this right, P5P acts like a PPI. This would explain some of the protective effects of P5P on stress-induced gastric ulcers in animals and men.
http://www.jbc.org/content/263/8/3652.long
"...Pig gastric membrane vesicles enriched in (H++K+)-ATPase were covalently modified with pyridoxal-5'- phosphate (PLP). The modification resulted in inhibition of K+-dependent ATP hydrolysis, formation of phosphoenzyme and ATP-driven H+-uptake catalyzed by (H+ +K+)-ATPase. ATP, ADP, and adenyl-5-ylimidodiphosphate were protective ligands, whereas Mg+ and K+ were not."
http://www.jbc.org/content/263/8/3652.long
"...Pig gastric membrane vesicles enriched in (H++K+)-ATPase were covalently modified with pyridoxal-5'- phosphate (PLP). The modification resulted in inhibition of K+-dependent ATP hydrolysis, formation of phosphoenzyme and ATP-driven H+-uptake catalyzed by (H+ +K+)-ATPase. ATP, ADP, and adenyl-5-ylimidodiphosphate were protective ligands, whereas Mg+ and K+ were not."