The inhibition of purified spinach ribulosebisphosphate carboxylase-oxygenase which occurs progressively during catalysis in vitro is caused by accumulation of at least two tight-binding inhibitors at the catalytic site. Reduction of these inhibitors with NaB3H4, followed by dephosphorylation, produced a mixture of xylitol and arabinitol, thus identifying one of them as D-xylulose 1,5-bisphosphate. It was formed during carboxylation, presumably by a stereochemically incorrect reprotonation of the 2,3-enediolate intermediate bound at the catalytic site. Under the conditions used, this epimerization occurred approximately once for every 400 carboxylation turnovers. Another inhibitor may be 3-keto-D-arabinitol 1,5-bisphosphate which would also be formed by misprotonation of the enediolate intermediate, but at C-2 rather than at C-3.
