Inactivation of red beet β-glucan synthase by native and oxidized phenolic compounds

Abstract

The effects of phenolic compounds on glucan synthase, a membrane-bound enzyme from red beet root, were examined. Different classes of phenolic compounds were screened in the absence and presence of polyphenoloxidase (PPO). At levels less than 1 mM, inactivation occurred with many of the compounds tested. However, in most cases oxidation by PPO was required. Coumarin, previously demonstrated to interfere with cell wall polysaccharide biosynthesis, was not inhibitory. Mechanistic studies utilizing catechol showed that phenolic inactivation could be protected against by PPO inhibitors and thiol protective reagents. However, once inactivation occurred, it could not be reversed. Ommission of thiols and polyvinylpyrolidone from homogenization buffers did not reduce glucan synthase levels of microsomal preparations. It appears that glucan synthase, a membrane-bound enzyme, is as susceptible to phenolic effects as cytosolic enzymes and in situ inactivation is a function of the availability of both endogenous phenolics and PPO.