DPNH peroxidase: effector activities of DPN.

At suboptimal H202 concentrations, DPNH inhibits the peroxidase activity of the flavoprotein, DPNH peroxidase, by converting the enzyme to an unstable intermediate that decays slowly to inactive enzyme. It is postulated that at concentrations of DPNH that are saturating for peroxidase activity, this unstable intermediate is responsible for most of the DPNH oxidation that is supported by alternate electron acceptors, such as 02 and menadione. DPN+ behaves as an activator by reversing the equilibria that lead to the unstable intermediate, thus converting the enzyme to the kinetically active complex that reduces H202. The data show that DPN+ binding will stimulate the peroxidase activity (by lowering the Km for H202) and simultaneously lead to strong inhibition of both the rate of enzyme inactivation and the rate at which DPNH is oxidized by alternate electron acceptors. INTRODUCTION DPNH peroxidase of Streptococcus faecaiis (1) is a dimer (2) containing 2 moles of FAD and one mole of a non-flavin electron acceptor per mole of enzyme (3). Evidence that the non-flavin acceptor is a disulfide group has been presented (3). Neutron activation analysis has shown that the enzyme does not contain selenium (2). In contrast, glutathione peroxidase, which has no heme or flavin prosthetic groups, is reported to contain one atom of Se per subunit (4). Previous work with DPNH peroxidase (3 ) suggested that the site at which H202 is reduced consists of a complex between the reduced non-flavin acceptor and enzyme bound FAD and DPN+. The present paper deals with the role of DPN+ as an activator of DPNH peroxidase activity and an inhibitor of several competing reactions. It is postulated that these multiple effects are brought