Although belonging to the widely investigated peroxidase superfamily, lactoperoxidase (LPO) and myeloperoxidase (MPO) share structural and functional features that make them peculiar with respect to other enzymes of the same group. A survey of the available literature on their catalytic intermediates enabled us to ask some questions that remained unanswered. These questions concern controver-sial features of theLPOandMPOcatalytic cycle, suchas the existence of Compound I and Compound II isomers and the identification of their spectroscopic properties
