The effect of polymerization of horseradish peroxidase on the peroxidase activity in the presence of excess H2O2: a background for a homogeneous enzyme immunoassay.

The phenol oxidation catalyzed by horseradish peroxidase (HRP) is slowed down by the presence of excess H2O2. This inhibition is due to accumulation of Compound III, which is a catalytically sluggish form of HRP. When HRP is polymerized through covalent bonds, Compound III becomes unstable and the peroxidase activity is less sensitive to excess H2O2. Under suitable experimental conditions, the phenol oxidation is increased by about 20-fold upon polymerization of the enzyme. This fact represents the principle of a homogeneous enzyme immunoassay reported by Hoshino et al. (J. Biochem. 97, 113-118 (1985)). The ratio of the peroxidase activities of monomeric and polymerized HRPs is 1 : 4 when phenol is replaced by resorcinol, and the difference is no larger when guaiacol and catechol are used as electron donors.