Kuwahara Y, Tamura M, Yamazaki I
J Biol Chem. 1982 Oct 10;257(19):11517-22.
Mg-substituted horseradish peroxidases were oxidized by K2IrCl6 or K3Fe(CN)6 to their porphyrin radical form and the 1:1 stoichiometric relationship was confirmed by spectrophotometric, fluorophotometric, and ESR titration methods. The values of E'0 for oxidations of Mg peroxidases A and C were both 0.63 V at pH 6 and depended on pH in the same way as postulated for the Compound I/Compound II couples of the corresponding enzymes. Unlike Zn peroxidase C, Mg peroxidase C was not directly oxidized by H2O2. The oxidation was catalyzed by the native peroxidase. Mg peroxidase C was photooxidized to the radical form faster than Zn peroxidase C, but its oxidation was accomplished by irreversible changes in the porphyrin in the early stage of reaction. The rate of reduction of the oxidized Mg peroxidases in the presence of various electron donors was measured at varying pH values and compared with the rate of Compound I reduction. A role of porphyrin as a site of electron transfer in the peroxidase catalysis was suggested.
镁取代的辣根过氧化物酶被六氯铱酸钾（K2IrCl6）或铁氰化钾（K3Fe(CN)6）氧化为其卟啉自由基形式，并且通过分光光度法、荧光光度法和电子自旋共振滴定法证实了1:1的化学计量关系。在pH 6时，镁过氧化物酶A和C氧化的E'0值均为0.63 V，并且其与相应酶的化合物I/化合物II对所假设的方式一样依赖于pH。与锌过氧化物酶C不同，镁过氧化物酶C不会被过氧化氢直接氧化。该氧化反应由天然过氧化物酶催化。镁过氧化物酶C比锌过氧化物酶C更快地光氧化为自由基形式，但其氧化是通过反应早期卟啉的不可逆变化完成的。在不同pH值下测量了各种电子供体存在时氧化的镁过氧化物酶的还原速率，并与化合物I的还原速率进行了比较。提出了卟啉在过氧化物酶催化中作为电子转移位点的作用。
