Nakajima R, Yamazaki I
J Biol Chem. 1979 Feb 10;254(3):872-8.
The oxygen-consuming oxidation of indole-3-acetic acid (IAA) occurred much faster in the presence of horseradish peroxidase C (neutral isoenzyme) than in the presence of horseradish peroxidase A (acidic isoenzyme). An intermediate oxidation product of IAA was found to be a hydroperoxide species that reacted with the ferric enzymes to form Compound I at second order rate constants of 6.8 X 10(3) M-1--S-1 for peroxidase A and 2.0 X 10(6) M-1--S-1 for peroxidase C at pH 4.4 The hydroperoxide concentration reached about one-half of the initial IAA concentration at the end of the oxygen-consuming reaction and then decreased slowly. The main intermediate of the enzyme observed during the oxygen-consuming reaction was Compound II, which oxidized IAA to its free radical at rate constants of 1.5 X 10(3) M-1--S1 for peroxidase A and 1.2 times 10(4) M-1--S-1 for peroxidase C at pH 4.4 The results supported the mechanism that the oxygen consumption occurs mainly through the reaction of oxygen with the IAA free radical formed from the peroxidatic oxidation of IAA. The ferric enzymes were not reduced by IAA under strict anaerobic conditions in the presence of carbon monoxide but were reduced upon addition to a small amount of oxygen or hydrogen peroxide to the systems. The results suggested that the ferric enzyme is reduced by the IAA free radical but not by IAA itself. From a comparison of reactivities of oxyperoxidase and Compound II we concluded that the catalytic cycle of ferrous and oxyperoxidases is not involved in the IAA oxidase reaction.
在辣根过氧化物酶C（中性同工酶）存在的情况下，吲哚 - 3 - 乙酸（IAA）的耗氧氧化反应比在辣根过氧化物酶A（酸性同工酶）存在时快得多。IAA的一种中间氧化产物被发现是一种氢过氧化物，它与铁酶反应形成化合物I，在pH 4.4时，过氧化物酶A的二级反应速率常数为6.8×10³ M⁻¹·s⁻¹，过氧化物酶C的二级反应速率常数为2.0×10⁶ M⁻¹·s⁻¹。在耗氧反应结束时，氢过氧化物浓度达到初始IAA浓度的约一半，然后缓慢下降。在耗氧反应过程中观察到的酶的主要中间体是化合物II，在pH 4.4时，它将IAA氧化为其自由基，过氧化物酶A的反应速率常数为1.5×10³ M⁻¹·s⁻¹，过氧化物酶C的反应速率常数为1.2×10⁴ M⁻¹·s⁻¹。这些结果支持了耗氧主要通过氧气与IAA过氧化物酶氧化形成的IAA自由基反应的机制。在严格厌氧条件下，一氧化碳存在时，铁酶不会被IAA还原，但向体系中加入少量氧气或过氧化氢后，铁酶会被还原。结果表明，铁酶是被IAA自由基还原，而不是被IAA本身还原。通过比较氧过氧化物酶和化合物II的反应活性，我们得出结论，亚铁过氧化物酶和氧过氧化物酶的催化循环不参与IAA氧化酶反应。
