Mura Anna, Pintus Francesca, Lai Paola, Padiglia Alessandra, Bellelli Andrea, Floris Giovanni, Medda Rosaria
Department of Applied Sciences in Biosystems, University of Cagliari, I-09042 Monserrato, Italy.
Biol Chem. 2006 May;387(5):559-67. doi: 10.1515/BC.2006.072.
The reaction of Euphorbia characias latex peroxidase (ELP) with hydrogen peroxide as the sole substrate was studied by conventional and stopped-flow spectrophotometry. The reaction mechanism occurs via three distinct pathways. In the first (pathway I), ELP shows catalase-like activity: H2O2 oxidizes the native enzyme to compound I and subsequently acts as a reducing substrate, again converting compound I to the resting ferric enzyme. In the presence of an excess of hydrogen peroxide, compound I is still formed and further reacts in two other pathways. In pathway II, compound I initiates a series of cyclic reactions leading to the formation of compound II and compound III, and then returns to the native resting state. In pathway III, the enzyme is inactivated and compound I is converted into a bleached inactive species; this reaction proceeds faster in samples illuminated with bright white light, demonstrating that at least one of the intermediates is photosensitive. Calcium ions decrease the rate of pathway I and accelerate the rate of pathways II and III. Moreover, in the presence of calcium the inactive stable verdohemochrome P670 species accumulates. Thus, Ca2+ ions seem to be the key for all catalytic pathways of Euphorbia peroxidase.
通过传统分光光度法和停流分光光度法研究了大戟乳胶过氧化物酶（ELP）与过氧化氢作为唯一底物的反应。反应机制通过三种不同途径发生。在第一种途径（途径I）中，ELP表现出过氧化氢酶样活性：H2O2将天然酶氧化为化合物I，随后作为还原底物，再次将化合物I转化为静止的铁酶。在过氧化氢过量的情况下，化合物I仍然形成并在另外两种途径中进一步反应。在途径II中，化合物I引发一系列循环反应，导致化合物II和化合物III的形成，然后回到天然静止状态。在途径III中，酶失活，化合物I转化为漂白的无活性物种；该反应在明亮白光照射的样品中进行得更快，表明至少有一种中间体是光敏的。钙离子降低途径I的速率并加速途径II和III的速率。此外，在有钙的情况下，无活性的稳定的绿血色原P670物种积累。因此，Ca2+离子似乎是大戟过氧化物酶所有催化途径的关键。
