Lactoperoxidase-catalyzed oxidation of thiocyanate: equilibria between oxidized forms of thiocyanate.
Lactoperoxidase-catalyzed oxidation of thiocyanate: equilibria between oxidized forms of thiocyanate.
复制标题
乳过氧化物酶催化的硫氰酸盐氧化：硫氰酸盐氧化形式之间的平衡。
DOI：
10.1021/bi00514a045
复制
发表时间：
1981
期刊：
影响因子：
2.9
通讯作者：
Thomas,EL
中科院分区：
文献类型：
--
作者：
Thomas,EL
埃德温湖托马斯摘要：研究了在pH 5-8范围内乳过氧化物酶催化的硫氰酸根离子（SCN-）的氧化，以获得次硫氰酸（HOSCN）或次硫氰酸根离子（OSCN-）作为主要观察到的产物。根据pH值对两个独立参数的影响，计算出HOSCN的pKa值为5.3：（1）HOSCN从HOSCN和OSCN”的水溶液中被萃取到有机溶剂中的程度;（2）HOSCN-OSCN”混合物的分解速率。计算HOSCN萃取到乙酸乙酯、1-辛醇和2-辛醇中的分配系数为5.0、2.1和2.3。HOSCN的鉴定通过氧化当量和SCN部分的萃取的2：1化学计量确认。SCN ~-促进了HOSCN ~--OSCN ~-的分解，改变了分解的表观动力学机理。在0.1-0.2 mM SCN ~（-1）存在下，HOSCN分解的二级速率常数为3 M ~（-1）s ~（-1），假定HOSCN的歧化为限速步骤。假设HOSCN与SCN-的反应是速率限制的，在IOmM SCN-下计算出2.6 X 10 - 3s-1的一级常数。许多pH缓冲剂也加速HOSCN-OSCN-的分解。这些试剂或SCN ′对HOSCN在有机溶剂中的萃取无影响。某些含氮化合物能稳定HOSCN-OSCN-混合物的氧化活性，这显然是通过降低游离HOSCN的浓度来实现的。磺胺类和芳香族亚胺的稳定作用与含有氮-硫氰酸盐（N-SCN）键的衍生物（硫代氰合磺酰胺和硫代氰铵）的形成一致。这些N-SCN衍生物保留了HOSCN或OSCN '的氧化当量，但在有机溶剂中的溶解度不同于HOSCN。结果表明，HOSCN以显著量形成，并且在与SCN的外分泌物、白细胞和甲状腺的过氧化物酶催化氧化一致的pH范围内可以相对稳定。中性HOSCN分子可能负责已归因于OSCN'阴离子的生物活性。此外，HOSCN-OSCN-的生物活性在低pH下可能大得多，条件是SCN-和培养基中其他组分的浓度有利于HOSCN的稳定性。e哺乳动物血红素蛋白过氧化物酶乳过氧化物酶、髓过氧化物酶和甲状腺过氧化物酶催化过氧化氢（H2 O2）对硫氰酸根离子（SCN '）的氧化。乳过氧化物酶、H2 O2和SCN ′在乳、唾液中以及可能还在眼泪中形成抗微生物系统（Wright & Tramer，1958; Zeldow，1963; Klebanoff等，1966;莫里森&艾伦，1966;托马斯等，1981年）。该系统有助于外分泌物的抗微生物活性。SCN'的氧化也可能有助于髓过氧化物酶介导的白细胞抗微生物活性。此外，SCN'与碘离子竞争作为甲状腺过氧化物酶的底物（Wood，1975）。高浓度的SCN'对甲状腺有毒性，但SCN'的氧化是否与抗甲状腺作用有关尚未确定。
Edwin L. Thomas abstract: Lactoperoxidase-catalyzed oxidation of thiocyanate ion (SCN") was studied in the pH range 5-8 so as to obtain either hypothiocyanous acid (HOSCN) or hypothiocyanite ion (OSCN") as the major observed product. A pKa value of 5.3 was calculated forHOSCN, from the effect of pH on two independent parameters:(1) the extent of extraction of HOSCN intoorganic solvents from aqueous mixtures of HOSCN and OSCN" and (2) the rate of decomposition of HOSCN-OSCN" mixtures. Partition coefficients of 5.0, 2.1, and 2.3 were calculated for extraction of HOSCN into ethyl acetate, 1-octanol, and 2-octanol. Identification of HOSCN was confirmed by the 2: 1 stoichiometry for extraction of ox-idizing equivalents and the SCN moiety. Decomposition of HOSCN-OSCN" was accelerated by SCN", which also changed the apparent kinetic mechanism of decomposition. A second-order rate constant of 3 M'1 s" 1 was calculated for decomposition of HOSCN inthe presence of 0.1-0.2 mM SCN", assuming the rate-limiting step to be the dismutation of HOSCN. A first-order constant of 2.6 X 10" 3 s" 1 was calculated at 10 mM SCN", assuming the reaction of HOSCN with SCN" to be rate limiting. A number of pH buffering agents also accelerated the decomposition of HOSCN-OSCN". These agents or SCN'had no effect on extraction of HOSCN intoorganic solvents. Certain nitrogenous com-pounds stabilized theoxidizing activity ofHOSCN-OSCN" mixtures, apparently by lowering the concentration of free HOSCN. Stabilization by sulfonamides and aromatic imines was consistent with formation of derivatives containing the nitrogen-thiocyanate (N-SCN) linkage (thiocyanatosulfon-amides and thiocyanatimines). These N-SCN derivatives retained the oxidizing equivalents of HOSCN or OSCN', but differed from HOSCN in their solubility in organic solvents. The results indicate that HOSCN is formed in significant amounts and can be relatively stable in the pH range consistent with peroxidase-catalyzed oxidation of SCN'inexocrine se-cretions, leukocytes, and the thyroid gland. The neutral HOSCN molecule may be responsible for biological activities that have been attributed to the OSCN'anion. Also, the biological activityof HOSCN-OSCN" may be much greater at low pH, provided that the concentrations of SCN'and other components of the medium favor the stabilityof HOSCN. e mammalian hemoprotein peroxidases lactoperoxidase, myeloperoxidase, and thyroid peroxidase catalyze the oxidation of thiocyanate ion (SCN') by hydrogen peroxide (H202). Lactoperoxidase, H202, and SCN'form an antimicrobial system in milk, saliva, and perhaps also in tears (Wright & Tramer, 1958; Zeldow, 1963; Klebanoff et al, 1966; Morrison & Allen, 1966; Thomas et al., 1981). This system contributes to the antimicrobial activity of exocrine secretions. Oxidation of SCN'may also contribute to the myeloperoxidase-mediated antimicrobial activity of leukocytes. Also, SCN'competes with iodide ion as a substrate forthyroid peroxidase (Wood, 1975). High levels of SCN'are toxic to the thyroid gland, though it has not been established that oxidation ofSCN'is reponsible for the antithyroid effect.