Modi S, Behere D V, Mitra S
Chemical Physics Group, Tata Institute of Fundamental Research, Bombay, India.
Biochim Biophys Acta. 1991 Oct 11;1080(1):45-50. doi: 10.1016/0167-4838(91)90110-l.
Horseradish peroxidase-catalysed oxidation of thiocyanate by hydrogen peroxide has been studied by 15N-NMR and optical spectroscopy at different concentrations of thiocyanate and hydrogen peroxide and at different pH values. The extent of the oxidation and the identity of the oxidized product of the thiocyanate has been investigated in the SCN-/H2O2/HRP system and compared with the corresponding data on the SCN-/H2O2/LPO system. The NMR studies show that (SCN)2 is the oxidation product of thiocyanate in the SCN-/H2O2/HRP system, and its formation is maximum at pH less than or equal to 4 and that the oxidation does not take place at pH greater than or equal to 6. Since thiocyanate does not bind to HRP at pH greater than or equal to 6 (Modi et al. (1989) J. Biol. Chem. 264, 19677-19684), the binding of thiocyanate to HRP is considered to be a prerequisite for the oxidation of thiocyanate. It is further observed that at [H2O2]/[SCN-] = 4, (SCN)2 decomposes very slowly back to thiocyanate. The oxidation product of thiocyanate in the SCN-/H2O2/LPO system has been shown to be HOSCN/OSCN- which shows maximum inhibition of uptake by Streptococcus cremoris 972 bacteria when hydrogen peroxide and thiocyanate are present in equimolar amounts (Modi et al. (1991) Biochemistry 30, 118-124). However, in case of HRP no inhibition of oxygen uptake by this bacteria was observed. Since thiocyanate binds to LPO at the distal histidine while to HRP near 1- and 8-CH3 heme groups, the role of distal histidine in the activity of SCN-/H2O2/(LPO, HRP) systems is indicated.
利用15N核磁共振和光谱学方法，研究了在不同硫氰酸盐和过氧化氢浓度以及不同pH值条件下，辣根过氧化物酶催化过氧化氢氧化硫氰酸盐的反应。在SCN-/H2O2/HRP体系中，研究了硫氰酸盐的氧化程度和氧化产物的特性，并与SCN-/H2O2/LPO体系的相应数据进行了比较。核磁共振研究表明，(SCN)2是SCN-/H2O2/HRP体系中硫氰酸盐的氧化产物，在pH小于或等于4时其生成量最大，而在pH大于或等于6时不发生氧化反应。由于在pH大于或等于6时硫氰酸盐不与HRP结合（莫迪等人，(1989)《生物化学杂志》264, 19677 - 19684），因此硫氰酸盐与HRP的结合被认为是硫氰酸盐氧化的先决条件。进一步观察到，当[H2O2]/[SCN-] = 4时，(SCN)2分解回硫氰酸盐的速度非常缓慢。已证明SCN-/H2O2/LPO体系中硫氰酸盐的氧化产物是HOSCN/OSCN-，当过氧化氢和硫氰酸盐等摩尔存在时，它对嗜热链球菌972细菌的摄取具有最大抑制作用（莫迪等人，(1991)《生物化学》30, 118 - 124）。然而，在HRP的情况下，未观察到该细菌的氧摄取受到抑制。由于硫氰酸盐在远端组氨酸处与LPO结合，而在靠近1-和8-CH3血红素基团处与HRP结合，这表明了远端组氨酸在SCN-/H2O2/(LPO, HRP)体系活性中的作用。
