Pruitt K M, Tenovuo J, Mansson-Rahemtulla B, Harrington P, Baldone D C
Biochim Biophys Acta. 1986 Apr 22;870(3):385-91. doi: 10.1016/0167-4838(86)90245-1.
The peroxidase-catalyzed oxidation of SCN- by H2O2 is an important in vivo reaction because it limits the accumulation of toxic H2O2 and provides significant concentrations of the antimicrobial agents, HOSCN and OSCN-. Data presented in this report suggest that the reaction: (Formula: see text) is in a state of dynamic equilibrium in vivo. Since OSCN- can form the weak acid HOSCN (pKa = 5.3), the equilibrium constant expression (Kox) for thiocyanate peroxidation is dependent on the concentration of hydrogen ions as well as the concentrations of H2O2, SCN-, HOSCN, OSCN- and water, and on the HOSCN ionization constant, Ka: (Formula: see text). The concentration of water is assumed to be constant and unaffected by the other components and is omitted from the Kox equation. The value of Kox was estimated from in vitro data to be 3.7 X 10(3) M-1 (S.D. = 0.8 X 10(3) M-1, n = 8). Using this value for Kox and observations of salivary concentrations of SCN- and HOSCN + OSCN- from several previous reports, the equilibrium concentrations of H2O2 in whole saliva were calculated to range from 8 to 13 microM. This range is consistent with reported estimates of 10 microM as the hydrogen peroxide tolerance limit for human cells.
过氧化氢酶催化的H2O2氧化SCN-是体内一个重要反应，因为它限制了有毒H2O2的积累，并产生了高浓度的抗菌剂HOSCN和OSCN-。本报告中的数据表明，反应：（公式：见正文）在体内处于动态平衡状态。由于OSCN-可形成弱酸HOSCN（pKa = 5.3），硫氰酸盐过氧化反应的平衡常数表达式（Kox）取决于氢离子浓度以及H2O2、SCN-、HOSCN、OSCN-和水的浓度，还取决于HOSCN的电离常数Ka：（公式：见正文）。假定水的浓度恒定且不受其他成分影响，因此在Kox方程中省略。根据体外数据估计Kox的值为3.7×10(3) M-1（标准差 = 0.8×10(3) M-1，n = 8）。利用该Kox值以及之前几份报告中唾液中SCN-和HOSCN + OSCN-浓度的观测结果，计算出全唾液中H2O2的平衡浓度范围为8至13微摩尔。该范围与报道的人类细胞过氧化氢耐受极限估计值10微摩尔一致。
