Thomas E L, Milligan T W, Joyner R E, Jefferson M M
Dental Research Center, University of Tennessee, Memphis 38163.
Infect Immun. 1994 Feb;62(2):529-35. doi: 10.1128/iai.62.2.529-535.1994.
In secreted fluids, the enzyme lactoperoxidase (LP) catalyzes the oxidation of thiocyanate ion (SCN-) by hydrogen peroxide (H2O2), producing the weak oxidizing agent hypothiocyanite (OSCN-), which has bacteriostatic activity. However, H2O2 has antibacterial activity in the absence of LP and thiocyanate (SCN-). Therefore, LP may increase antibacterial activity by using H2O2 to produce a more effective inhibitor of bacterial metabolism and growth, or LP may protect bacteria against the toxicity of H2O2 by converting H2O2 to a less-potent oxidizing agent. To clarify the role of LP, the antibacterial activities of H2O2 and the LP-H2O2-SCN- system were compared by measuring loss of viability and inhibition of bacterial metabolism and growth. The relative toxicity of H2O2 and the LP system to oral streptococci was found to depend on the length of time that the bacteria were exposed to the agents. During incubations of up to 4 h, the LP system was from 10 to 500 times more effective than H2O2 as an inhibitor of glucose metabolism, lactic acid production, and growth. However, if no more H2O2 was added, the concentration of the inhibitor OSCN- fell because of slow decomposition of OSCN-, and when OSCN- fell below 0.01 mM, the bacteria resumed metabolism and growth. In contrast, the activity of H2O2 increased with time. H2O2 persisted in the medium for long periods of time because H2O2 reacted slowly with the bacteria and streptococci lack the enzyme catalase, which converts H2O2 to oxygen and water. After 24 h of exposure, H2O2 was as effective as the LP system as an inhibitor of metabolism. H2O2 also caused a time-dependent loss of viability, whereas the LP system had little bactericidal activity. The concentration of H2O2 required to kill half the bacteria within 15 s was 1.8 M (6%) but fell to 0.3 M (1%) at 2 min, to 10 mM (0.03%) at 1 h, and to 0.2 mM (0.0007%) with a 24-h exposure. The results indicate that if high levels of H2O2 can be sustained for long periods of time, H2O2 is an effective bactericidal agent, and the presence of LP and SCN- protects streptococci against killing by H2O2. Nevertheless, the combination of LP, H2O2, and SCN- is much more effective than H2O2 alone as an inhibitor of bacterial metabolism and growth.
在分泌液中，乳过氧化物酶（LP）催化过氧化氢（H₂O₂）将硫氰酸根离子（SCN⁻）氧化，生成具有抑菌活性的弱氧化剂次硫氰酸（OSCN⁻）。然而，在没有LP和硫氰酸盐（SCN⁻）的情况下，H₂O₂也具有抗菌活性。因此，LP可能通过利用H₂O₂产生一种更有效的细菌代谢和生长抑制剂来增强抗菌活性，或者LP可能通过将H₂O₂转化为一种活性较低的氧化剂来保护细菌免受H₂O₂的毒性。为了阐明LP的作用，通过测量细菌活力丧失以及细菌代谢和生长的抑制情况，比较了H₂O₂和LP - H₂O₂ - SCN⁻体系的抗菌活性。结果发现，H₂O₂和LP体系对口腔链球菌的相对毒性取决于细菌接触这些试剂的时间长短。在长达4小时的孵育过程中，作为葡萄糖代谢、乳酸产生和生长的抑制剂，LP体系比H₂O₂有效10至500倍。然而，如果不再添加H₂O₂，由于OSCN⁻的缓慢分解，抑制剂OSCN⁻的浓度会下降，当OSCN⁻浓度降至0.01 mM以下时，细菌恢复代谢和生长。相比之下，H₂O₂的活性随时间增加。H₂O₂在培养基中能长时间存在，因为H₂O₂与细菌反应缓慢，而且链球菌缺乏将H₂O₂转化为氧气和水的过氧化氢酶。暴露24小时后，H₂O₂作为代谢抑制剂与LP体系效果相当。H₂O₂还会导致细菌活力随时间丧失，而LP体系几乎没有杀菌活性。在15秒内杀死一半细菌所需的H₂O₂浓度为1.8 M（6%），但在2分钟时降至0.3 M（1%），1小时时降至10 mM（0.03%），24小时暴露后降至0.2 mM（0.0007%）。结果表明，如果能长时间维持高浓度的H₂O₂，H₂O₂是一种有效的杀菌剂，并且LP和SCN⁻的存在可保护链球菌免受H₂O₂的杀灭。然而，LP、H₂O₂和SCN⁻的组合作为细菌代谢和生长的抑制剂比单独使用H₂O₂要有效得多。
