乳過氧酵素系統（LPO system）是人體口腔中天生具有對抗微生物的保護系統。本實驗之目的在研究乳過氧酵素（LPO），硫氰酸鈉（NaSCN）及過氧化氫（H2O2）之濃度對乳過氧酵素系統可能的影響。突變形鏈球菌株先在血瓊膠培養基上生長，取其上數個菌落再種入TSYG培養液內，在攝氏37度下生長一夜，經離心收集的細菌置於恒溫（攝氏37度）滴定儀內測試其產酸速率。酸鹼值維持在7.0。反應液內包含320μg/ml細菌（S.mutans），2 mM酸鹼緩衝液，18 mM KCL，0.4 mM MgCl2，20 mM葡萄糖與不同濃度之過氧化氫、硫氰酸鈉及乳過氧酵素。結果顯示當H2O2濃度增加時LPO及SCN-濃度亦須增加才能達到抑制S.mutans產酸的效果。適當的H2O2濃度可抑制S.mutans產酸之效力，但過大H2O2濃度反而會減少抑制S.mutans產酸之效果。結論：LPO system所造成抑制S.mutans產酸速率的作用會受到個別因子的影響。

Lactoperoxidase (LPO) catalyzes the oxidation of thiocyanate (SCN-) by hydrogen peroxide (H2O2) yielding the antimicrobial substance hypothiocyanite (OSCN-). Sugar transport and metabolism of S. mutans are inhibited by the LPO system. This study examined the effect of LPO, SCN-, and H2O2 concentration on inhibition of acid production by S. mutans 10449 fermenting glucose. Cells were grown anaerobically at 37℃ in TSYG broth overnight, harvested and examined in a pH state at pH 7.0 and 37℃. Reaction mixtures contained cells (ca. 320g/ml dry wt.), 2 mM KPi, 18 mM KCl, 0.4 mM MgCl2, 20 mM glucose. Results showed that increased H2O2 concentrations need more LPO and SCN- concentration to inhibit acid production of S. mutans . In the same concentration of LPO and SCN-, adequate H2O2 concentration could inhibit acid production of S. mutans but lower or more H2O2 concentrations reduced the inhibition effect. In conclusion, The effects of LPO system on the inhibition of acid production of S. mutans dependent on LPO, SCN- and H2O2 concentrations.