Formation of cyanogen iodide by lactoperoxidase

•Cyanogen iodide (ICN) is produced by the lactoperoxidase (LPO)–H2O2–SCN–/I− system.•ICN formation by LPO depends on pH and SCN−/I− ratio.•Oxidised iodine species react with SCN− resulting in ICN formation.•Myeloperoxidase can also catalyse the ICN formation.

The haem protein lactoperoxidase (LPO) is an important component of the anti-microbial immune defence in external secretions and is also applied as preservative in food, oral care and cosmetic products. Upon oxidation of SCN− and I− by the LPO–hydrogen peroxide system, oxidised species are formed with bacteriostatic and/or bactericidal activity. Here we describe the formation of the inter(pseudo)halogen cyanogen iodide (ICN) by LPO. This product is formed when both, thiocyanate and iodide, are present together in the reaction mixture. Using 13C nuclear magnetic resonance spectroscopy and gas chromatography–mass spectrometry we could identify this inter(pseudo)halogen after applying iodide in slight excess over thiocyanate. The formation of ICN is based on the reaction of oxidised iodine species with thiocyanate. Further, we could demonstrate that ICN is also formed by the related haem enzyme myeloperoxidase and, in lower amounts, in the enzyme-free system. As I− is not competitive for SCN− under physiologically relevant conditions, the formation of ICN is not expected in secretions but may be relevant for LPO-containing products.

Graphical abstractLactoperoxidase is involved in anti-microbial defence in secretions and as an active component in home-health products. Upon activation by H2O2, it oxidises SCN− or I−. Here we describe the formation of the oxidation product cyanogen iodide. It is formed when SCN− and I− are simultaneously present in the reaction mixture.Figure optionsDownload full-size imageDownload as PowerPoint slide