Hydroperoxide-lyase activity in mint leaves: Volatile C6-aldehyde production from hydroperoxy-fatty acids

Abstract

The extraction of 13-hydroperoxide-lyase activity from mint leaves as well as its use for C6-aldehyde production was studied in this work. The enzyme cleaves 13( S)-hydroperoxy-C18 fatty acids into C6-aldehyde and C12-oxo-acid. Two mint species were tested: Mentha veridis and Mentha pulegium. The headspace injection method coupled to gas chromatography was used for volatile compound analysis. The optimal conditions for temperature and pH were, respectively, 15 and 7 °C. We also studied the specific synthesis of hexanal and hexenals respectively from 13( S)-hydroperoxy-linoleic acid and 13( S)-hydroperoxy-linolenic acid. Considerable quantities of aldehyde (up to 2.58 μmol) were produced after 15 min of cleavage reaction in 2 ml stirred at 100 rpm, especially in presence of extract of M. veridis. The conversion yields decreased from 52.5% as maximum to 3.3% when using initial hydroperoxide concentrations between 0.2 and 15 mM. An unsaturated aldehyde, the 3( Z)-hexenal was produced from 13( S)-hydroperoxy-linolenic acid. The 3( Z)-isomer was unstable and isomerized in part to 2( E)-hexenal. In this work, we observed a very limited isomerization of 3( Z)-hexenal to 2( E)-hexenal, since the reaction and the volatile purge were carried out successively in the same flask without delay or any contact with the atmosphere. These aldehydes contribute to the fresh green odor in plants and are widely used in perfumes and in food technology. Their importance increases especially when the starting materials are of natural biological origin as used in this work. GC–MS analysis allowed the identification of the products.