Substrate specificity, hydroperoxide lyase

Major products of the lipoxygenase reaction are 9- and 13-hydroperoxides of IV and V, VI, VII, VIII and IX. Hydroperoxide lyase utilizes the 9- and/or 13-hydroperoxides (VI, VII, VIII and IX). Based on substrate specificity, hydroperoxide lyase is classified into three types. The first type is 9-hydroperoxide-specific. 

Substrate Specificity. Hydroperoxide dehydrase from sunflower cotyledons was about four-fold more active wiA 13-hydroperoxylinoleic acid Aan 13-hydroperoxylinolenic acid. In contrast, leaf hydroperoxide lyase was more Aan 10-fold more active wiA 13-hydroperoxylinolenic acid Aan wiA 13-hydroperoxylinoleic acid. However, root hydroperoxide lyase showed no preference between Ae two substrates, again suggesting Aat roots have a different form of Ae enzyme Aan leaves. NeiAer 9-hydroperoxylinoleic nor 9-hydroperoxylinolenic acids were suitable substrates for any of Ae hydroperoxide metabolizing enzymes in sunflower. 

The rate of hydroperoxide consumption, corresponding to lyase and dehydrase activities, was determined in different parts of the grain during germination (Fig. 1). Hydroperoxide lyase activity was measured in different parts of the grain but no activity was detected using the specific spectrophotometric assay. This indicates that hydroperoxide lyase is absent or exhibits low activity in the extracts (competition for the hydroperoxide substrate between hydroperoxide lyase and dehydrase may occur in the incubation media). Consequently, the consumption of hydroperoxides corresponds mainly to the dehydrase activity. 

Fatty acid hydroperoxide lyase is one of the enzymes responsible for volatile Cft- and C9-aldehyde formation from linoleic and linolenic acid. This enzyme cleaves 9- and/or 13-hydroperoxides derived from linoleic and linolenic acid. The enzyme is distributed in a wide range of plant species in membrane bound forms both chloroplastic and non-chloroplastic. Three types of hydroperoxide lyases have been reported 9-hydroperoxide-specific, 13-hydroperoxide-specific and nonspecific. Other properties of the hydroperoxide lyase including substrate specificity and reaction mechanism are discussed in this review. 

The most common substrates for lipoxygenases in plants are linoleic and linolenic acids. Their oxidation, catalysed by lipoxygenases, yields 13- and 9-hydroperoxy fatty acids. Some lipoxygenases mainly catalyse the formation of (13S)-hydroperoxides, while less specific lipoxygenases produce a higher proportion of (13J )-hydroperoxides. Other lipoxygenases catalyse the formation of optically active 9-hydroperoxides. The 13-hydroperoxides of linoleic and linolenic acids, by the action of hydroperoxide lyases, produce aldehydes with six carbon atoms (Figure 3.60) that... 

Important. The substrate specificities and the product specificities of these enzymes determine the composition of volatile aldehydes formed from llnolelc acid and llnolenlc acid. The present report describes occurrence of, and substrate and product specificities of, lipoxygenase and hydroperoxide lyase In leaves, particularly In tea leaves. 

Substrate and product specificities of chloroplast hydroperoxide lyase. 

Hydroperoxide dehydrase activity was determined at 25 C by monitoring the decrease in absorbance at 234 nm [5] with 10 fjM of 13-hydroperoxide of linoleic acid as substrate. Hydroperoxide lyase activity was determined by a specific spectrophotometric assay [7], using 40 /iM of 13-hydroperoxide of linoleic acid as substrate. The solution of 13-hydroperoxide of... 

---