Soybean lipoxygenase catalysed oxygenation of unsaturated fatty acid encapsulated in cyclodextrin

The linoleic or arachidonic acid entrapped in cyclodextrin (alpha, beta or gamma) serves as an excellent substrate for soybean lipoxygenase-1 catalysis. At pH 9.0 the Km values for the beta-cyclodextrin encapsulated arachidonic acid, referred herein as encapsulated substrate, and the Tween-20 dispersed substrate were 7.7 microM and 7.5 microM, respectively. However, the Vmax values for alpha- and beta-cyclodextrin solubilized substrates were lower in comparison with the Tween-20 dispersed substrate. Interestingly, the pH-activity profile for the enzyme towards cyclodextrin encapsulated arachidonic acid showed optimum around 7.5, while that towards Tween-20 dispersion showed the expected broad optimum in the alkaline range (8.5-10.0). The activity with encapsulated substrate at pH 7.5 was at least 5-fold higher than that obtained with Tween-20 dispersed substrate at the corresponding pH. Similar results were obtained using linoleic acid. The second order rate constant, Kcat/Km, for the encapsulated substrate was an order of magnitude higher when compared to the Tween-20 dispersed substrate. The plot of v obtained at pH 9.0, against S gave hyperbolic curves for both the encapsulated as well as the Tween-20 dispersed substrates, whereas at pH 7.5, the curve for cyclodextrin encapsulated arachidonic acid appeared initially concave and then at higher concentrations of the substrate sigmoidal. The positional specificity of soybean lipoxygenase remained unaltered, however.