β-Carotene and Capsanthin Co-oxidation by Lipoxygenase. Kinetic and Thermodynamic Aspects of the Reaction

Abstract

Carotenoid pigment co-oxidation by lipoxygenase has been studied by factorially combining three pigments: β-carotene, capsanthin, and diesterified capsanthin; two environmental conditions: presence or absence of light; three oxidant conditions in the same reaction medium: presence of both lipoxygenase and linoleic acid, presence of linoleic acid only, and absence of both; and four temperatures: 15, 25, 35, and 45 °C. Independently of the presence or absence of enzyme, diesterified capsanthin and β-carotene were the most unstable pigments, and both exhibited similar lability, although nonesterified capsanthin showed a great stability. This different behavior among pigments could be related with their polarity. The overall reaction clearly follows a first-order kinetic model and the thermodynamic parameters reveal that the co-oxidation reactions of carotenoids are multiple isokinetic forms of just one reaction. In all of the cases studied, enzymatic or nonenzymatic decoloring of carotenoids is a theoretical reaction that can be explained as a loss of conjugation in a sequence of conjugated double bonds. The source of pro-oxidant compounds, the environmental conditions, the reaction medium, and the differences in pigment structure are external factors that modify the kinetic parameters of the reaction and consequently the thermodynamic parameters, but not the reaction pathway. Keywords: Carotenoids; lipoxygenase; decoloring; degradation; kinetic