Protective role of native bovine serum albumin and alpha-unsaturated fatty acids on catechin oxidation

Catechin, a common ingredient of tea and coffee is a colourless water soluble compound which undergoes spontaneous oxidation by aerial O2, heat and also by UV exposure or ammonium persulfate (APS) treatment resulting in a noticeable change to yellow colouration (λmax = 440 nm). Several α-unsaturated fatty acids (ascorbic, maleic, sorbic but not acrylic), βmercapto ethanol (BME) and native bovine serum albumin (BSA) block the colouring process whereas the alkylated BSA fails to do so, confirming the reducing role of thiol (–SH) group(s) within the protein molecule. But alkylation does not change either the affinity (Kd) or the capacity (Bmax) of catechin binding at pH 7.5 (Kd ~ 0.8 nM and Bmax ~ 4.0 mol/mol BSA). Interestingly, a large increment in Bmax is observed at pH 4.5. Additionally, the native BSA acts as a non-competitive inhibitor during oxidation of catechin as analyzed by the Lineweaver-Burke plot. The oxidation by APS follows the Michaelis equation and the rate (Vmax) is favourable at pH 8.5 (order → pH: 8.5 > 7.5 > 10.5) whereas the activation energy (∆E) deduced from Arrhenius equation shows that the reaction is favourable at higher pH. These incidents insist to hypothesize that APS, UV illumination and aerial oxygenation produce the same yellow poly-phenolic species as proven by the identical spectral changes associated with any of these procedures. The reducing agents interfere by blocking the crucial oxidative pathway.