Kinetic studies of lycopene isomerization in a tributyrin model system at gastric pH

A semi-preparative HPLC method was developed in order to isolate and purify the 13-cis-lycopene isomer in tomato-based materials. The result was compared with the naturally predominant all-trans-lycopene isomer, in terms of stability to gastric pH at physiological temperature in a tributyrin model system. Kinetic experiments confirmed that lycopene isomerization is a reversible reaction, and under these conditions the all-trans isomer is more stable than the 13-cis isomer. In addition, it was found that at gastric pH 13-cis-lycopene would predominantly isomerize to the all-trans form rather than undergo oxidation/breakdown. A simulation based on the rate constants calculated in the kinetic study indicated that at gastric pH the lycopene isomeric distribution aimed toward an equilibrium characterized by approx 16% 13-cis-, 16% 9-cis-, and 68% all-trans-lycopene. This study suggests that pH-driven isomerization in the stomach is at least partially responsible for the relatively high cis-lycopene proportion found in vivo.