Identification and quantification of apo-lycopenals in fruits, vegetables, and human plasma

Abstract

Research has suggested that lycopene may be metabolized by eccentric cleavage, catalyzed by beta-carotene oxygenase 2, resulting in the generation of apo-lycopenals. Apo-6'-lycopenal and apo-8'-lycopenal have been reported previously in raw tomato. We now show that several other apo-lycopenals are also present in raw and processed foods, as well as in human plasma. Apo-lycopenal standards were prepared by in vitro oxidation of lycopene, and a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method using atmospheric pressure chemical ionization in negative mode was developed to separate and detect the apo-6'-, apo-8'-, apo-10'-, apo-12'-, apo-14'-, and apo-15'-lycopenal products formed in the reaction. Hexane/acetone extracts of raw tomato, red grapefruit, watermelon, and processed tomato products were analyzed, as well as plasma of individuals who had consumed tomato juice for 8 weeks. Apo-6'-, apo-8'-, apo-10'-, apo-12'-, and apo-14'-lycopenals were detected and quantified in all food products tested, as well as plasma. The sum of apo-lycopenals was 6.5 microg/100 g Roma tomato, 73.4 microg/100 g tomato paste, and 1.9 nmol/L plasma. We conclude that several apo-lycopenals, in addition to apo-6'- and -8'-lycopenal, are present in lycopene-containing foods. In addition, the presence of apo-lycopenals in plasma may derive from the absorption of apo-lycopenals directly from food and/or human metabolism.

Figure 1

Series of apo-lycopenals produced after ozone treatment of lycopene.

Figure 2

An HPLC-MS chromatogram of the products resulting from the oxidation of lycopene with ozone acquired on a quadropole-time-of-flight mass spectrometer (QTof Premier, Micromass, Beverley, MA). The chromatogram displays the sum of ion intensities for the radical anions of lycopene and apo-lycopenals in APcI negative (note: HPLC gradient slightly modified from that used for blood plasma analysis – see methods). 1 = all-trans-lycopene (536.44), 1’ = cis-lycopene isomers (536.44), 2 = apo-6′-lycopenal (442.32), 3 = apo-8′-lycopenal (416.31), 4 = apo-10′-lycopenal (376.28), 5 = apo-12′-lycopenal (350.26), 6 = apo-14′-lycopenal (310.23), and 7 = apo-15′-lycopenal (284.21).

Figure 3

Standard calibration curves for the quantification of lycopene (○), apo-6′-lycopenal (●), apo-8′-lycopenal (□), and apo-12′-lycopenal (■)

Figure 4

HPLC-MS/MS analysis of lycopene and apo-lycopenals in the blood plasma extract of one representative individual. (A) MS chromatogram of the collection of SRM channels for all-trans lycopene (536>467), apo-6′- (442>373), -8′- (416>347), and -12′-lycopenal (350>281) standards and (B-G) lycopene and apo-lycopenals in blood plasma. Parent radical anion m/z values are shown in the upper right of each trace, with the respective transition to M-69 daughter ions that were monitored.