Proposed Mechanisms for the Formation of Synthetic and Naturally Occurring Metabolites of Lycopene in Tomato Products and Human Serum

Oxidation of lycopene (I) with m-chloroperbenzoic acid has been previously shown to yield lycopene 1,2-epoxide (II) and lycopene 5,6-epoxide (III) as major products and lycopene 1,2;5,6-diepoxide (IV), lycopene 1,2;5‘,6‘-diepoxide (V), lycopene 5,6;5‘,6‘-diepoxide (VI), and lycopene 1,2;1‘,2‘-diepoxide (VII) as minor products. Similarly, the acid-catalyzed rearrangement products of these epoxides have been identified as 2,6-cyclolycopene-1,5-epoxide A (VIII) and B (IX), 2,6-cyclolycopene-1,5-diol A (X) and B (XI), 1,16-didehydro-2,6-cyclolycopen-5-ol (XII), and lycopene 1,6;2,5-diepoxide (XIII). On the basis of the stereochemistry of the observed products (VIII−XI), here, it is proposed that the acid-catalyzed rearrangement of lycopene 5,6-epoxide may be proceeded by SN1- and/or SN2-type mechanisms, whereas an SN2-type mechanism is preferred for the rearrangement of lycopene 1,2-epoxide. Details of the acid-catalyzed rearrangement of lycopene epoxides by SN1- and SN2-type reactions are presented. Although II and VII−XI are present in tomato products at low concentrations, only X and XI are found in human serum. Possible pathways leading to the formation of the synthetic compounds and the oxidative metabolites of lycopene in tomato products and human serum are discussed. Keywords: 1,16-Didehydro-2,6-cyclolycopen-5-ol; 2,6-cyclolycopene-1,5-diol; 2,6-cyclolycopene-1,5-epoxide; lycopene 1,6;2,5-diepoxide; lycopene 1,2-epoxide; lycopene 5,6-epoxide; lycopene oxidation products; new serum carotenoids; carotenoid metabolites