Characterization of products formed during the autoxidation of β-carotene

The anticarcinogenic actions of carotenoids such as β-carotene has been frequently ascribed to their antioxidant properties. However, very little is actually known about the nature of the antioxidant reaction or the products that are formed. β-Carotene was exposed to either spontaneous autoxidation conditions or to radical-initiated autoxidation conditions. The products were separated by reverse-phase HPLC, and individual peaks were characterized with an on-line diode array detector. Carbonyl products were isolated and characterized by several procedures, including borohydride reduction to the corresponding alcohols, derivatization with O-ethyl-hydroxylamine to the corresponding O-ethyl-oximes of the carbonyls, and analysis by GC-MS. Under the conditions of the experiments, the formation of a homologous series of carbonyl products was demonstrated, including β-apo-13-carotene, retinal, β-apo-14′-carotenal, β-apo-12′-carotenal, and β-apo-10′-carotenal. Several very hydrophobic compounds were formed, which have not been previously identified. In addition, the products of NaOCl-treatment of β-carotene were analyzed, and shown to be significantly different from the autoxidation products. This type of product analysis should be useful in determining the nature of the oxidants reacting with β-carotene in vivo.