The β-Carotene-Oxygen Copolymer: its Relationship to Apocarotenoids and β-Carotene Function

Abstract

{beta}-Carotene spontaneously copolymerizes with molecular oxygen to form a {beta}-carotene-oxygen copolymer compound ("copolymer") as the main product, together with small amounts of many apocarotenoids. Both the addition and scission products are interpreted as being formed during progression through successive free radical {beta}-carotene-oxygen adduct intermediates. The product mixture from full oxidation of {beta}-carotene, lacking both vitamin A and {beta}-carotene, has immunological activities, some of which derive from the copolymer. However, the copolymers chemical makeup is unknown. A chemical breakdown study shows the compound to be moderately stable but nevertheless the latent source of many small apocarotenoids. Although the copolymer alone is only slightly affected by heating at 100{degrees}C for 4 h, in methanol solution it is significantly degraded by hydrochloric acid or sodium hydroxide, liberating many apocarotenoids. GC-MS analysis with mass-spectral library matching identified a minimum of 45 structures, while more than 90 others remain unassigned. Thirteen products are Generally Recognized as Safe (GRAS) human flavor agents. Newly identified products include various small keto carboxylic acids and dicarboxylic acids, several of which are central metabolic intermediates. Also present are the dialdehydes glyoxal and methyl glyoxal, recently reported as {beta}-carotene metabolites in plants. Although both compounds at higher concentrations are known to be toxic, at low concentration methyl glyoxal has been reported to be potentially capable of activating an immune response against microbial infection. In plants, advantage is taken of the electrophilic reactivity of specific apocarotenoids derived from {beta}-carotene oxidation to activate protective defenses. Given the copolymer occurs naturally and is a major product of non-enzymatic {beta}-carotene oxidation in stored plants, by partially sequestering apocarotenoid metabolites the copolymer may serve to limit potential toxicity and maintain low cellular apocarotenoid concentrations for signaling purposes. In animals the copolymer may serve as a systemic source of apocarotenoids.