Antioxidant Activity of β-Carotene Compounds in Different in Vitro Assays

beta-Carotene (BC) is the most abundant carotenoid in human diet, almost solely as (all-E)-isomer. Significant amounts of (Z)-isomers of BC are present in processed food as well as in mammalian tissues. Differences are described for the activity of various BC isomers in forming retinal and protecting against cancer and cardiovascular diseases. Eccentric cleavage of BC leads to degradation products such as carotenals. A variety of negative consequences were published for the non-vitamin A active BC metabolites, such as inducing the carcinogenesis of benzo[a] pyrene, impairing mitochondrial function, or increasing CYP activity. To increase the knowledge on the antioxidant activity, a variety of BC isomers and metabolites were tested in various in vitro assays. In the present study, no ferric reducing activity (FRAP assay) was observed for the BC isomers. Between the major BC isomers (all-E, 9Z, and 13Z) no significant differences in bleaching the ABTS(center dot+) (alpha TEAC assay) or in scavenging peroxyl radicals (ROO center dot) generated by thermal degradation of AAPH (using a chemiluminescence assay) were detected. However, the (15Z)-isomer was less active, maybe due to its low stability. The degradation to beta-apo-carotenoids increased FRAP activity and ROO center dot scavenging activity compared to the parent molecule. Dependence on chain length and character of the terminal function was determined in alpha TEAC assay with following order of increasing activity: beta-apo-8'-carotenal < beta-apo-8'-carotenoic acid ethyl ester < 6'-methyl-beta-apo-6'-carotene-6'-one (citranaxanthin). The results indicate that BC does not lose its antioxidant activity by degradation to long chain breakdown products.