Stability of bacterial carotenoids in the presence of iron in a model of the gastric compartment - Comparison with dietary reference carotenoids

Recently isolated spore-forming pigmented marine bacteria, Bacillus indicus HU36 and Bacillus firmus GB1 are sources of carotenoids (∼fifteen distinct yellow and orange pigments and ∼thirteen distinct pink pigments, respectively). They are glycosides of oxygenated lycopene derivatives (apo-lycopenoids) and are assumed to be more heat- and gastric-stable than common carotenoids. In this study, the oxidation by O2 of the bacterial carotenoids was initiated by free iron (FeII and FeIII) or by heme iron (metmyoglobin) in a mildly acidic aqueous solution mimicking the gastro-intestinal compartment and compared to the oxidation of the common dietary carotenoids β-carotene, lycopene and astaxanthin. Under these conditions, all bacterial carotenoids appear more stable in the presence of heme iron vs. free iron. Carotenoid autoxidation initiated by FeII is relatively fast and likely involves reactive oxygen-iron species derived from FeII and O2. By contrast, the corresponding reaction with FeIII is kinetically blocked by the slow preliminary reduction of FeIII into FeII by the carotenoids. The stability of carotenoids toward autoxidation increases as follows: β-carotene < lycopene < astaxanthin < HU36 and GB1. In particular, bacterial carotenoids react more quickly than reference carotenoids with FeIII, but much more slowly than the reference carotenoids with FeII. This reaction is correlated with the structure of the carotenoids, which can have opposite effects in a micellar system: bacterial carotenoids with electro-attracting terminal groups have a lower reducing capacity than β-carotene and lycopene. However, their polar head favours their location close to the interface of micelles, in closer contact with oxidative species. Kinetic analyses of the iron-induced autoxidation of astaxanthin and HU36 carotenoids has been performed and gives insights in the underlying mechanisms.