Carotenoid Properties Define Primary Biological Actions and Metabolism Defines Secondary Biological Actions

Abstract

The biological properties of carotenoids that can be observed and measured in animals, including humans, in plants, in cells grown in culture, in the microbial worlds, or in the classical “test tube” experiments should all be related to their chemical and physical properties (Britton, 1995; Krinsky, 1993). For example, beta-carotene and lycopene, shown in Figure 1, are both C₄₀H₅₆ hydrocarbons which share many similar properties. They are very hydrophobic, each have 11 conjugated double bonds, although in the case of lycopene, they are all in a linear polyene chain, whereas with β-carotene, only 9 double bonds are in the linear chain, and 2 additional conjugated double bonds are in the rings. As determined by x-ray crystal analysis of beta-carotene, the ring double bonds are slightly out of plane because of the steric hindrance of the methyl group substituents on and near the ring. In addition, because lycopene is an acyclic compound, it has 2 more non-conjugated double bonds at the termini of the molecule. As will be discussed in section 3, these two carotenoids have different biological actions.