Carotenoid—Protein Complexes

Abstract

The existence in nature of lipochromes solubilized by attachment to protein was known long before lipochromes were chemically characterized as carotenoids. The water-solubility of various pigments from invertebrates was first demonstrated in 1883 by Merejkowsky [1] who also showed that solutions of these pigments underwent profound changes when heated or treated with acid, alkali or alcohol with the liberation of the red zoöerythrin (astaxanthin) (1.1). Newbigin [2,3] at the turn of the century felt that the lipochrome was attached to an unstable organic base but it was not until the 1920s that the non-pigment component was identified as a protein [4–6]. There followed a descriptive period in which the existence of many carotenoproteins was established [see e.g. 7–10]; these pigments are listed in Table 1.1. More detailed studies developed which involved protein purification, the identification of the carotenoid present and the measurement of the absorption spectrum of the complex. The observations at this level are summarized in Table 1.2. During this time it was also becoming clear that two main types of pigments existed, one in which the pigment was bound to the protein in stoicheiometric amounts in non-covalent linkages, and one in which the carotenoid was dissolved in the lipid component of a lipoprotein or lipoglycoprotein. The first group represents true carotenoproteins whilst the latter are better described as carotenoid-lipoproteins or carotenoid-lipoglycoproteins.