True carotenoproteins are defined as complexes in which a carotenoid and a protein are in stoichiometric combination. Very few carotenoproteins have been purified, but their presence in extracts may often be presumed from spectroscopic evidence. Carotenoids have been found in stoichiometric association with simple proteins, lipoproteins and glycoproteins. Many lipoproteins also occur in which carotenoids form part of the lipid prosthetic group. Although the relationship between the carotenoid and the remainder of the complex may in these cases not appear to be stoichiometric, it often shows some evidence of selectivity.

Carotenoproteins are widely distributed among the invertebrate phyla. They are found mainly in exoskeleton or ectoderm and in eggs and ovaries. Their isolation by standard techniques of protein chemistry is facilitated by their characteristic spectral properties.

The ease with which carotenoid prosthetic groups are removed from apoproteins suggests that covalent linkages are not involved in the complex formation. The union of carotenoid with apoprotein has been shown, in at least two cases, to stabilize the carotenoid against photo-oxidation and the protein against denaturative changes of tertiary structure. In crustacyanin, the carapace pigment of the lobster, there is clear evidence that the carotenoid is responsible for establishing the very complex quaternary structure.

Only astaxanthin, astaxanthin esters and canthaxanthin have been unequivocally identified as carotenoid prosthetic groups of true carotenoproteins. Experiments in the specificity of interaction of a series of carotenoids with the apoprotein of crustacyanin have indicated the requirement of one or both of the 4- and 4′-keto groups of the carotenoid for interaction with the protein.

It is suggested that, at least in this case, the carotenoid-protein interaction might occur between carotenoid keto groups and basic residues of the protein in such a way as to provide a lock on the tertiary configuration.

Carotenoproteins may participate in a variety of functions including protective coloration, photosensitivity, electron transport and enzymic activity. They appear, furthermore, to play some part in development.