Metabolism of three stereoisomers of astaxanthin in the fish, rainbow trout and tilapia.

In rainbow trout (Salmo gairdneri), the dietary astaxanthin diesters were mostly absorbed and accumulated in their integuments keeping their configurations, and partially metabolized to (3R, 3'R)-zeaxanthin (major) and/or (3R, 3'S)-zeaxanthin (medium) and/or (3S,3'S)-zeaxanthin (minor). In tilapia (Tilapia nilotica), the three stereoisomers of astaxanthin diesters were promptly metabolized to only (3S,3'S)-astaxanthin, and subsequently to (3R,3'R)-zeaxanthin and/or (3R,3'S)-zeaxanthin and/or (3S,3'S)-zeaxanthin at an invariable ratio, 4:1:0.3. The above facts indicate that the conversion from 3S- to 3R-configuration was carried out in vivo, and vice versa, and that astaxanthins were reductively metabolized to zeaxanthins in both the fish.