LAUSR

The binding of carotenoids to the myofibrillar protein F-actin purified from the white muscle of Atlantic salmon (Salmo salar L.) was studied using in vitro reconstitution. The binding of astaxanthin and canthaxanthin was saturable, and analysis revealed the presence of a single carotenoid-binding site. The dissociation constants (Kd) for actin prepared from 2.5 Kg FW fish were 1.04 ± 0.13 μg carotenoid mg-1 actin and 0.54 ± 0.11 μgmg-1 for astaxanthin and canthaxanthin, respectively. The saturation binding level (Bmax) for astaxanthin was 1.39 ± 0.07 μgmg-1 and 1.04 ± 0.08 μgmg-1 for canthaxanthin. These values were higher for F-actin prepared from organic and small (~0.5 Kg FW) salmon than for non-organic and larger, mature fish. The structural specificity of carotenoid binding revealed a preference for carotenoids that possess a keto group at C-4 on the end-group of the molecule, but the presence of hydroxyl groups at C-3 or C-4 reduced overall binding efficiency. The study suggests that the ability of myofibrillar proteins to bind carotenoids is not a limiting factor governing the deposition of carotenoids in the muscle of salmonids.