Isolation of apolipoproteins from carotenoid-carrying lipoprotein in the serum of chum salmon, Oncorhynchus keta

Carotenoid-carrying lipoprotein (CCL) was rapidly isolated from the high density lipoprotein (HDL) fraction of the upstream migrating male chum salmon (Oncorhynchus keta) by a single-step density gradient ultracentrifugation. The two apolipoproteins (Mr = 24,000 and 12,000; designated apo-I and apo-II, respectively) were readily dissociated and separated in 0.1% SDS by gel filtration chromatography. Prominent features of the amino acid composition in the CCL included the relative high levels of glutamic acid, alanine, leucine, and lysine, and the low cysteine content. Apo-I, as well as the CCL, was rich in glutamic acid, alanine, leucine, and lysine. Compared to the amino acid composition of apo-I, apo-II included relatively high levels of glycine and tyrosine, and low threonine, serine, and arginine contents. When the intact CCL particle was treated with trypsin, apo-I was rapidly proteolyzed, while apo-II was resistant. However, both apo-I and apo-II isolated from the CCL particle were readily digested with trypsin. This suggested that a different structural arrangement rather than the amino acid compositions of the apolipoproteins was associated with the limited trypsin digestion of the CCL particle. Apo-II may be sheltered from the aqueous environment and lie partly within the CCL particle. The properties of both the HDL fraction and apolipoproteins from pink salmon (Oncorhynchus gorbuscha) were similar to those of the CCL from chum salmon.