I have previously described [Babin (1987) J. Biol. Chem. 262, 4290-4296] the apolipoprotein composition of the major classes of trout plasma lipoproteins. The present work describes the use of an isopycnic density gradient centrifugation procedure and sequential flotation ultracentrifugation to show: (1) the presence of intermediate density lipoproteins (IDL) in the plasma, between 1.015 and 1.040 g/ml; (2) the existence of a single type of Mr 240,000 apoB-like in the low density lipoproteins (LDL, 1.040 less than p less than 1.085 g/ml); (3) the presence of apoA-I-like (Mr 25,000) in the densest LDL; (4) the adequacy of 1.085 g/ml as a cutoff between the LDL and high density lipoproteins (HDL); (5) the accumulation of Mr 55,000 and 76,000 apolipoproteins and apoA-like apolipoproteins in the 1.21 g/ml infranatant. The fractionation of trout lipoprotein spectrum thus furnishes the distribution of the different lipoprotein classes and leads to the description of the constituent apolipoproteins, which account for about 36% of circulating plasma proteins in this species.
