Catfish plasma lipoproteins: In vivo studies of apoprotein synthesis and catabolism

Abstract

AbstractThree distinct lipoprotein classes, very‐low‐density (VLDL), low‐density (LDL), and high‐density (HDL), were isolated from plasma of channel catfish, Ictalurus punctatus. HDL was the major lipoprotein class, accounting for 22–32% of the total plasma protein. The major peptide of VLDL and LDL was an apoB‐like protein (molecular mass 250,000 daltons). When a protease inhibitor was added to the plasma, there was no evidence of apoC‐ or apoE‐like proteins in catfish VLDL. The major peptide of HDL in male catfish was an apoAI‐like peptide (molecular mass 25,000 daltons). The major peptide of the HDL of female catfish undergoing vitellogensis had a molecular mass of 92,000 daltons. Plasma lipoprotein metabolism was investigated by measurement of plasma titers and the synthesis and catabolic rates of apoVLDL, apoLDL, and apoHDL. Apoprotein synthesis rates (ks) were measured in fed, 3‐day‐fasted, and 20‐day‐fasted channel catfish by continuous infusion of 14C‐tyrosine. In vivo synthesis rates of plasma apoproteins have not been previously measured. Analysis of data required the derivation of a series of equations that accounted for the kinetics of secretion of plasma proteins and their equilibration with an extravascular compartment. The plasma titer of apoVLDL was closely correlated to dietary intake, although apoVLDL synthesis rates did not significantly change with fasting. ApoLDL plasma titers significantly decreased after a 20‐day fast but not after a 3‐day fast. Fasting resulted in a significant increase in apoLDL synthesis. The apoHDL titer and synthesis rates were not significantly affected by fasting. Male and female fish have similar apoVLDL and apoLDL titers and synthesis rates. ApoHDL plasma titers and synthesis rates were significantly higher in females (14.0 ± 1.6 mg ml−1; ks = 34.9% ± 15.3% day−1) than in males (9.0 ± 1.6 mg ml−1; ks = 8.7% ± 2.6% day−1). The higher apoHDL titer and synthesis rate in female fish were due to the presence of a peptide (molecular mass 92,000 daltons) not present in males.