title: Turnover of 125I-VLDL and 131I-LDL apolipoprotein B in rabbits fed diets containing casein or soy protein.
abstract: Rabbits fed low-fat, cholesterol-free, semi-purified diets containing casein developed a marked hypercholesterolemia compared to rabbits fed a similar diet containing soy protein (plasma cholesterol 281 +/- 31 vs. 86 +/- 9 mg/dl; P less than 0.05). Turnover studies (three per dietary group) were carried out in which homologous 125I-labeled VLDL and 131I-labeled LDL were injected simultaneously into casein- (n = 8) or soy protein- (n = 9) fed rabbits. ApoB-specific activities were determined in VLDL, IDL and LDL isolated from the pooled plasma of two or three rabbits per dietary group. The production rate of VLDL apoB (1.20 +/- 0.3 vs. 1.09 +/- 0.1 mg/h per kg) was similar for the two dietary groups. The fractional catabolic rate of VLDL apoB was lower for the casein group (0.15 +/- 0.03 vs. 0.23 +/- 0.01.h-1; 0.05 less than P less than 0.10). Although the pool size of VLDL apoB was higher in the casein group (8 +/- 2 vs. 5 +/- 0.3 mg/kg), this value did not reach statistical significance. For LDL apoB, the increased pool size in casein-fed rabbits (30 +/- 5 vs. 5 +/- 1 mg/kg; P less than 0.01) was associated with a decreased fractional catabolic rate (0.03 +/- 0.005 vs. 0.08 +/- 0.008.h-1; P less than 0.01) and a 2-fold increase in the production rate of LDL apoB (1 +/- 0.3 vs. 0.4 +/- 0.06 mg/kg per h; 0.05 less than P less than 0.10) compared to rabbits fed soy protein. Analysis of precursor-product relationships between the various lipoprotein fractions showed that casein-fed rabbits synthesized a higher proportion of LDL apoB (95% +/- 2 vs. 67% +/- 2; P less than 0.001) independent of VLDL catabolism. These results support the concept that the hypercholesterolemia in casein-fed rabbits is associated with impaired LDL removal consistent with a down-regulation of LDL receptors. These changes do not occur when the casein is replaced by soy protein.
authors.affiliation: Department of Biochemistry, University of Western Ontario, London, Canada.
authors.forename: P
authors.initials: P
authors.lastname: Khosla
authors.forename: S
authors.initials: S
authors.lastname: Samman
authors.forename: K K
authors.initials: KK
authors.lastname: Carroll
authors.forename: M W
authors.initials: MW
authors.lastname: Huff
mesh_terms: D000818:Animals; D001055:Apolipoproteins B / Q000097:blood*; D002364:Caseins / Q000008:administration & dosage / Q000494:pharmacology*; D002784:Cholesterol / Q000097:blood; D004044:Dietary Proteins / Q000494:pharmacology*; D007457:Iodine Radioisotopes; D007700:Kinetics; D008074:Lipoproteins / Q000097:blood; D053419:Lipoproteins, IDL; D008077:Lipoproteins, LDL / Q000097:blood*; D008079:Lipoproteins, VLDL / Q000097:blood*; D008297:Male; D014674:Plant Proteins, Dietary / Q000008:administration & dosage / Q000494:pharmacology*; D011817:Rabbits; D030262:Soybean Proteins; D014280:Triglycerides / Q000097:blood
chemical_list: D001055:Apolipoproteins B; D002364:Caseins; D004044:Dietary Proteins; D007457:Iodine Radioisotopes; D008074:Lipoproteins; D053419:Lipoproteins, IDL; D008077:Lipoproteins, LDL; D008079:Lipoproteins, VLDL; D014674:Plant Proteins, Dietary; D030262:Soybean Proteins; D014280:Triglycerides; D002784:Cholesterol