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Non-stedy-state studies of low-density-liproprotein turnover in familial hypercholesterolaemia
Insights
Familial hypercholesterolaemia involves an intrinsic defect in low-density lipoprotein (LDL) catabolism. Even after plasma exchange, fractional catabolic rates remain constant, indicating impaired LDL breakdown in affected individuals.
Area of Science:
- Metabolic disorders
- Cardiovascular research
- Lipoprotein metabolism
Background:
- Familial hypercholesterolaemia (FH) is a genetic disorder characterized by extremely high levels of low-density lipoprotein (LDL) cholesterol.
- Understanding the precise mechanisms of LDL metabolism is crucial for developing effective treatments for FH.
Purpose of the Study:
- To investigate the non-steady-state turnover of LDL apoprotein B (LDL-apo-B) in patients with familial hypercholesterolaemia.
- To determine if an intrinsic defect in LDL catabolism exists in FH, particularly in homozygous individuals.
Main Methods:
- Utilized 131I-labelled LDL-apo-B to study turnover in four FH patients (three homozygotes, one heterozygote).
- Measured fractional catabolic rates (FCR) and absolute catabolic rates (ACR) using urinary and whole-body radioactivity counting.
- Assessed LDL-apo-B synthesis by combining ACR with daily plasma LDL increments.
Main Results:
- Fractional catabolic rates (FCR) for LDL-apo-B remained stable despite significant reductions in LDL pool size achieved through plasma exchange.
- This stability in FCR indicates an intrinsic defect in LDL catabolism in FH patients.
- LDL-apo-B synthesis rates were markedly elevated in homozygous FH patients compared to the single heterozygote, who exhibited normal synthesis.
Conclusions:
- The findings confirm an intrinsic defect in LDL catabolism as a key feature of familial hypercholesterolaemia.
- The study highlights the utility of combining plasma exchange with whole-body radioactivity counting for analyzing lipoprotein turnover and pool size dynamics.
Abstract:
1. The non-steady-state turnover of low-density lipoprotein (LDL), labelled in its apoprotein moiety (apo-B) with 131I, was determined in four patients with familial hypercholesterolaemia, three of them homozygotes. 2. The fractional and absolute catabolic rates (FCR and ACR) of LDL-apo-B were determined by relating the excretion of radioactivity, measured in urine in vitro and by whole-body counter in vivo, to plasma radioactivity and to LDL specific radioactivity respectively. 3. The FCR remained relatively constant, even after marked reduction of LDL pool size by means of plasma exchange. This confirms the existence of an intrinsic defect in LDL catabolism in familial hypercholesterolaemia. 4. LDL-apo-B synthesis, determined by summing the ACR and the daily increment in plasma LDL, was much higher in the three homozygotes than in the one heterozygote, in whom the synthetic rate was normal. 5. These results illustrate the usefulness of combining plasma exchange and whole-body radioactivy counting as a means of examining the relationship between the turnover and pool size of a 131I-labelled protein, such as LDL.