In vivo metabolism of apolipoprotein S in humans. Comparison with apolipoprotein A-I metabolism

High-density lipoprotein apolipoprotein A-I kinetics: comparison of radioactive and stable isotope studies

To compare the kinetic determinants of high-density lipoprotein (HDL) apolipoprotein A-I (apoA-I) concentration in lean normolipidaemic subjects using radioisotope and stable isotope studies. We pooled data from 16 radioisotope and 13 stable isotope studies to investigate the kinetics of apoA-I in lean normolipidemic individuals. We also examined the associations of HDL kinetic parameters with age, sex, body mass index (BMI) and concentrations of apoA-I, triglycerides, HDL cholesterol and low-density lipoprotein (LDL) cholesterol. Lean subjects from radioisotope and stable isotope studies were matched for age, gender, BMI and lipid profile. The apoA-I concentration was significantly lower in the radioisotope group than the stable isotope group (P = 0.031). There was no significant difference in HDL apoA-I fractional catabolic rate (FCR) and production rate (PR) between the groups. In the radioisotope group, HDL apoA-I FCR was significantly associated with apoA-I and HDL cholesterol concentrations (r = -0.681, P < 0.001 and r = -0.542, P < 0.001, respectively), whereas in the stable isotope group, only HDL apoA-I PR was significantly associated with apoA-I concentration (r = 0.455, P = 0.004). Our findings suggest that HDL apoA-I FCR is the primary determinant of apoA-I concentrations in lean subjects in studies using radiotracer techniques. By contrast, HDL apoA-I PR is the primary determinant of apoA-I concentration in lean subject in studies employing stable isotope methods. These discrepancies may be reconciled by differences in methodologies and/or study population characteristics.

Apolipoprotein C-II and C-III metabolism in a kindred of familial hypobetalipoproteinemia

Three affected members of a kindred with asymptomatic hypobetalipoproteinemia (HBL) showed low levels of triglycerides, low-density lipoprotein (LDL)-cholesterol, and apolipoproteins (apo) B, C-II, and C-III. Turnover of iodine-labeled apo C-II and apo C-III associated in vitro to plasma lipoproteins was studied after intravenous injection. Radioactivity in plasma and lipoproteins (95% recovered in high-density lipoprotein [HDL] density range) and in 24-hour urine samples was observed for 16 days. A parallelism of the slowest slopes of plasma decay curves was observed between apo C-II and apo C-III, indicating a partial common catabolic route. Urine/plasma radioactivity ratio (U/P) varied with time, suggesting heterogeneity of metabolic pathways. A new compartmental model using the SAAM program was built, not only fitting simultaneously plasma and urine data, but also taking into account the partial common metabolism of lipoprotein particles (LP) containing apo C-II and apo C-III. The low apo C-II and C-III plasma concentrations observed in HBL compared with normal resulted from both an increased catabolism and a reduced synthesis, these changes being more marked for apo C-III. The modifications in the rate constants of the different pathways calculated from the new model are in favor of an increased direct removal of particles following the fast pathway, likely in the very-low-density lipoprotein (VLDL) density range.

Variations in apolipoproteins serum amyloid A, A-I, A-II, and C-III in severely head-injured patients

In five severely head-injured patients we determined the plasma concentrations of apolipoproteins serum amyloid A, A-I, A-II, C-III, and B, prealbumin and C-reactive protein on day 1, 5, 10 and 15 after head injury where possible. A dramatic increase in apolipoprotein serum amyloid A up to a mean plasma level of 0.764 g/l was accompanied by a considerable decrease in apolipoprotein A-I, apolipoprotein A-II and apolipoprotein C-III concentrations. The variations observed by immunological methods were confirmed by two-dimensional gel electrophoresis performed on plasma and different lipoprotein fractions. In addition to its association with high density lipoproteins, apolipoprotein serum amyloid A was also found with lipoproteins of low and very low density. Two-dimensional electrophoresis also showed the presence of several different serum amyloid A-peptides not seen in plasmas from healthy subjects. We propose that apolipoprotein serum amyloid A may be responsible for the decrease of the main HDL apolipoproteins in head-injured patients.

Characterization of anti-apolipoprotein A-I monoclonal antibodies and their use in the measurement of apolipoprotein A-I by a two-site enzyme immunoassay

Six monoclonal antibodies raised against pure apo A-I or high density lipoprotein were characterized for epitope specificity by enzyme and radioimmunoassays, immunodiffusion and affinity chromatography. The six antibodies were classified into three groups according to the region of apo A-I they reacted with. The antibody VI10H, from group II, appeared to recognize a region fully exposed on native HDL-apo A-I, whereas group I comprised antibodies specific for a partially masked region. Group III comprised only one antibody. Use of the non-ionic detergent Tween 20 in the immunoassays permitted antibodies from the three groups to react with their respective epitope on native HDL-apo A-I. An antibody from group I (V4F) was chosen as the first antibody and VI10H, the antibody showing the highest affinity, was chosen for the anti-A-I-peroxidase conjugate in a two-site enzyme immunoassay.

Non-competitive enzyme-linked immunosorbent assay for human apolipoprotein SAA or S

Purified antibodies against human apolipoprotein S were used to measure apo S levels in human plasma by a sandwich ELISA technique. The method developed was highly sensitive and specific. A reliable calibration of a plasma standard was obtained by reassociating the purified protein with plasma HDL. A significant correlation (r = 0.69) was found between plasma apo S and CRP values. This correlation coefficient reached 0.97 when the plasma samples were selected according to additional clinical and laboratory criteria.