HDL Cholesterol Measurement Unaffected by Serum Amyloid A

High-density lipoprotein cholesterol (HDL) is known as the anti-atherogenic lipoprotein. Epidemiological studies have shown that low plasma concentrations of HDL are associated with an increased risk of cardiovascular events. HDL-cholesterol (HDL-C) is the only standardized parameter available to estimate the plasma HDL concentration.

Serum amyloid A (SAA) is a highly conserved, acute phase protein that is predominantly synthesized by the liver. During acute inflammation, SAA concentrations in the serum can increase to up to 1000-fold of the basal levels. Under these conditions, SAA displaces apolipoprotein A-I (apoA-I) in HDL and becomes the major apolipoprotein of circulating HDL.

Clinical biochemists from the Tokyo Medical and Dental University (Tokyo, Japan) and their colleagues obtained anonymized serum samples with various C-reactive protein (CRP) concentrations from 248 patients without clinically apparent liver disease. The SAA concentration of the serum samples were measured by latex agglutination turbidimetric immunoassay, and the serum samples were stratified into three groups, based on their SAA concentrations: low SAA (SAA ≤ 8 μg/mL, n = 94), middle SAA (8 < SAA ≤ 100 μg/mL, n = 37), and high SAA (SAA > 100 μg/mL, n = 117).

The team performed all assays on an automatic analyzer, JCA-BM8000 series. SAA and CRP were measured by the latex agglutination turbidimetric immunoassay using the LZ test ‘Eiken’ SAA and CRP-latexX2, respectively. HDL-C, low-density lipoprotein cholesterol (LDL-C), total cholesterol (TC) and triglyceride (TG) levels were determined using commercial assays. The evaluation of HDL particle size was done with whole lipoproteins isolated by ultracentrifugation from randomly selected serum samples of patients with low or high SAA concentrations and was dialyzed against PBS and subjected to native-gel electrophoresis (native-PAGE) using 8% polyacrylamide gels.

The team reported that HDLs obtained from patients with low SAA concentrations were separated into their general particle sizes and classified as HDL2 and HDL3 by native-gel electrophoresis. On the other hand, HDLs obtained from patients with high SAA concentrations occasionally showed distributions different from the typical sizes of HDL2 and HDL3, such as extremely small or large particles. Nevertheless, HDL-C concentrations measured using the homogeneous assay was strongly correlated with those measured using the ultracentrifugation method, regardless of the SAA concentrations. However, the ratios of HDL-C concentrations obtained by the homogeneous assay to those obtained by the ultracentrifugation method for patients with high SAA concentrations were significantly lower than those of patients with low SAA concentrations.

The authors concluded that structural diversity, such as HDL particle size and SAA distribution, was observed in HDLs obtained from patients with inflammation; however, the HDL-C values measured by homogeneous assay were consistent with those obtained by ultracentrifugation method, regardless of SAA concentrations, an indicator of the presence or absence of inflammation. The study was published in the January 2019 issue of the journal Clinical Biochemistry.

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Tokyo Medical and Dental University