2.2 Lipids, Lipoproteins, Apolipoproteins, and Coronary Heart Disease Risk Assessment

Lipids, Lipoproteins, Apolipoproteins, and Coronary Heart Disease Risk Assessment

Lipids are water-insoluble biological organic molecules that play essential structural, metabolic, and signaling roles in human physiology. Because lipids are insoluble in aqueous solutions, they are transported throughout the vascular compartment packaged within macromolecular micellar assemblies termed lipoproteins. Derangements in lipid and lipoprotein metabolism represent the central pathophysiological basis for atherosclerotic cardiovascular disease (ASCVD). Medical technologists must master lipid biochemistry, apolipoprotein functions, analytical calculations, and diagnostic dyslipidemia classifications.

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1. Lipid Chemistry and Biological Classification

1. Fatty Acids: Linear hydrocarbon chains terminating in a polar carboxyl group. Classified by hydrocarbon chain length and degree of unsaturation (saturated without double bonds; monounsaturated with one double bond; polyunsaturated with two or more double bonds). Essential fatty acids (linoleic and $\alpha$-linolenic acids) must be obtained from dietary sources.
2. Triglycerides (Triacylglycerols): Composed of one molecule of glycerol esterified with three fatty acid molecules. Represent the primary storage form of metabolic energy in adipose tissue ($>90\%$) and the principal constituent of exogenous dietary fat.
3. Cholesterol: A 27-carbon alicyclic steroid alcohol containing a cyclopentanoperhydrophenanthrene four-ring nucleus. Essential structural component of eukaryotic cell membranes, determining membrane fluidity and lipid raft integrity. Serves as the biochemical precursor for bile acids, vitamin D, and all steroid hormones (adrenocortical glucocorticoids, mineralocorticoids, and gonadal sex steroids). In circulating plasma, cholesterol exists in two distinct pools:
   • Free (Unesterified) Cholesterol (~30%): Amphipathic, with its polar $3\beta$-hydroxyl group positioned on the external monolayer of lipoproteins.
   • Cholesteryl Esters (~70%): Hydrophobic nonpolar core lipids, formed when free cholesterol is esterified at carbon-3 with a fatty acid via the enzyme Lecithin-Cholesterol Acyltransferase (LCAT) in plasma or Acyl-CoA:Cholesterol Acyltransferase (ACAT) intracellularly.
4. Phospholipids: Amphipathic diglycerides containing a glycerol (or sphingosine) backbone, two hydrophobic fatty acid chains, and a polar hydrophilic head group esterified with phosphoric acid and an organic base (e.g., choline forming phosphatidylcholine / lecithin; ethanolamine forming phosphatidylethanolamine; sphingomyelin). Constitute the outer amphipathic monolayer of lipoproteins and biological membranes.

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2. Lipoprotein Architecture, Classes, and Electrophoretic Mobility

Lipoproteins are spherical pseudomicellar particles containing a neutral hydrophobic core of triglycerides and cholesteryl esters surrounded by an amphipathic surface monolayer of phospholipids, unesterified free cholesterol, and apolipoproteins.

Physical and Electrophoretic Properties

Under preparative ultracentrifugation, lipoproteins separate based on hydrated density (inversely related to lipid content and directly related to protein content). On agarose gel electrophoresis in barbital buffer at $\text{pH } 8.6$, lipoproteins migrate according to net surface charge.

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3. Apolipoproteins and Reverse Cholesterol Transport

Apolipoproteins are the specialized protein moieties embedded on the lipoprotein surface. They maintain structural integrity, serve as ligands for cell-surface endocytic receptors, and act as essential activators or inhibitors of regulatory vascular enzymes.

Functional Spectrum of Major Apolipoproteins

• Apolipoprotein A-I (Apo A-I): The major structural protein of HDL ($~70\%$ of HDL protein). Serves as the definitive physiological activator of LCAT. Clinically quantified as an objective index of anti-atherogenic, cardioprotective capacity.
• Apolipoprotein B-48 (Apo B-48): Synthesized exclusively by mucosal enterocytes of the small intestine via post-transcriptional RNA editing of Apo B mRNA (introducing a premature stop codon, yielding 48% of the full-length protein). Lacks the LDL receptor-binding domain. Serves as the obligate structural scaffold for dietary chylomicrons.
• Apolipoprotein B-100 (Apo B-100): Synthesized in hepatocytes as the full-length 550 kDa protein. Present on VLDL, IDL, and LDL. Functions as the critical recognition ligand for the hepatic LDL receptor (Apo B/E receptor). Because each atherogenic particle contains exactly one molecule of Apo B-100, total plasma Apo B-100 measurement reflects the absolute particle number of circulating atherogenic lipoproteins.
• Apolipoprotein C-II (Apo C-II): Essential physiological cofactor and activator of endothelial Lipoprotein Lipase (LPL). In circulating capillaries of adipose tissue and muscle, LPL hydrolyzes core triglycerides in chylomicrons and VLDL to release free fatty acids and glycerol. Hereditary deficiency of Apo C-II leads to severe hyperchylomicronemia.
• Apolipoprotein C-III (Apo C-III): Inhibits lipoprotein lipase and hepatic uptake of remnant lipoproteins; elevated levels correlate with hypertriglyceridemia.
• Apolipoprotein E (Apo E): High-affinity ligand for the hepatic remnant receptor (LDL receptor-related protein, LRP) and the LDL receptor, mediating rapid endocytosis of chylomicron remnants and IDL. Polymorphic with three major alleles: $\epsilon 2$, $\epsilon 3$, and $\epsilon 4$:
  • Apo E2: Defective binding to LDL receptors. Homozygosity ($\text{Apo E2/E2}$) produces Type III Familial Dysbetalipoproteinemia.
  • Apo E4: Associated with elevated plasma LDL cholesterol and represents a major genetic risk factor for late-onset Alzheimer's disease.

Reverse Cholesterol Transport (RCT)

Reverse cholesterol transport is the physiological pathway by which excess unesterified cholesterol is extracted from peripheral cells (including arterial wall macrophage foam cells) and routed to the liver for biliary excretion.

1. Efflux: Cellular free cholesterol is pumped across the macrophage plasma membrane via ABCA1 (ATP-Binding Cassette transporter A1) to lipid-poor Apo A-I, generating nascent pre-$\beta$ HDL. Mature HDL receives additional cholesterol via ABCG1.
2. Esterification: LCAT (activated by Apo A-I) transfers a fatty acyl group from phosphatidylcholine to free cholesterol, generating hydrophobic cholesteryl esters that sequester into the particle core, converting discoidal HDL into spherical mature $\text{HDL}_3$ and $\text{HDL}_2$.
3. Hepatic Clearance: Cholesteryl esters are delivered to the liver either directly through binding to hepatic Scavenger Receptor Class B Type I (SR-BI) or indirectly via Cholesteryl Ester Transfer Protein (CETP), which exchanges HDL cholesteryl esters for triglyceride molecules from VLDL and LDL.

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4. Independent and Emerging Cardiovascular Risk Biomarkers

• Lipoprotein(a) [Lp(a)]: An LDL-like particle in which apolipoprotein B-100 is covalently linked by a disulfide bond to the carbohydrate-rich glycoprotein apolipoprotein(a). Apo(a) exhibits high structural homology to plasminogen, containing multiple kringle-IV repeats. It competitively inhibits plasminogen binding to fibrin, blocking tissue plasminogen activator (tPA)-mediated plasmin generation and blunting fibrinolysis while simultaneously promoting atherogenesis. Plasma Lp(a) levels are $>90\%$ genetically determined, highly refractory to lifestyle or statin therapy, and represent an independent cardiovascular risk factor.
• High-Sensitivity C-Reactive Protein (hs-CRP): An acute-phase pentraxin synthesized by hepatocytes under interleukin-6 stimulation. In the absence of overt systemic infection or trauma, baseline hs-CRP reflects low-grade chronic vascular endothelial inflammation and atherosclerotic plaque instability:
  • Low Risk: $< 1.0\text{ mg/L}$
  • Average Risk: $1.0\text{ to }3.0\text{ mg/L}$
  • High Cardiovascular Risk: $> 3.0\text{ mg/L}$ (Note: hs-CRP values $> 10.0\text{ mg/L}$ indicate acute systemic infection, tissue injury, or autoimmune flares, invalidating cardiovascular risk assessment).

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5. Specimen Requirements and Pre-Analytical Protocol

• Fasting Requirement: Classic teaching (and most board items) specifies a 9 to 12-hour overnight fast for the lipid profile. Total cholesterol changes little after a meal, but triglycerides and calculated LDL-C are distorted by dietary chylomicrons. Current cardiovascular guidelines accept non-fasting profiles for routine screening and call for a fasting repeat when non-fasting triglycerides exceed about $400\text{ mg/dL}$.
• Posture & Tourniquet: Prolonged tourniquet application ($> 1\text{ minute}$) or transitioning from a recumbent to an upright seated position causes intravascular hemoconcentration of large non-diffusible lipoprotein macromolecules, falsely increasing cholesterol and triglyceride concentrations by $10\%\text{ to }15\%$. Venipuncture should occur after the patient has been seated quietly for 5 minutes.
• Serum vs. Plasma: Serum is the primary clinical specimen. If EDTA anticoagulated plasma is utilized, osmotic shifts draw water from erythrocytes into plasma, diluting large lipoproteins and causing measured values to be approximately $3\%$ lower than corresponding serum values (a conversion factor of $1.03$ is applied).

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6. The Friedewald Formula and Methodological Boundaries

Historically, direct quantification of LDL cholesterol required preparative ultracentrifugation (beta-quantification). The Friedewald calculation was developed to estimate LDL-C from routine lipid profile components:

$\text{Total Cholesterol} = \text{HDL-C} + \text{LDL-C} + \text{VLDL-C}$

Assuming that in the fasting state, plasma VLDL contains a relatively constant triglyceride-to-cholesterol ratio of $5:1$ by weight:

$\text{VLDL-C} = \frac{\text{Triglycerides}}{5} \quad (\text{in mg/dL})$

$\text{LDL-C} = \text{Total Cholesterol} - \text{HDL-C} - \frac{\text{Triglycerides}}{5} \quad (\text{in mg/dL})$

In SI units ($\text{mmol/L}$), the formula utilizes a denominator of $2.22$:

$\text{LDL-C} = \text{Total Cholesterol} - \text{HDL-C} - \frac{\text{Triglycerides}}{2.22} \quad (\text{in mmol/L})$

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7. NCEP ATP III Clinical Reference Cutoffs

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8. Fredrickson Classification of Dyslipidemias

• Type I (Familial Hyperchylomicronemia): Inactivating mutations in the Lipoprotein Lipase (LPL) or Apo C-II genes. Marked accumulation of chylomicrons. Standing plasma shows a thick creamy top layer over a crystal-clear infranate. Manifests with eruptive xanthomas, hepatosplenomegaly, and recurrent acute pancreatitis, without accelerated atherosclerosis.
• Type IIa (Familial Hypercholesterolemia): Autosomal dominant mutations in the LDL receptor gene (or Apo B-100 / PCSK9 gain of function). Elevated LDL with normal VLDL. Standing plasma is completely clear. Clinically marked by severe premature coronary artery disease, tendon xanthomas (Achilles tendon), and xanthelasma.
• Type IIb (Familial Combined Hyperlipidemia): Overproduction of hepatic Apo B-100. Concomitant elevation of LDL and VLDL. Standing plasma is clear to slightly hazy. High coronary risk.
• Type III (Familial Dysbetalipoproteinemia / Broad-Beta Disease): Homozygosity for Apo E2/E2, preventing normal hepatic receptor-mediated clearance of chylomicron remnants and IDL. Characterized by accumulating IDL and $\beta$-VLDL. Standing plasma is turbid, occasionally with a faint cream layer. Pathognomonic physical findings include palmar xanthomas (xanthoma striatum palmare) and tuberoeruptive xanthomas.
• Type IV (Familial Hypertriglyceridemia): Increased hepatic secretion of VLDL with delayed catabolism. Standing plasma is turbid / milky throughout with no floating cream layer. Strongly associated with obesity, insulin resistance, and type 2 diabetes.
• Type V (Mixed Hyperlipidemia): Combined elevation of chylomicrons and VLDL. Standing plasma demonstrates a floating creamy top layer over a turbid infranate. High risk of acute pancreatitis.