Question

The lipoprotein responsible for VLDL secretion from liver is

• A. Apo B-40
• B. Apo B-100
• C. Apo A-I
• D. Apo A-II

Hint:

Remember:
- Apo B-48 is intestinal (chylomicrons).
- Apo B-100 is hepatic (VLDL, IDL, LDL).
They are synthesized from the exact same gene but edited differently in different tissues.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Apolipoproteins are the protein components of lipoprotein complexes.
They help solubilize hydrophobic lipids, maintain the structural integrity of lipoprotein particles, and bind to specific cell-surface receptors to regulate lipid clearance from the bloodstream.

Step 2: Detailed Explanation:
Apolipoprotein B (Apo B) is a large, hydrophobic glycoprotein that exists in two primary isoforms: Apo B-48 and Apo B-100.
These two isoforms are produced from a single gene via a tissue-specific post-transcriptional mRNA editing process mediated by the enzyme cytidine deaminase.
- Apo B-48: In the enterocytes of the small intestine, mRNA editing introduces a premature stop codon. This results in a shorter protein ($48\%$ of the full-length sequence) that lacks the LDL-receptor binding domain. It is required for the assembly and secretion of chylomicrons, which transport dietary lipids.
- Apo B-100: In hepatocytes of the liver, the mRNA is not edited, resulting in the translation of the full-length protein. Apo B-100 contains the ligand domain that binds to the LDL receptor. It serves as the primary structural protein required for the assembly, packaging, and secretion of Very Low-Density Lipoproteins (VLDL) into the circulation.
VLDL particles transport endogenously synthesized triglycerides and cholesterol from the liver to peripheral tissues.
If Apo B-100 synthesis or assembly is disrupted, VLDL cannot be secreted, causing triglycerides to accumulate within hepatocytes, which can lead to hepatic steatosis (fatty liver).

Step 3: Final Answer:
Apolipoprotein B-100 is the protein responsible for the secretion of VLDL from the liver.