Which of the following processes allows apolipoprotein B to be synthesized in the liver as a 100-kDa protein and a 48-kDa protein in the intestine?
**Core Concept**
Apolipoprotein B (ApoB) is a crucial apolipoprotein involved in lipid metabolism. It exists in two main isoforms: ApoB100, synthesized in the liver, and ApoB48, synthesized in the intestine. The synthesis of these isoforms is a critical process in lipid digestion and transport.
**Why the Correct Answer is Right**
The correct answer is **Alternative splicing**. ApoB100 is synthesized through the use of a single gene through alternative splicing, resulting in a 100-kDa protein. In contrast, ApoB48 is produced by the removal of the C-terminal 2/3 of the ApoB100 mRNA, leading to a 48-kDa protein. This process is facilitated by the enzyme **apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3 (APOBEC-3)** in the intestine.
**Why Each Wrong Option is Incorrect**
* **Option A:** **Transcriptional regulation** is not directly responsible for the synthesis of ApoB100 and ApoB48. While transcriptional regulation affects the overall expression of the ApoB gene, alternative splicing is the key process in generating the two isoforms.
* **Option B:** **Translation** is the process by which mRNA is translated into a protein, but it does not account for the different sizes of ApoB100 and ApoB48. Alternative splicing is the critical step in generating these isoforms.
* **Option D:** **Mutation** can affect the expression or function of proteins, but it is not responsible for the synthesis of ApoB100 and ApoB48. Alternative splicing is the mechanism that generates these two isoforms from a single gene.
**Clinical Pearl / High-Yield Fact**
Apolipoprotein B is a key marker of atherosclerotic cardiovascular disease risk. Elevated levels of ApoB100 are associated with increased risk of cardiovascular events, whereas ApoB48 is primarily involved in lipid digestion and absorption in the intestine.
**Correct Answer: C. Alternative splicing**