Which amino acids are absorbed in enterocytes through Meister cycle?
**Core Concept:** The Meister cycle, also known as the Transporter-Enzyme-Peptide cycle, is a process by which amino acids are absorbed in enterocytes (intestinal epithelial cells) through various transporters and enzymes. This process enables the absorption of essential amino acids, which are essential for protein synthesis and other cellular functions.
**Why the Correct Answer is Right:** The correct answer refers to the specific amino acids absorbed through the Meister cycle. In this case, the correct answer is:
**Correct Answer: Option D - L-Amino acids**
The Meister cycle involves several transporters and enzymes, including:
1. Amino acid transporters: These transport the amino acids across the apical (cell surface) membrane of the enterocytes. In the context of the Meister cycle, this includes the L-type amino acid transporters (LATs).
2. Peptide transporters: These transport di- and tripeptides across the basolateral (cell interior) membrane. The correct answer, L-amino acids, includes both di- and tripeptides.
3. Peptide transaminases: These enzymes catalyze the formation of α-keto acids and α-aminos from di- and tripeptides.
4. Amino acid transaminases: These enzymes catalyze the formation of α-keto acids and α-aminos from free amino acids.
5. Peptide-γ-glutamyl transpeptidase: This enzyme is responsible for the formation of γ-glutamyl peptides from amino acids and di- and tripeptides.
**Why Each Wrong Option is Incorrect:**
A. **Option A - Dipeptides**: Dipeptides are absorbed through a different mechanism, the dipeptide transporters, which are not part of the Meister cycle.
B. **Option B - Tripeptides**: Tripeptides are absorbed through peptide transporters, not the Meister cycle.
C. **Option C - Amino acid transporters**: While amino acid transporters are involved in the process, the correct answer (L-amino acids) includes di- and tripeptides as well.
**Clinical Pearls:**
Understanding the Meister cycle is crucial for understanding intestinal amino acid and peptide absorption. This process is essential for maintaining amino acid homeostasis in the body, particularly during starvation or malnutrition, when the body relies on the breakdown of proteins to maintain essential amino acid levels.
**Why the Correct Answer is Right:**
The Meister cycle is a crucial part of the process of amino acid and peptide absorption in enterocytes. L-amino acids are absorbed through this cycle due to the involvement of amino acid transporters, peptide transaminases, and peptide-γ-glutamyl transpeptidase. This process ensures efficient absorption of amino acids, particularly during starvation or malnutrition, when the body relies on the breakdown of proteins to maintain essential amino acid levels.