Скачать или смотреть 17. Transport of Ammonia to the Liver | Nitrogen Metabolism | Medical Biochemistry | USMLE Step 1

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Transport of Ammonia to the Liver | Nitrogen Metabolism | Medical Biochemistry | MBBS | USMLE Step 1

👋 Hey future doctors and biochemistry learners! Did you know that ammonia (NH₃) produced during amino acid breakdown is highly toxic to the brain and must be transported safely to the liver for detoxification? 🧠☠️ The body uses clever strategies to prevent ammonia buildup, and this process is a high-yield concept for MBBS, NEET PG, FMGE, and USMLE Step 1 exams.

In this lecture, we’ll break down exactly how nitrogen is transported from peripheral tissues to the liver, where it enters the urea cycle for safe excretion. 🧬💧

🌟 Pathways of Ammonia Transport

1️⃣ Glutamine Pathway 🚚

In most tissues, ammonia is combined with glutamate → Glutamine (enzyme: glutamine synthetase).

Glutamine acts as a non-toxic carrier of ammonia in the blood.

In the liver and kidney, glutaminase releases ammonia for the urea cycle or acid-base balance.

2️⃣ Alanine (Glucose-Alanine or Cahill Cycle) 🔄

In muscle, amino groups from breakdown of proteins are transferred to pyruvate → Alanine.

Alanine travels to the liver, where it is transaminated back to pyruvate.

Pyruvate is used for gluconeogenesis, while the amino group enters the urea cycle.

This cycle links muscle protein breakdown ↔ liver glucose production.

3️⃣ Direct Ammonia Transport (minor role)

Free ammonia can move via the blood, but only in very small amounts due to toxicity.

🩺 Clinical Correlations

Hyperammonemia → results from defects in ammonia transport or urea cycle → confusion, vomiting, cerebral edema, coma.

Glutamine accumulation → toxic to the brain (seen in hepatic encephalopathy).

Cahill cycle importance → explains muscle wasting in chronic illness and starvation.

Urea cycle disorders → often present in infancy with severe hyperammonemia.

🎯 Exam Integration

Alanine and Glutamine are the major nitrogen carriers in blood.

Glutamine synthetase and glutaminase are key enzymes.

Linking concepts: transamination → ammonia transport → urea cycle.
👉 This is a favorite for case-based USMLE & NEET PG questions.

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