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Fundamentals intermediate

Amino Acid Transporters

Learn about the SLC family of amino acid transporters, system A/B/L classification, brain transport mechanisms, and intestinal absorption processes.

By Wikipept Community | 3 min read
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Amino Acid Transporters

Amino acid transporters are membrane proteins essential for cellular amino acid uptake and homeostasis. They belong to the Solute Carrier (SLC) family and play critical roles in nutrition, brain function, and cellular metabolism.

Solute Carrier (SLC) Family

The SLC family includes over 400 members organized into 65 families. Key amino acid transporter families include:

SLC1 family (neutral amino acid transporters):

  • SLC1A1-5: High-affinity glutamate transporters
  • SLC1A4-5: Neutral amino acid transporters

SLC7 family (light chains of heterodimeric transporters):

  • SLC7A5 (LAT1): Large neutral amino acid transporter
  • SLC7A11 (xCT): Cystine/glutamate antiporter

SLC38 family (system A and N transporters):

  • SLC38A1-2: System A transporters (neutral amino acids)
  • SLC38A3-5: System N transporters (glutamine, histidine, asparagine)

System A/B/L Classification

Historically, amino acid transport systems were classified by their substrate specificity:

System A (SLC38A family):

  • Neutral amino acids: alanine, serine, proline
  • Sodium-dependent
  • Broad substrate specificity
  • Important in brain and placenta

System ASC:

  • Alanine, serine, cysteine
  • Sodium-independent
  • Broad specificity

System B⁰,⁺:

  • Broad specificity for neutral and basic amino acids
  • Sodium-independent
  • Important in intestine and kidney

System L (SLC7A5/LAT1):

  • Large neutral amino acids: leucine, isoleucine, valine
  • Sodium-independent
  • Exchanges amino acids
  • Critical for brain amino acid supply

Brain Transport Mechanisms

The blood-brain barrier (BBB) restricts amino acid entry into the brain. Key transporters include:

LAT1 (SLC7A5):

  • Primary transporter for large neutral amino acids
  • Essential for tryptophan (serotonin precursor) entry
  • Exchanges intracellular glutamine for extracellular amino acids

System A transporters:

  • Supply small neutral amino acids
  • Important for neuronal function
  • Regulated by insulin and other signals

Transport competition:

  • Amino acids compete for transport at the BBB
  • High protein meals can affect brain amino acid levels
  • This explains why tryptophan supplementation may increase brain serotonin

Intestinal Absorption

The small intestine absorbs amino acids via multiple transport systems:

Apical membrane (lumen to cell):

  • System B⁰,⁺: Broad specificity, sodium-independent
  • SLC38A2: System A, sodium-dependent
  • PEPT1: Di- and tripeptide transporter (H⁺-coupled)

Basolateral membrane (cell to blood):

  • LAT2: System L, bidirectional exchange
  • y⁺L system: Cationic amino acid exchange
  • System asc: Broad specificity

Practical Learning Tip

Mnemonic: “SALT for Transport” - Remember the three main systems: System A (small neutrals), System L (large neutrals), and System B⁰,⁺ (broad specificity). The ‘T’ stands for Transport, helping you remember these are the key systems.

Clinical Significance

Transporter dysfunction causes several diseases:

  • Hartnup disease: Defective neutral amino acid transport in kidney and intestine
  • Cystinuria: Defective dibasic amino acid transport
  • Lysinuric protein intolerance: Defective cationic amino acid transport

Understanding amino acid transport is crucial for nutrition, pharmacology, and developing therapies for metabolic disorders.