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Functional Characterization of Na+/H+ Exchangers of Intracellular Compartments Using Proton-killing Selection to Express Them at the Plasma Membrane
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Molecular and functional identification of sodium ion-dependent, high affinity human carnitine transporter OCTN2

I Tamai1, R Ohashi, J Nezu

  • 1Faculty of Pharmaceutical Sciences, Kanazawa University, 13-1 Takara-machi, Kanazawa 920-0934, Japan.

The Journal of Biological Chemistry
|August 1, 1998
PubMed

Insights

Researchers identified OCTN2 as the human carnitine transporter responsible for primary carnitine deficiency. This high-affinity, sodium-dependent transporter is crucial for carnitine uptake in vital tissues.

Area of Science:

  • Molecular Biology
  • Human Physiology
  • Genetics

Background:

  • Primary carnitine deficiency results from defects in plasma membrane carnitine transporters, leading to severe symptoms.
  • The specific molecular identity of these carnitine transporters remained unknown.

Purpose of the Study:

  • To identify the molecular basis of carnitine transport defects.
  • To characterize the function of the identified transporter, OCTN2, in carnitine uptake.

Main Methods:

  • Screened a human kidney cDNA library to identify a carnitine transporter homologue.
  • Examined the function of the identified OCTN2 cDNA by expressing it in HEK293 cells.
  • Analyzed substrate specificity and transport kinetics of OCTN2-mediated carnitine uptake.

Main Results:

  • Identified and cloned OCTN2 cDNA, encoding a 557-amino acid polypeptide with high similarity to OCTN1.
  • Northern blot analysis revealed strong OCTN2 expression in kidney, skeletal muscle, heart, and placenta.
  • Expressed OCTN2 demonstrated sodium-dependent L-carnitine uptake with high affinity (Km = 4.34 microM), distinguishing it from other organic cation transporters.

Conclusions:

  • OCTN2 is identified as a high-affinity, sodium-dependent carnitine cotransporter in humans.
  • The tissue distribution and functional characteristics of OCTN2 align with known carnitine transport activity.
  • OCTN2 plays a critical physiological role in carnitine transport, and its defects likely cause primary carnitine deficiency.

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