A novel nonopioid action of enkephalins: competitive inhibition of the mammalian brain high affinity L-proline transporter.

Fremeau RT, Velaz-Faircloth M, Miller JW, Henzi VA, Cohen SM, Nadler JV, Shafqat S, Blakely RD, Domin B
Mol Pharmacol. 1996 49 (6): 1033-41

PMID: 8649341

The high affinity L-proline transporter (PROT) is a member of the family of Na+ (and Cl-)-dependent plasma membrane transport proteins that comprises transporters for several neurotransmitters, osmolytes, and metabolites. The brain-specific expression of PROT in a subset of putative glutamatergic pathways implies a specialized function for this novel transporter and its presumed natural substrate L-proline in excitatory synaptic transmission. However, definitive studies of the physiological role(s) of high affinity L-proline uptake have been precluded by the lack of specific uptake inhibitors. Here, we report that Leu- and Met-enkephalin and their des-tyrosyl derivatives potently and selectively inhibited high affinity L-proline uptake in rat hippocampal synaptosomes and in PROT-transfected HeLa cells. High concentrations of the opiate receptor antagonist naltrexone did not block the inhibitory actions of these peptides, arguing against an involvement of opioid receptors. Des-tyrosyl-Leu-enkephalin elevated the apparent K(m) of L-proline transport in transfected HeLa cells without altering the V(max). PROT-transfected HeLa cells did not accumulate [3H]Leu-enkephalin above background levels, demonstrating that enkephalins are not substrates for PROT. These findings indicate that enkephalins competitively inhibit mammalian brain PROT through a direct interaction with the transporter protein at or near the L-proline binding site. The high potency and specificity of des-tyrosyl-Leu-enkephalin make this compound a useful tool for elucidating the structure-function properties and physiological role(s) of PROT.

MeSH Terms (14)

Amino Acid Transport Systems, Neutral Animals Brain Enkephalins HeLa Cells Humans Male Membrane Transport Modulators Membrane Transport Proteins Naltrexone Proline Rats Rats, Sprague-Dawley Structure-Activity Relationship

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