Pharmacologically distinct binding sites in rat brain for [3H]thyrotropin-releasing hormone (TRH) and [3H][3-methyl-histidine(2)]TRH.
We have used a directed peptide library, in which the histidyl residue of thyrotropin-releasing hormone (TRH) was systematically replaced by a series of 24 natural and unnatural amino acids, to characterise TRH binding sites in rat brain cortex. This was achieved by measuring the ability of library peptides to compete with [3H][3-Me-His(2)]TRH or [3H]TRH binding to rat cortical homogenates. [3H][3-Me-His(2)]TRH was observed to bind to a single population of high-affinity, low-capacity sites (K(d): 4.54+/-0.62 nM, N=5; B(max): 4.38+/-0.21 fmol/mg wet weight tissue, N=5), consistent with them being central TRH receptors. Displacement studies showed TRH to bind to these sites with an apparent K(i) of 22 nM. K(i) values for the library peptides at [3H][3-Me-His(2)]TRH-labelled sites varied from 10(-3) to 10(-9)M; the potency order was: [3-Me-His(2)]>His>Thi>Leu,Phe,Asn>Gln, Arg, Thr, Ala, HomoPhe. All other replacements had K(i) values >10(-4)M. [3H]TRH was observed to label a single population of low-affinity, high-capacity sites (K(d): 7.55+/-1.23 microM, N=6; B(max): 3.40+/-0.63 pmol/mg wet weight tissue, N=6). The affinities of the synthetic peptides for [3H]TRH-labelled sites did not correlate with their affinities for [3H][3-Me-His(2)]TRH-labelled sites (r=0.33, N=18, P>0.1). They did, however, correlate significantly with previously reported binding affinities for TRH-degrading ectoenzyme (r=0.72, N=12, P<0.01). These results strongly indicate that the identity of the low-affinity, [3H]TRH-labelled site is the membrane-bound enzyme, TRH-degrading ectoenzyme, not a subpopulation of TRH receptors. They also provide the first comprehensive description of the influence of the histidyl residue in TRH on binding of TRH to brain receptors.
Kelly
Julie A
JA
Department of Biochemistry, Trinity College Dublin, Ireland. kellyja@tcd.ie
Slator
Gillian R
GR
O'Boyle
Kathy M
KM
D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D008297:Male; D011761:Pyrrolidonecarboxylic Acid / Q000031:analogs & derivatives; D051381:Rats; D017207:Rats, Sprague-Dawley; D013973:Thyrotropin-Releasing Hormone / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000494:pharmacology; D014316:Tritium
D014316:Tritium; C014109:thyrotropin-releasing hormone, 3-Me-; D013973:Thyrotropin-Releasing Hormone; D011761:Pyrrolidonecarboxylic Acid