Iodination of (Tyr11)somatostatin yields a super high affinity ligand for somatostatin receptors in GH4C1 pituitary cells

Journal Article · Tue Nov 01 00:00:00 EST 1988 · Mol. Pharmacol.; (United States)

OSTI ID:6611596

Presky, D H; Schonbrunn, A

GH4C1 cells are a clonal strain of rat pituitary tumor cells which contain high affinity receptors for the inhibitory neuropeptide somatostatin (SRIF). In contrast to other peptides that bind to specific receptors on these cells, receptor-bound (125I-Tyr1)SRIF does not undergo rapid endocytosis. Rather, partial degradation to 125I-tyrosine occurs concomitantly with the dissociation of (125I-Tyr1)SRIF from cell surface receptors. In this study we characterize the binding, biological activity and receptor-mediated degradation of (125I-Tyr11)SRIF, a SRIF analog that is radiolabeled in the center of the molecule. The binding of trace concentrations of (125I-Tyr11)SRIF (less than 50 pM) required 6 hr to reach equilibrium at 37 degrees compared with the 60 min required for (125I-Tyr1)SRIF. Analysis of the kinetics of (125I- Tyr11)SRIF binding showed that the rate constant for association (kon = 1.7 x 10(8) M-8min-1) was similar to that for (125I-Tyr1)SRIF (0.8 x 10(8) M-1min-1). However, the two radioligands exhibited markedly different dissociation kinetics; the koff for (125I-Tyr11)SRIF was 0.002 min-1 compared with the value of 0.02 min-1 for (125I-Tyr1) SRIF. In agreement with its much slower rate of dissociation, (125I-Tyr11)SRIF bound to the SRIF receptor with higher affinity (Kd = 70 pM) than did (125I-Tyr1)SRIF (Kd = 350 pM). However, the apparent ED50 for (I-Tyr11)SRIF to inhibit cAMP accumulation (1.9 +/- 0.4 nM) was greater than the ED50 for SRIF (0.19 +/- 0.04 nM). The low potency of (I-Tyr11)SRIF probably resulted from the fact that subsaturating concentrations of this peptide did not achieve equilibrium binding during the 30-min incubation used to assay biological activity. As previously reported for (125I-Tyr1)SRIF, receptor-bound (125I-Tyr11)SRIF was not internalized and was released from the cells as a mixture of intact (125I-Tyr11)SRIF (30%) and the degradation product 125I-tyrosine (65%).

• Research Organization: Harvard Medical School, Boston, MA (USA)
• OSTI ID: 6611596
• Journal Information: Mol. Pharmacol.; (United States), Vol. 34:5
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
PITUITARY GLAND
NEOPLASMS
SOMATOSTATIN
BIOCHEMICAL REACTION KINETICS
AMP
DOSE-RESPONSE RELATIONSHIPS
IODINATION
IODINE 125
LIGANDS
RATS
RECEPTORS
TRACER TECHNIQUES
TUMOR CELLS
TYROSINE
AMINO ACIDS
ANIMAL CELLS
ANIMALS
BETA DECAY RADIOISOTOPES
BODY
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
DAYS LIVING RADIOISOTOPES
DISEASES
ELECTRON CAPTURE RADIOISOTOPES
ENDOCRINE GLANDS
GLANDS
HALOGENATION
HYDROXY ACIDS
INTERMEDIATE MASS NUCLEI
IODINE ISOTOPES
ISOTOPE APPLICATIONS
ISOTOPES
KINETICS
MAMMALS
MEMBRANE PROTEINS
NUCLEI
NUCLEOTIDES
ODD-EVEN NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANS
PROTEINS
RADIOISOTOPES
REACTION KINETICS
RODENTS
VERTEBRATES
550201* - Biochemistry- Tracer Techniques
550901 - Pathology- Tracer Techniques