A key functional role for the insulin-like growth factor 1 N-terminal pentapeptide.
In order to elucidate the role of the N-terminus of insulin-like growth factor 1 (IGF-1) with respect to its biological properties, we chemically synthesized analogues of IGF-1 truncated by one to five amino acid residues from the N-terminus. In a bioassay that measured the stimulation of protein synthesis in rat L6 myoblasts, the concentrations required to produce a half-maximal response were: IGF-1, 13 ng/ml; des-(1)-IGF-1, 10 ng/ml; des-(1-2)-IGF-1, 13 ng/ml; des-(1-3)-IGF-1, 1.5 ng/ml; des-(1-4)-IGF-1, 5.1 ng/ml; des-(1-5)-IGF-1, 1200 ng/ml. When tested for their abilities to compete with 125I-IGF-1 binding to L6 myoblasts at 3 degrees C, the concentrations required for 50% competition were: IGF-1, des-(1)-IGF-1 and des-(1-2)-IGF-1, 20 ng/ml; des-(1-3)-IGF-1, 14 ng/ml; des-(1-4)-IGF-1, 40 ng/ml; des-(1-5)-IGF-1, greater than 1000 ng/ml. Receptor-binding experiments at 25 degrees C, however, gave results suggesting that the myoblasts were secreting a binding protein selective for the three longest peptides. This interpretation was confirmed by binding studies with medium conditioned by the L6 myoblasts as well as binding protein purified from MDBK-cell-conditioned medium. In both cases IGF-1, des-(1)-IGF-1 and des-(1-2)-IGF-1 competed for tracer IGF-1 binding at least 60-fold better than did the three shorter peptides. The results obtained account for the increased potency of des-(1-3)-IGF-1 and des-(1-4)-IGF-1, since their activities are not attenuated by the binding protein, and the relatively lower potency of des-(1-4)-IGF-1 is a consequence of this peptide binding less well to the L6-myoblast receptor.
Bagley
C J
CJ
Department of Biochemistry, University of Adelaide, South Australia.
May
B L
BL
Szabo
L
L
McNamara
P J
PJ
Ross
M
M
Francis
G L
GL
Ballard
F J
FJ
Wallace
J C
JC
D000818:Animals; D001667:Binding, Competitive; D001681:Biological Assay; D007334:Insulin-Like Growth Factor I / Q000031:analogs & derivatives / Q000502:physiology*; D009842:Oligopeptides / Q000378:metabolism*; D014176:Protein Biosynthesis; D051381:Rats; D013002:Somatomedins / Q000502:physiology*
D009842:Oligopeptides; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I