Molecular basis for species and ligand specificity of a monoclonal antibody raised against human IGF-I

The anti-hIGF-I monoclonal antibody, alpha-sm1.2, was found to have substantial crossreactivity with human and rat IGF-II, but recognized rat IGF-I only when this ligand was present at very high concentration. (E50 for hIGF-I approximately 3.5 ng/tube vs. approximately 12,000 ng/tube for rat IGF-I). In the context of previous studies to define the epitope(s) of alpha-sm1.2, these findings point to the critical importance of aspartic acid at residue 20 in the B domain in determining the species and ligand specificity of this antibody. Previous studies using this antibody in rodent tissues may require reinterpretation in the light of these findings.