Heterogeneity of 125I-labeled epidermal growth factor

Highly purified epidermal growth factor (EGF) was iodinated, and the labeled product with the same isoelectric point as underivatized EGF was isolated by isoelectric focusing. When the 125I-labeled EGF was analyzed by reverse-phase chromatography, the resulting profile of 125I activity was much broader than the profile obtained with underivatized EGF. Rechromatography of 125I-EGF fractions indicated that our highly-purified labelled EGF was indeed heterogeneous. Analysis of each HPLC column fraction demonstrated that degradation of EGF had not occurred. The column fractions containing 125I-EGF were pooled into five groups for analysis of cell binding characteristics. Scatchard plot analysis of the five 125I-EGF pools revealed markedly different binding behaviors. In contrast, they had equal potency in stimulating DNA synthesis, within the sensitivity of our assay. Specific activity measurements indicated that the five HPLC pools of 125I-EGF had varying numbers of 125I atoms per EGF molecule. The heterogeneity of the highly purified 125I-EGF and the binding characteristics of the 125I-EGF subfractions pose serious implications for all workers who use iodinated ligands for receptor binding studies.