ONTOGENY AND CHARACTERIZATION OF EPIDERMAL GROWTH-FACTOR RECEPTORS ON THE FETAL AREA OF THE SHEEP PLACENTA

Studies of the binding of I-125-labelled epidermal growth factor (EGF) to the ovine placenta were carried out on days 50, 90-100 and 140 of pregnancy. Membrane fractions were purified from the fetal area of the cotyledon. Two classes of binding sites were found. Their dissociation constants (K(d)) were not significantly different for the three stages of pregnancy considered (high-affinity K(d) 54 70.2 pmol/l; low-affinity K(d) 12.2 to 19 nmol/l). However, the number of high-affinity binding sites on days 90-100 was significantly (P < 0.01) greater (146 +/- 19 fmol/mg protein) than on either day 50 or day 140 (respectively 74.2 +/- 1-26 and 56.3 +/- 5.6 fmol/mg protein). Affinity cross-linking studies followed by SDS-PAGE under reducing conditions demonstrated that the major part of the EGF was specifically cross-linked to a protein of molecular weight of 150 kDa and to lesser extent to 180 kDa and 130 kDa proteins. Membranes prepared from unfrozen tissues, in the presence of sodium iodoacetate to reduce endogenous enzymatic conversion of the 180 kDa form to the 150 and 130 kDa forms, still exhibited a major EGF-binding protein of 150 kDa. The occurrence of an increased number of EGF receptors at the period of rapid cotyledonary growth which coincides with the increase in placental hormonal secretions suggests that EGF has a role in the development of the ovine placenta.