AbstractThe disuccinimidy! suberate, affinity‐labeling procedure, and proteolytic mapping techniques have been employed to characterize further the human placental receptors for insulin and basic somatomedin. Electrophoretic analysis of the basic somatomedin receptor, selectively crosslinked to 125I basic somatomedin in the presence of excess native insulin revealed, under reducing conditions, major labeled constituents of 270‐280 and 125‐140 kd, substantiating our previous work employing a photoaffinity labeling reagent. Affinity labeling also demonstrated the presence of less intensely labeled components with apparent molecular weights of 40 and 45 kd but failed to reveal a distinct 90‐ to 100‐kd species observed in parallel experiments with insulin. In the absence of β‐mercaptoethanol, all components specifically labeled with 125I basic somatomedin migrated in the 300‐ to 400‐kd range. In comparison, selective affinity labeling of the insulin receptor in the presence of excess native basic somatomedin revealed components, upon electrophoresis under reducing conditions, with apparent molecular weights of 270‐280, 125‐140, 90‐100, and 40 kd. The major insulin‐labeled component (125‐140 kd) comigrated with the major constituent (125‐140 kd) selectively labeled with basic somatomedin. When digestion was performed prior to solubilization, chymotryptic and tryptic proteolysis of the membrane‐localized selectively labeled insulin, and basic somatomedin receptors yielded quite similar gel electrophoretic maps. However, when digestion was done subsequent to solubilization, chymotryptic and tryptic proteolysis of selectively labeled insulin and basic somatomedin receptors solubilized in SDS yielded similar but not identical gel electrophoretic maps. We conclude that the receptors for basic somatomedin and insulin are highly homologous structures with respect to their disulfide crosslinked composition, and with respect to the size of the major components detected by selective affinity‐labeling procedures. Nevertheless, the detection of electrophoretically distinct labeled receptor components upon analysis of specifically labeled intact or proteolytically digested receptors points to subtle differences between the polypeptide compositions of the two receptors.