Size discrepancy between somatomedin-C and insulin receptors

Somatomedin-C (Sm-C) and insulin receptors have similar size and structure. Each receptor is a heterotetramer composed two alpha- and two beta-subunits. Sodium dodecyl sulfate (SDS)-polyacrylamide gel autoradiographic studies of affinity labeled receptor preparations demonstrated that each has a whole receptor of greater than 350,000 daltons, a half-receptor of 220,000 daltons, and binding subunit (alpha-subunit) of about 140,000 daltons. Using SDS-polyacrylamide disc gels and double labeling techniques, we demonstrated that the 125I affinity labeled alpha-subunit of the Sm-C receptor from human placenta was 8,000 daltons smaller than the 131I affinity labeled insulin receptor alpha-subunit. Further double label studies demonstrated that [125I]insulin and [131I]insulin cross-linked to placental insulin receptors precisely comigrated on SDS-disc gels, indicating that the difference between insulin and Sm-C alpha-subunits is not an artifact of the system. The difference in ligand size (Sm-C, 7,500 daltons; insulin, 5,700 daltons) would minimize the observed difference and is clearly not the cause of the difference in size observed. These studies provide further evidence in support of two functionally and physically distinct receptor molecules for insulin and Sm-C in human placenta.