Dithiothreitol stimulates insulin receptor autophosphorylation at the juxtamembrane domain

Autophosphorylation of insulin receptors from human placental membranes prepared with iodoacetamide was more than double that of control receptors prepared without iodoacetamide. Dithiothreitol (DTT) treatment of control receptors prepared without iodoacetamide resulted in increased autophosphorylation. However, DTT was without effect on insulin receptors prepared with iodoacetamide. Phosphopeptide analysis showed that while 32P-labeling of all of the phosphopeptides was increased in insulin receptors from membranes prepared with iodoacetamide, two additional phosphopeptides were detected and identified as deriving from the juxtamembrane domain, containing tyrosine residue 960. Similar results were produced by DTT treatment of control insulin receptors. These observations suggest that a thiol(s) may be involved in insulin receptor autophosphorylation in the juxtamembrane domain.