High molecular weight forms of the insulin receptor

The insulin receptor of liver, adipose, and placental plasma membranes was photoaffinity labeled with radioiodinated N epsilon B29-(monoazidobenzoyl)insulin. Three specifically labeled bands of 450, 360, and 260 kilodaltons (kDa) were identified in each tissue by polyacrylamide gel electrophoresis of the membranes solubilized in sodium dodecyl sulfate (SDS). The 360- and 260-kDa bands corresponded to partially reduced forms of the 450-kDa band. The distribution of radioactivity between the three insulin receptor bands was dependent on the tissue, the purity of the receptor preparation, and the conditions of solubilization in SDS. The 360- and 260-kDa bands became more prominent in each tissue with an increasing time of solubilization in SDS. However, with a short solubilization time in SDS, the 450-, 360-, and 260-kDa bands of the receptor were distributed approximately in a ratio of 85:15:0 in all three tissues. Inclusion of sulfhydryl alkylating reagents during solubilization in SDS altered this ratio to about 95:5:0. We conclude that the 450-kDa band represents the predominant form of the photolabeled insulin receptor and that the 260-kDa and probably the 360-kDa form as well were generated during the experimental manipulations preceding identification of the receptor. However, the appearance of the 360- and 260-kDa bands was not due to reductant present in SDS or buffer solutions and could not be accounted for by proteolytic degradation of the receptor. Furthermore, purification of the receptor over 2000-fold did not prevent the appearance of the 360- and 260-kDa bands.