Oligosaccharide Heterogeneity of Insulin Receptors. Comparison of N-Linked Glycosylation of Insulin Receptors in Adipocytes and Brain*

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Abstract
We tested the hypothesis that the molecular weight discrepancy between insulin receptors in brain and adipocytes is due to differences in glycosylation by treating photoaffinity-labeled insulin receptors from both tissues with endo-.beta.-N-acetylglucosaminidase of (Endo F) and analyzing the products of sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Endo F removed glycans from the adipocyte 125-kilodalton (kDa) subunit and the brain 115-kDa subunit in a manner dependent upon the concentration of enzyme and time of incubation. At a maximally effective concentration of Endo F, the adipocyte .alpha.-subunit was reduced from 125-kDa to 100-kDa and the brain .alpha.-subunit from 115-kDa to 100-kDa. We also examined the type of oligosaccharides present in both .alpha.-subunits by treating the proteins with endo-.beta.-N-acetylglucosaminidase H (Endo H), which selectively removes high mannose residues, and neuraminidase. Endo H treatment reduced the apparent molecular weight of both the adipocyte and brain .alpha.-subunits. In both receptors, the deglycosylated product obtained with Endo H was larger than that generated by Endo F. The adipocyte .alpha.-subunit demonstrated a shift in mobility on sodium dodecyl sulfate gels after neuraminidase treatment, whereas the brain .alpha.-subunit did not. We conclude from these studies that 1) The discrepancy in apparent molecular weight of .alpha.-subunits in brain and adipocytes is due to differences in N-linked glycosylation; 2) high mannose and complex type oligosaccharides are present in both receptor types; and 3) the complex oligosaccharides in the adipocyte .alpha.-subunit are terminated in a manner different from the complex glycans of the brain .alpha.-subunit.

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