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Membrane Sialic Acid and the Mechanism of Insulin Action in Adipose Tissue Cells

EFFECTS OF DIGESTION WITH NEURAMINIDASE

From the ¹ From the Departments of Medicine, and Pharmacology and Experimental Therapeutics, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

Mild digestion of isolated adipose tissue cells with Clostridium perfringens neuraminidase (10 to 20 mµg of purified enzyme per ml, 15 min at 37°) results in enhanced glucose transport by these cells. This effect appears to mimic closely the physiological effect of insulin on glucose transport by normal cells, and there is evidence to suggest that these two apparently different stimuli may affect similar membrane structures.

Digestion of fat cells with higher concentrations of neuraminidase (about 1 µg of purified enzyme per ml, 15 min, 37°) causes a fall in the enhanced basal rate of glucose transport described above and abolishes the effects of insulin on glucose transport and on lipolysis. Such enzymatic digestion, however, has no effect on the total quantity of specific cell membrane receptor for insulin or in the affinity of the insulin-receptor complex. The biological and the receptor-binding properties of insulin are thus dissociated. The loss of insulin-stimulated glucose transport caused by neuraminidase digestion does not reverse spontaneously by prolonged incubation of the cells after digestion.

Neuraminidases from C. perfringens, Vibrio cholerae and influenza virus were purified by affinity chromatography using selective agarose adsorbents containing an oxamic acid derivative. The three enzymes, at low concentration, can destroy the glucose transport effects of insulin on fat cells. However, the specific patterns of digestion observed with the three different enzymes are not identical, probably as a result of the differences in substrate specificity of the enzymes. Differently linked sialic acid residues at the cell surface may participate differently in various membrane functions.

The effects of digesting fat cells with various combinations of C. perfringens neuraminidase and trypsin on glucose oxidation and the patterns of release of sialic acid and sialyl peptides suggest that the biological effects resulting from digesting cells with these enzymes are a consequence of cleavage of similar portions of the same or closely related membrane structures.

Sialic acid at the cell surface is intimately involved in the processes concerned with transmitting the effects of insulin on membrane glucose transport and lipolysis, but it is not involved in the recognition of insulin by the receptor. Despite the unessential role of sialic acid in the function of insulinrecognition by the receptor, this moiety may nonetheless be a constituent of a glycoprotein insulin receptor.

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				G. V. Bennett and P. Cuatrecasas Insulin Receptor of Fat Cells in Insulin-Resistant Metabolic States Science, May 19, 1972; 176(4036): 805 - 806. [Abstract] [PDF]