Carbohydrate chains of human thyrotropin are differentially susceptible to endoglycosidase removal on combined and free polypeptide subunits
The accessibility of the asparagine-linked carbohydrate chains of human thyrotropin (hTSH) and free ..cap alpha.. and ..beta.. subunits was investigated by their susceptibility to endoglycosidases H and F as well as to peptide:N-glycosidase F. Iodinated hTSH or subunits were incubated with a commercial enzyme preparation containing both endoglycosidase F and N-glycosidase F activities and further analyzed by sodium dodecyl sulfate gel electrophoresis followed by quantitative autoradiography. The authors show that, working at the optimum of the N-glycosidase activity, the relative amount of endoglycosidase required for half-deglycosylation was 20-fold higher for native hTSH than for the reduced and dissociated subunits. Under nondenaturing conditions, the 18K ..beta.. subunit of hTSH could be readily deglycosylated to a 14K species while the 22K ..cap alpha.. subunit was largely resistant. However, both subunits were converted to an apoprotein of similar apparent molecular weight of 14K following reduction of disulfide bonds. In contrast, the free ..cap alpha.. subunit of human choriogonadotropin appeared fully sensitive to carbohydrate removal under nonreducing conditions despite the presence of a partially deglycosylated 18K intermediate at low concentration of endoglycosidase. Similarly, both hTSH-..cap alpha.. and hTSH-..beta.. could be completely deglycosylated after acid dissociation of the native hormone. While all three carbohydrate chains of hTSH are sensitive to pure peptide:N-glycosidase F, only one on ..cap alpha.. and the single oligosaccharide present on ..beta.. in hTSH appeared to be cleaved by pure endoglycosidase F. These findings indicate that while the carbohydrate chain on ..beta.. is not involved in ..cap alpha..BETA association, the oligosaccharides on ..cap alpha.. are hindered when hTSH subunits are combined.
- Research Organization:
- CNRS, Marseille, France
- OSTI ID:
- 5721258
- Journal Information:
- Biochemistry; (United States), Vol. 26:18
- Country of Publication:
- United States
- Language:
- English
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