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 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. We 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 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 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-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 alpha and the single oligosaccharide present on beta in hTSH appeared to be cleaved by pure endoglycosidase F. Interestingly, one of the two carbohydrate chains present on alpha was also found to be susceptible to endoglycosidase H.(ABSTRACT TRUNCATED AT 250 WORDS)