Structures of high-mannose oligosaccharides of mouse thyrotropin: differential processing of alpha- versus beta-subunits of the heterodimer.

We have determined the structures of high mannose (Man) oligosaccharide units of the alpha- and beta-subunits of mouse TSH heterodimers and of free alpha-subunits. Mouse thyrotropic tumor tissue, or pituitaries from euthyroid or hypothyroid mice, were incubated with D-[2-3H]Man, homogenized, and incubated in the presence or absence of pH 3 buffer to dissociate heterodimers. Highly enriched TSH beta-subunits, or TSH heterodimers, were obtained using anti-TSH beta serum and free alpha-subunits were subsequently obtained using anti-LH alpha-serum. High Man units released by endoglycosidase H were analyzed by paper chromatography. At 1 and 3 h oligosaccharides with 8 or 9 Man residues predominated in all subunits. In all three tissue types, units with 9 Man tended to accumulate in TSH beta-subunits, whereas Man-trimming from Man9GlcNAc to Man8GlcNAc proceeded more rapidly in TSH alpha-subunits, and in free alpha-subunits. The rate of TSH beta-subunit processing was also a function of tissue type; the rate in euthyroid pituitary tissue equaled that in tumor tissue and was greater than that in hypothyroid pituitary tissue, suggesting that processing may be physiologically regulated. Hypothyroid calf serum slowed TSH alpha-subunit processing in euthyroid pituitaries. Thus, the previously reported slower processing of Man9GlcNAc to Man8GlcNAc in heterodimers as compared to free alpha-subunits is in large part due to accumulation of Man9GlcNAc in TSH beta-subunits.