Bovine Luteinizing Hormone Study of the Primary Structure around the Carbohydrate Attachment Sites of the Luteinizing Hormone α‐Subunit

1 The α‐subunit of bovine luteinizing hormone is electrophoretically heterogeneous. Two subfractions of this subunit, designated LHα A and LHα B , were isolated and found homogeneous by electrophoresis. 2 Bovine LHα A was cleaved by both cyanogen bromide and trypsin. Two distinct groups of homologous glycopetides were defined. The first group corresponds to the carboxy‐terminal portion of the subunit. The sequence accounting for the major peptide population is the following: Gly‐Asx‐Val‐Arg‐ Glx Val ‐Glx‐Asx‐ His Ser ‐ Thr‐Glx‐Cys‐His‐ Ser His ‐Cys‐Thr‐Cys‐Tyr‐Tyr‐His‐Lys‐Ser‐ COOH. Ambiguities in the amino‐terminal sequence are observed at the fifth, eighth and thirteenth position. Another population of peptides lacks three (or four) amino acids at the carboxy‐terminal end. The second group of glycopeptides corresponds to an internal portion of the subunit. The most abundant peptide population exhibits the following sequence: Asx‐Thr‐Thr‐Ser‐Ala‐Glx‐Cys‐Ile‐Cys‐Val‐Ala‐Lys‐COOH. A variant population exhibits the sequence Asx‐Ile‐Thr‐Ser‐Ala‐. 3 Two sites are proposed for the attachment of the polysaccharide prosthetic groups, i.e. the tripeptide Asx‐ His Ser ‐Thr and the tripeptide Asx‐ Ile Thr ‐Thr, corresponding to the carboxy‐terminal portion and to the inner part of the subunit, respectively. 4 Within both groups of glycopeptides, a relationship was observed between the type of residue × occupying the α position at the carbonyl side of Asx in the tripeptide sequence Asx‐X‐Thr and the composition of the polysaccharide side chain. 5 Our data document the degree of homology existing between LHα and the common type of subunit present in the thyroid‐stimulating hormone molecule ( i.e. the α‐subunit).