CIRCULAR-DICHROISM OF HUMAN PITUITARY LUTEINIZING-HORMONE AND ITS GLYCOPEPTIDES - CURVE RESOLUTION AND BAND ASSIGNMENTS TO PEPTIDE CHROMOPHORE, AROMATIC RESIDUES, DISULFIDES, AND N-ACETYLATED AMINO-SUGARS

Abstract

The circular dichroic (CD) spectrum of human pituitary luteinizing hormone (hLH) was determined between 195-320 nm and resolved into gaussian constituents. Below 230 nm the CD spectrum is characterized by a negative extremum at 207 nm with a shoulder at 217 nm. Resolution into gaussian constituents of the 200-230 nm CD spectrum resulted in 2 resolved negative bands, one at 206 nm and the other at 215 nm. The latter band is assigned to .beta.-structure which is estimated to be about 25%. The 206 nm resolved band is assigned to the N-acetylated carbohydrate groups (e.g., N-acetyl glucosamine, galactosamine and neuraminic acid). This is based partly on the evidence that the CD spectrum of the hLH glycopeptide fraction (prepared by a pronase digestion of s-carboxymethylated hLH) exhibited a negative extremum at 207.5 nm, which is close to the resolved 206 nm band in hLH. Above 230 nm the CD spectrum is characterized by a negative extremum at about 275 nm. Most of the ellipticity in this region is attributed to the disulfides in hLH. Both strong acid (0.1 N HCl) and concentrated guanidine hydrochloride (4 M) affect the ellipticity in the vicinity of 275 nm, but only the latter (as well as concentrated urea) has a major effect on the CD spectrum below 230 nm indicating extensive conformational changes. There is, however, some loss of .beta.-structure in 0.1 N HCl. It appears that the conformation of the hLH subunits in these subunit-dissociating agents is rather different. There was no dramatic change in the magnitude of the 207 nm extremum of native hLH between 10-50.degree. C.