Circular Dichroism of Human Pituitary Luteinizing Hormone and Its Glycopeptides

Abstract

The circular dichroic (CD) spectrum of the glycoprotein hormone, human pituitary luteinizing hormone (hLH), has been 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 two resolved negative bands, one at 206 nm and the other at 215 nm. The latter band is assigned to β‐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 β‐structure in 0.1 N HCl. Thus, 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°C.

Structure–Function Relationships of Glycoprotein Hormones and Their Subunits’ Ancestors

“…The circular dichroism (CD) analyses of GPHs had shown a long time ago that they exhibit a limited amount of periodic (secondary) structures: only ~4–8% α-helix and ~30% β-sheet and β-turn (24–30). From these data, the rest of the molecule was expected to be non-periodic.…”