Facile separation of the subunits of ovine and bovine gonadotrophins by high-performance liquid chromatography: microheterogeneity and biological activity

The alpha and beta subunits of ovine and bovine gonadotrophins which undergo instant dissociation in 0.1% (v/v) trifluoroacetic acid in water could be separated into their multiple components by reverse-phase high-performance liquid chromatography on analytical and preparative scales. Several batches of highly purified ovine LH (oLH) separated into four alpha components (alpha 1 to alpha 4) and five beta components (beta 1 to beta 5). While the major alpha component (alpha 3) remained the same in all, the beta subunit was variable in all these preparations of oLH of equal biological and immunoreactivity. Various manipulations, such as prolonged incubation in dilute acid or alkali or conditions prone to air oxidation, did not change elution patterns or produce interconversions among the alpha or beta components, suggesting that microheterogeneity arose in the pituitary itself or during its collection. The subunits obtained on a preparative scale were of high yield and quality and recombined (100%) to produce fully active LH or FSH. Chemical deglycosylations in which 75-80% of the peripheral sugars were removed shifted the elution of all components (alpha as well as beta) slightly but modifications to the protein backbone by other reagents changed the profile of the alpha subunit. Obtaining the various alpha and beta components in high yield would permit a complete analysis of gonadotrophin microheterogeneity.

Determination of subunit contact-associated epitopes of the beta-subunit of human follicle-stimulating hormone

Three different experimental approaches were used to assess the regions on the beta-subunit of human FSH (hFSH beta) that may be altered or masked by its association with the alpha-subunit of hFSH (hFSH alpha) in the heterodimeric hFSH molecule. In a direct approach, we tested whether synthetic peptides corresponding to hFSH beta sequences 1-20, 16-36, 33-53, 49-67, 66-85, 81-100, and 98-111 inhibited association of hFSH alpha and hFSH beta in an enzyme-linked immunosorbent assay. Synthetic peptides-(81-100), -(98-111), and -(66-85) caused greater than 50% inhibition of subunit association, whereas other peptides showed 26% or less inhibition. These data suggested that the C-terminal sequences of hFSH beta, particularly 81-100, are at a subunit interface with hFSH alpha in heterodimeric hFSH. In another approach we reasoned that antibodies with a higher affinity for free hFSH beta than for heterodimeric hFSH bind to epitopes on hFSH beta that are masked or altered by hFSH alpha subunit. To test this hypothesis, epitopes of hFSH beta were mapped using synthetic peptides of hFSH beta sequences, three monoclonal antibodies (3G3, 4D5, and 4G8), and a polyclonal antiserum (NIDDK anti-hFSH beta). Compared to 3G3 all the other antibodies exhibited minimal reactivity with hFSH, but bound strongly to hFSH beta. The epitope-mapping data with both 4D5 and NIDDK anti-hFSH beta identified peptide 81-100, which was not recognized by 3G3. The epitope map with 4G8 identified the same three peptides as with 3G3. However, in the case of 4G8 its reactivity with peptide 33-53 was the least, whereas it was ranked first for 3G3. Since both 3G3 and 4G8 had an identical affinity for hFSH beta, it was hypothesized that sequences in peptide 33-53 may be altered or masked by hFSH alpha. To test this, we determined the specificity of anti-hFSH beta-(33-53) peptide antiserum for hFSH beta and hFSH in an enzyme-linked immunosorbent assay. The antipeptide antiserum bound strongly to free hFSH beta and weakly to hFSH, suggesting that part of the sequence in peptide-(33-53) was masked or altered by association with hFSH alpha in heterodimeric hFSH. Taken together, the subunit association studies, the epitope-mapping data, and the specificity of anti-hFSH beta-(33-53) peptide antiserum have suggested that sequences in peptide-(81-100) and -(33-53) are masked or conformationally altered by hFSH alpha in heterodimeric hFSH.

N-linked oligosaccharides on free alpha interfere with its ability to combine with human chorionic gonadotropin-beta subunit

The purpose of the study was to examine the role of N-linked oligosaccharides in preventing combination of free alpha molecules with human chorionic gonadotropin (hCG)-beta subunit to form the intact hormone, hCG. Culture media from JEG cells incubated in the presence or absence of Swainsonine were filtered on Sephadex G-100, and free alpha was identified by radioimmunoassay. Swainsonine interferes with glycosylation by inhibiting alpha-mannosidase II, resulting in formation of hybrid structures. Approximately 50% of the free alpha molecules from Swainsonine-treated cells (Swainsonine pool 2) had an apparent molecular size that was smaller than that of free alpha from control cells. The oligosaccharides of control alpha molecules were resistant to endo-beta-N-acetylglucosaminidase H treatment. In contrast, virtually all of the Swainsonine free alpha molecules contained endo-beta-N-acetylglucosaminidase H-sensitive oligosaccharides. Swainsonine free alpha and control free alpha molecules were incubated with an excess of hCG-beta subunit, followed by chromatography on Sephadex G-100. Each fraction was assayed by radioimmunoassay for intact hCG and for alpha. Less than 10% of control free alpha molecules combined with hCG-beta. In contrast, 54% of Swainsonine alpha pool 2 and 40% of Swainsonine alpha pool 1 combined with beta to form hCG. Thus, modulation of N-linked oligosaccharide processing converted free alpha molecules to forms that can combine with hCG-beta. These results indicate that the inability of a substantial portion of control free alpha molecules to combine with hCG-beta is due to the presence of N-linked oligosaccharide structures that interfere with combination.