Metabolism of gonadotropins: comparisons of the primary structures of the human pituitary and urinary LH beta cores and the chimpanzee CG beta core demonstrate universality of core production

Abstract

The gonadotropins are a family of closely related heterodimeric glycoprotein hormones homologous in structure to disulfide-knot growth factors. Metabolic proteolytic processing in vivo of this disulfide cross-linked region results in urinary excretion of a residual highly stable core structure. The primary structure of the pituitary form of the hLH core was reported earlier, but it has proved difficult to isolate the urinary core, although antibodies to the pituitary core demonstrated its presence. By conventional and immunoaffinity methods, the urinary core has been isolated and its structure determined by both chemical and mass spectrometric methods. The urinary hLH core is the same as the pituitary-extracted hLH core, 6-40 disulfide bridged to 55-93, except that the pituitary core is more heterogeneous containing also 49-93. These findings imply a dual origin of urinary cores, both directly from a secreting tissue and by kidney processing of circulating hormone. We also found that pregnant chimpanzees excrete a CG core with a primary structure identical to that of the human CG core of pregnancy. In conclusion, gonadotropin core generation and urinary excretion of nearly identical gonadotropin metabolites is common among primates. Although possible biological functions of these core fragments remain unproven, they have diagnostic utility because of their stability and abundance.

Differential expression of human chorionic gonadotropin (hCG) glycosylation isoforms in failing and continuing pregnancies: preliminary characterization of the hyperglycosylated hCG epitope

“…Lyophilized hCG C5 free subunit (20 µg) and hCG preparation 863 (87 µg) were reduced, alkylated, and separated on reverse phase HPLC as described earlier (Birken et al 2001).…”

Identification of post-translational modifications resulting from LHbeta polymorphisms by matrix-assisted laser desorption time-of-flight mass spectrometric analysis of pituitary LHbeta core fragment

“…MALDI-TOF MS of reduced and carboxymethylated or alkylated LH cf revealed the presence of two non-glycosylated peptides spanning not only amino acids 55-93 but also 49-93 (Birken et al 2001). Following DTT reduction, our MALDI-TOF MS data revealed the presence of only a single non-glycosylated peptide, 55-93, in our sample of pooled LH cf.…”

“…The primary structure of the glycosylated peptide has been determined to be 6-40 by MALDI-TOF MS following N-glycanase digestion (O'Connor et al 1998, Birken et al 2001. Subsequent analysis suggested that LH cf is more heterogeneous in the length of its constituent peptides than hCG cf.…”

“…Structural analysis suggests that LH cf is more heterogeneous in the length of its constituent peptides than hCG cf. Matrix-assisted laser desorption/ ionisation (MALDI) time-of-flight (TOF) mass spectrometry (MS) of reduced, carboxymethylated and deglycosylated LH cf has revealed the presence of two non-glycosylated peptides spanning not only amino acids 55-93 but also 49-93 (Birken et al 2001). The primary structure of the glycosylated peptide, not subjected to MS, has previously been determined to be 6-40 (Birken et al 1993, O'Connor et al 1998.…”

Patterns of LHβcf among women in health and disease

“…Therefore, hLHβcf may be produced both from LH extracted from the blood during passage through the kidney and also directly by secretion by pituitary tissue into the blood and be very rapidly excreted in urine. (Birken et al, 1993;Birken et al, 1996;Birken et al, 1999;Birken et al, 2001). …”