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Chemical synthesis of the precursor-specific region of pre-proparathyroid hormone.

Parathyroid hormone (PTH), an 84-amino acid peptide, is initially biosynthesized intracellularly in a precursor form termed pre-proparathyroid hormone (PreProPTH), which contains an NH2-terminal extension, or leader sequence. Using the solid phase method, a 30amino acid peptide representing the precursor-specific region of Pre-ProPTH was chemically synthesized. DTyrosine amide was placed at the COOH terminus to provide resistance to exopeptidase degradation and a site for radioiodination. [3H]Glycine and [14C]alanine were incorporated at positions near the COOH and NH2 termini of the peptide, respectively, to monitor purification of the synthetic product. Purification of the synthetic peptide was performed by gel filtration and ion exchange chromatography under denaturing conditions. Homogeneity of the purified peptide was assessed by multiple criteria including amino acid analysis, Edman sequence analysis, sodium dodecyl sulfate-gel electrophoresis, isoelectric focusing, and thin layer chromatography, as well as by analyses of tryptic peptides prepared from the synthetic precursor-specific peptide. By these criteria, greater than 90% of the synthetic product consisted of a single homogeneous peptide representing the complete sequence of 30 amino acids. Special difficulties encountered during synthesis and purification of this peptide seemed related to the marked hydrophobicity of the precursor-specific sequence; inasmuch as these issues may be pertinent to synthesis of leader sequences of other secreted proteins, the problems encountered and solutions chosen to overcome them are reviewed. Availability of the precursor-specific peptide of Pre-ProPTH should facilitate investigation of the mechanism of Pre-ProPTH attachment to, transport across, and proteolytic cleavage by rough endoplasmic reticulum, and generation of antisera for use in immunolocalization studies and immunoassays for detection of the precursor-related peptides, if present, in tissue and blood.