Radioimmunoassay for Proparathyroid Hormone

Abstract

A sensitive radioimmunoassay has been developed to bovine proparathyroid hormone which employs an antiserurm produced by immunization of rabbits with a synthetic 18-amino acid peptide fragment of the bovine prohormone. The synthetic prohormone fragment was prepared by solid-phase synthesis and consists of the amino-terminal 12 residues of bovine parathyroid hormone plus the hexapeptide sequence of the bovine prohormone. The antiserum is uniquely specific to detection of the prohormone or synthetic fragments that preserve the peptide linkage between the prohormone region and amino-terminal hormonal sequences. Bovine proparathyroid hormone of natural origin, as well as synthetic fragments incorporating the prohormone hexapeptide sequence and amino-terminal sequences of bovine parathyroid hormone, are bound by the antiserum, but the hexapeptide, intact bovine or human hormone and amino-terminal hormonal fragments alone are not detected. The assay readily measures prohormone in extracts of bovine but not human parathyroids, a finding that may be explained by the fact that human proparathyroid hormone differs from bovine by a substitution of serine for alanine at position number one and leucine for phenylalanine at position seven of the prohormone sequence. Thus, the assay should be particularly useful in studies of the intracellular localization of bovine proparathyroid hormone and the mechanism of intracellular transport and conversion of proparathyroid hormone to parathyroid hormone. In preliminary studies, it was found that proparathyroid hormone was undetectable in peripheral or parathyroid effluent blood from calves, a finding which reflects either a low rate of prohormone secretion or a rapid cleavage of the prohormone in blood once secreted.