Chemistry And Biochemistry

A. Parathyroid Hormone

Parathyroid hormone (PTH) is an 84-amino acid-containing protein that is secreted by the chief cells of the parathyroid gland. PTH is a single-chain polypeptide of 84 amino acids (molecular weight = 9300) with no cysteine residues and hence no disulfide bridges. There is a preponderance of basic residues (arginine + lysine), conferring an overall positive charge on the molecule. A distinctly hydrophobic region is found in the central portion of the sequence (residues 31-43), including the single tyrosine residue, which is the site of iodine-125 labeling for radioimmunoassay procedures. A fragment of the intact hormone consisting of the first 28-34 amino acids at the N-terminal region of the molecule is sufficient for the peptide to exert its entire spectrum of characteristic biological effects.

Because PTH represents a secreted protein, it is bio-synthesized as a larger precursor by the parathyroid gland (see Figure 1-11). Two precursor species have been identified, including a prepro-PTH with 31 additional amino acids added onto the N-terminal region of PTH, representing the primary translation product of the mRNA in the ribosomal fraction of the parathyroid gland. This prepro-PTH then is secreted into the cisterna of the rough endoplasmic reticulum, where it is processed within seconds into a form known as pro-PTH. This occurs by the removal of the NH2-terminal methionyl residue and the next 24 amino acids (i.e., residues -30 through -7) after biosynthesis. By 20 min after synthesis, pro-PTH reaches the Golgi region where it is stored in vesicles and converted into PTH by removal of the NH2-terminal hexapeptide. PTH is stored in the secretory granule until it is released into circulation in response to a fall in the blood concentration of calcium. These steps are summarized schematically in Figure 1-11. Through the application of recombinant DNA techniques, a cDNA probe to the prepro-PTH messenger RNA has been prepared and used to evaluate cloned genomic DNA sequences containing the gene for PTH.

The introduction of radioimmunoassay techniques has permitted the detection of circulating levels of PTH and its changing concentration in disease states. Three predominant peptide species have been identified in human plasma. The first appears to be identical to the intact or native form of the PTH hormone as isolated from parathyroid tissue; this has 84 amino acids and a molecular weight of ~9500. There are two other populations of PTH fragments, one with a molecular weight of ~7000 and one that is smaller, having a molecular weight in the range of 4500. Conflicting views are held on the importance of these three peripheral forms of circulating PTH. One view is that only the 9500 molecular weight material is biologically active, with the smaller species representing degradation products. The other view is that one of the smaller species is the major biologically active form of the hormone. There is evidence that the liver Kupfer cells play an important role in the metabolism of the intact PTH molecule. The half-life of the intact PTH molecule in normal human plasma is only 20 min.

B. Parathyroid Hormone-Related Protein

Parathyroid hormone-related protein (PTHrP) was originally purified and an N-terminal amino sequence was obtained from human tumors associated with humoral hypercalcemia of malignancy (HHM); this information permitted its molecular cloning and study of the total amino acid sequence. PTHrP is a 141-amino-acid protein; a comparison of the structures of PTH and PTHrP is presented in Figure 9-3. Of the first 13 amino acids, 8 are identical and 3 more represent conservative amino acid substitutions. The close structural homology of PTHrP with PTH over the first 13 amino

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