Luke L. H. Chu

Biosynthesis of proparathyroid hormone and parathyroid hormone: Chemistry, physiology and role of calcium in regulation

The discovery of bovine proparathyroid hormone is reviewed, and recent findings regarding the structure of the prohormone are presented. Sequence analyses on two prohormone preparations have established that the NH2-terminal region-of the bovine prohormone is the hexapeptide, Lys-Ser-Val-Lys-Lys-Arg, attached to the NH*-terminal Ala of the hormone form. The sequence of the COOH-terminal region of the prohormone is not yet known. Our present data on the human prohormone suggest that it consists of a similar NH*-terminal hexapeptide. The newly synthesized peptides are separable from the bulk of the preexisting hormone by either deoxycholate extraction or sucrose gradient centrifugation, which permits studies of the subcellular distribution of the prohormone and hormone; from such studies a proposal is made concerning the intracellular locales of synthesis and conversion of prohormone to hormone. Glands from rats receiving a low calcium intake convert prohormone to hormone more efficiently than do glands from animals receiving a normal high calcium intake. A model depicting the various sites at which calcium may affect the production of hormone is proposed.

Biosynthesis of Proparathyroid Hormone and Parathyroid Hormone by Human Parathyroid Glands

Human parathyroid glands obtained at autopsy were incubated with [(3)H]leucine and [(3)H]lysine. After incubation, nonradioactive parathyroid tissue of either human or bovine origin was added. Radioactive parathyroid hormone and proparathyroid hormone were isolated from the gland and medium by organic solvent and salt fractionation, trichloroacetic acid precipitation, Sephadex G-100 gel filtration, and carboxymethyl cellulose column chromatography. The human hormonal peptides were identified in the ion-exchange column eluates by their relatively high levels of radioactivity, their elution positions, and their immunoreactivity to anti-PTH antiserum. The time-course of radioactive amino acid incorporation into these peptides and a brief incubation of the gland with radioactive amino acids, followed by various lengths of incubation with nonradioactive amino acids, indicated that a precursor-product relationship exists for the two peptides. An alternate method for isolation of the hormone and prohormone, which involves separation of peptides by urea-polyacrylamide gel electrophoresis, confirmed the identities of the human parathyroid hormone and proparathyroid hormone.