Parathyroid Cell

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FIGURE 1-11 Scheme depicting the proposed intracellular pathway of the biosynthesis of parathyroid hormone. Preproparathyroid hormone (pre-pro-PTH), an initial product of synthesis on the ribosomes, is converted to proparathy roid hormone (pro-PTH) by the removal of (1) the NHr terminal methionyl residues and (2) the NH2-terminal sequence (- 29 through - 7) of 23 amino acids during or within seconds after synthesis, respectively. The conversion of pre-pro-PTH probably occurs during transport of the polypeptide into the cisterna of the rough endoplasmic reticulum. By 20 min after synthesis, pro-PTH reaches the Golgi region and is converted into PTH by (3) removal of the NH2-terminal hexapeptide. PTH is stored in the secondary granule until it is released into circulation in response to a fall in the blood concentration of calcium. The times needed for these events are given below the scheme. Modified from Habener, J. F., Kemper, B. W., Rich, A., and Potts, J. T. (1977). Biosynthesis of parathyroid hormone. Recent Prog. Horm. Res. 33,287.

a prohormone is the immediate product of the translation process, while if two steps are involved, then a pre-pro version of the hormone is the immediate translation product. The function of the pre sequence or pre-pro sequence is to facilitate the insertion of the nascent peptide into the membrane of the rough endoplasmic reticulum (RER) immediately followed by movement of the entire peptide into the cisterna of

Parathyroid gland

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