PTH SECRETORY PROTEIN-I PROCESSING & SECRETION

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The parathyroid gland and its hormone, parathormone (PTH), play a key role in Ca++ homeostasis. Recently a second major protein of the parathyroid gland has been recognized, named secretory protein-I (SP-I), whose secretion, as that of PTH, is closely regulated by Ca++. SP-I is glycosylated and has a molecular weight of about 70 kD. It is chemically similar to chromogranin A (CGA)--a major protein of the adrenal medulla that is cosecreted with chromaffin granule contents. It has been detected by immunological means in several other endocrine cells including thyroid C-cells with calcitonin, pancreatic islets with SRIF and pituitary with TRF. SP-I may play a specific role in the processing and/or secretion of peptide hormones in those cells in which it has been detected. This hypothesis will be tested in the parathyroid gland by correlating the synthesis, glycosylation and other covalent modifications to SP-I with the intracellular processing and secretion of PTH. We will determine a) if the SP-I that coexists with PTH in the "new" granule pool differs chemically from that in the "mature" granule pool and if such a difference can account for the ability of the two hormone pools to be differentially elicited for secretion; b) if the proteolytic activity of SP-I noted in purified preparations is inherent, and if it participates in the calcium-dependent and calcium-independent intracellular degradation of PTH; and c), if SP-I exerts a physiological action on bone, kidney and vasculature--tissues that are targets of PTH. Cow, pig and rat parathyroids will be incubated for various times and conditions (e.g., secretagogues, high and low calcium, inhibitors of respiration, protein synthesis, glycosylation) with radioactive amino acids, sugars, phosphate and fatty acids to label PTH and SP-I at various stages of their processing. PTH, SP-I and related fragments will be isolated from medium, crude membrane fractions and purified secretory granules and assayed for amount, size, structure, degree of glycosylation, etc. In the case of the granules, the intercalation of SP-I with, and possible protrusion through, the granule membrane will be tested since this could account for the "coding" of the PTH and SP-I in multiple secretory pools. In order to extend this concept to other endocrine tissues, we will determine which granules of the pituitary contain SP-I; if SP-I is cosecreted from rat pancreatic islet cells and with which hormones; and if immunoactive SP-I detected in these and other endocrine tissues is physically equivalent to SP-I and CGA.