Chromogranin A (CGA), which is cosecreted from the parathyroid gland with PTH in response to low extracellular calcium, can be processed to amino- terminal peptides that, in turn, inhibit PTH secretion. The synthetic peptide KCIVEVISDTLSKPSPMPVSKECFE [CGA-(16-40)] is active in inhibiting secretion from freshly isolated or cultured bovine parathyroid cells. Peptide analogs in which alanine is substituted for classes of residues between the two cysteines have been synthesized and tested for biological activity. Substitution of the lysine, serine, or threonine residues by alanines does not greatly diminish biological activity. However, when the prolines are replaced by alanines or when glutamic acid and aspartic acid are replaced by alanines, the peptides do not effectively inhibit PTH secretion. Tests of synthetic peptides in which the individual glutamate or aspartate residues have been replaced showed that glutamate 37, followed by aspartate 24, are more critical for biological activity. Further experiments have shown that residues 11-15 in the natural CGA sequence do not enhance the biological activity of CGA-(16-40), whereas adding residues 6-10 restores full biological activity compared to that of CGA-(1-40). Circular dichroism experiments with CGA-(16-40) and the alanine substitution analogs show significant differences only for the peptide in which the three prolines are replaced. The inactive peptide with two glutamic acids and one aspartic acid replaced by alanine residues has the same circular dichroism spectrum as some of the peptides that are fully active. The N-terminal CGA sequences may tolerate many changes without alteration of biological activity. However, there are specific amino acid residues that are required for biological function.
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