title: Low-molecular-weight constituents of isolated insulin-secretory granules. Bivalent cations, adenine nucleotides and inorganic phosphate.
abstract: The concentrations of Zn2+, Ca2+, Mg2+, Pi and adenine nucleotides were determined in insulin-secretory granules prepared from a transplantable rat insulinoma. Differential and density-gradient centrifugation analyses revealed that Zn2+ in this tissue was principally localized in the secretory granule, a second major fraction being found in association with cytosolic proteins. Pi was principally recovered in the latter fraction, whereas Ca2+ and Mg2+ were more widely distributed. Intragranular ion-distribution experiments suggested that Zn2+ was complexed mainly to insulin and its precursor forms and remained in the granule in an insoluble state. The Zn2+/insulin ratio (0.54) was greater than that expected for insulin molecules having two centrally co-ordinated Zn2+ atoms/hexamer, but less than the maximal Zn2+-binding capacity of the molecule. Most of the granular Ca2+, Mg2+ and Pi was released in a soluble form when granules were disrupted by sonication. Simulation in vitro of the ionic composition of the granule suggested that up to 90% of its Ca2+ was complexed to Pi and adenine nucleotides. Granular macromolecules also bound Ca2+, as shown by equilibrium-dialysis studies of granule lysates. However, such binding was displaced by Mg2+. Examination of the efflux of Ca2+ from granules incubated in iso-osmotic suspensions at 37 degrees C suggested that the passive permeability of the granule membrane to Ca2+ was very low. Nevertheless, more than 50% of the granular Ca2+ was rapidly released in an ionized form on hypo-osmotic or detergent-induced disruption of the granule membrane. This may represent a potentially mobilizable pool of Ca2+ in vivo.
mesh_terms: D000227:Adenine Nucleotides / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002413:Cations, Divalent / Q000378:metabolism*; D002498:Centrifugation; D002499:Centrifugation, Density Gradient; D003594:Cytoplasmic Granules / Q000201:enzymology / Q000378:metabolism*; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007340:Insulinoma / Q000378:metabolism; D010190:Pancreatic Neoplasms / Q000378:metabolism; D010710:Phosphates / Q000378:metabolism*; D051381:Rats
chemical_list: D000227:Adenine Nucleotides; D002413:Cations, Divalent; D007328:Insulin; D010710:Phosphates; D002118:Calcium