title: Effects of pH and Ca2+ on monomer-dimer and monomer-tetramer equilibria of chromogranin A.
abstract: Chromogranin A is a high capacity, low affinity Ca2+ binding protein which undergoes Ca2+- and pH-dependent conformational changes, and has recently been suggested to play a Ca2+-buffering role in the secretory vesicle of adrenal medullary chromaffin cell, the major inositol 1,4,5-trisphosphate-sensitive intracellular Ca2+ store of chromaffin cell (Yoo, S.H., and Albanesi, J.P. (1990) J. Biol. Chem. 265, 13446-13448). In the present study, it is shown that chromogranin A exists in a monomer-dimer equilibrium at pH 7.5 and in a monomer-tetramer equilibrium at pH 5.5. The pH appears monomer-tetramer equilibrium at pH 5.5. The pH appears to be a necessary and sufficient factor determining the types of oligomers formed. Although Ca2+ did not change the type of oligomerization, it had a very significant effect on the values of the thermodynamic parameters characterizing the associations. The delta G0 values for a monomer-dimer equilibrium were -7 to -8 kcal/mol, while those for a monomer-tetramer equilibrium were -20 to -23 kcal/mol. At pH 5.5, the values of delta H0, delta S0, and delta C0p were large and negative in the absence of Ca2+ and large and positive in the presence of 35 mM Ca2+, implying markedly different reaction mechanisms. Extrapolation of the results to 37 degrees C and 1 mM chromogranin A suggests that chromogranin A is virtually 100% tetramer at pH 5.5 in the presence of 35 mM Ca2+ but is 96% dimer at pH 7.5 in the absence of Ca2+, the two conditions resembling those seen in vivo. These results suggest that chromogranin A is mostly dimer in the endoplasmic reticulum and cis-Golgi area and is essentially all tetramer in the vesicle.
authors.affiliation: Laboratory of Cellular Biology, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: S H
authors.initials: SH
authors.lastname: Yoo
authors.forename: M S
authors.initials: MS
authors.lastname: Lewis
mesh_terms: D000311:Adrenal Glands / Q000737:chemistry; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent; D002417:Cattle; D002837:Chromaffin Granules / Q000737:chemistry; D053379:Chromogranin A; D002864:Chromogranins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D015544:Inositol 1,4,5-Trisphosphate / Q000737:chemistry; D013696:Temperature; D013816:Thermodynamics; D014461:Ultracentrifugation
chemical_list: D002413:Cations, Divalent; D053379:Chromogranin A; D002864:Chromogranins; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium