SOLUBLE SEROTONIN AND CATECHOLAMINE BINDING-PROTEINS IN THE BOVINE ADRENAL-MEDULLA

The soluble serotonin-binding proteins (SBP) present in the adrenal me dulla and in chromaffin cells, are very similar to those reported for the bovine brain and retina. Binding of [H-3]serotonin and [H-3]dopami ne to these SBP is increased by Fe2+ but not by Fe3+. At an optimal co ncentration of Fe2+ (0.1 mM) these proteins behave as a single class o f non-cooperative sites for [H-3]serotonin (B(max) = 124+/-28 pmol/mg protein, K(D) = 0.51+/-0.13 muM) and [H-3]dopamine (B(max) = 685+/-118 pmol/mg protein, K(D) = 0.46+/-0.06 muM). Binding of [H-3]dopamine is also increased by Cu2+ and Mn2+, but to a lesser extent than by Fe2+. Catecholamines are good competitors for [H-3]serotonin binding (K(i) = 0.31 muM for dopamine, 0.6 muM for adrenaline and 0.9 muM for noradr enaline). The serotonin binding proteins from adrenal medulla elute in the void volume of a Sephacryl 100 HR gel filtration column, reflecti ng aggregation, and migrate mainly with an apparent molecular weight o f 45 kDa in native polyacrylamide gel electrophoresis experiments. Sub cellular localization studies and release experiments suggest that SBP are not present in chromaffin granules, but in the cytosol of purifie d chromaffin cells. The present data suggest that these proteins must have other functions than storing monoamines in synaptic vesicles.