IN VITRO Ca2+ DEPENDENT PROTEOLYSIS IN BRAIN TISSUES: A POSSIBLE SOURCE OF ERROR IN TRANSMITTER METABOLISM STUDIES

—The effects of Ca²⁺ ions on the metabolism of ³H]serotonin and ³H]-labelled catecholamines have been examined in hippocampal slices or synaptosomes. The formation of ³H]-5 hydroxyindoles (³H]serotonin + ³H]-5 hydroxyindoleacetic acid) from ³H]tryptophan and that of ³H]-labelled catecholamines from ³H]tyrosine were increased when Ca²⁺ was omitted from the incubating medium. However, the total synthesis of 5-HT from tryptophan and that of catecholamines from tyrosine did not seem to be significantly changed. Altered formation of tritiated amines were due to changes in the specific activities of respective precursor amino acids. This reflected altered sizes of the free amino acid pools caused by Ca²⁺-dependent in vitro proteolysis. This must be taken into consideration when studying in vitro Ca²⁺ dependency of neutrotransmitter metabolism.