GC/MS analysis of thermally degraded neuromelanin from the human substantia nigra.

Neuromelanin (NM) is a complex polymer pigment found in catecholaminergic neurons of the human brain. The structure, formation pathway, and physiological function of NM have not yet been clarified, but interest in this polymer has been sparked by the suggestion that NM is involved in cell death in Parkinson's disease. In the current study, pyrolysis-gas chromatography/mass spectrometry analysis was applied for structural investigation of NM isolated from the human substantia nigra, using synthetic eumelanin and pheomelanin-type pigments as reference materials. None of the heterocyclic, sulfur-containing compounds being characteristic thermal degradation products of cysteinyldopamine-derived units of synthetic pheomelanin standard was detected in the pyrolysates of natural NM. The results suggest that nigral pigment isolated from normal brain tissue does not contain benzothiazine-type monomer units. Pyrolytic experiments in the presence of a derivatizing agent allowed identification of high levels of saturated and monounsaturated straight-chain C14-C18 fatty acids and led to the conclusion that a part of a lipid component is chemically bound to the NM macromolecule. The nigral pigment was also shown to be tightly associated with an isoprenoid-type compound.