Mitochondrial functions of polymelanosomes

Neuromelanin and its possible protective and destructive properties

The function of neuromelanin is not known, but some properties of the pigment suggest a protective action. Its unique ability to accumulate and retain several compounds, such as various amines and a number of metals, may protect the pigment-containing neurons from high exposure to harmful substances. This possible mechanism of protection may however in certain instances be of a double-edged nature, as accumulation of neurotoxic agents with a high melanin affinity may cause toxic concentrations in the neuro-melanin-containing cells. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) seems to be such a compound, as it has been found to preferentially destroy neuromelanin-containing cells. The degree of MPTP neurotoxicity seems to be related to the amount of neuromelanin present in the particular species. It is possible that also manganese, which is known to cause an extrapyramidal disorder resembling Parkinson's disease, causes injury to neuromelanin-bearing neurons due to its melanin affinity. This mechanism may be involved in other forms of chemically induced Parkinsonism and possibly also in idiopathic Parkinson's disease, although the offending agent remains to be discovered.

Autoradiography of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP): uptake in the monoaminergic pathways and in melanin containing tissues

A recently discovered neurotoxic compound, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, has been found to cause a parkinsonian-like syndrome in man and monkey, but not in laboratory animals such as rat, mouse and guinea pig. MPTP seems to selectively destroy the melanin containing dopaminergic cells in pars compacta of substantia nigra. Lower mammalian species do not have melanin in these cells, which indicates that the presence of neuromelanin may be of importance for the development of MPTP-induced lesions. By means of whole-body autoradiography of 3H-MPTP in mice, accumulation and retention was observed in the dopaminergic pathways, in locus caeruleus and in structures in the medulla oblongata and spinal cord. A high uptake was also seen in melanin-containing tissues such as in the eyes of pigmented mice. MPTP has earlier been found to have high affinity in vitro for dopamine melanin, which is similar to the pigment in substantia nigra. The typical features of the MPTP-induced neurotoxicity with destruction of pigmented nerve cells and development of parkinsonism may be to to accumulation and retention of MPTP and its metabolites in these cells. In species with pigmented nerve cells, such as man and monkey, the accumulation may be much more pronounced because of the melanin affinity of MPTP and its metabolites.

Melanin affinity of manganese

The melanin affinity of manganese was studied in vitro using melanin isolated from beef eyes or human hair, and synthetic dopamine melanin, which is known to be structurally similar to the melanin present in the pigmented nerve cells in the human substantia nigra. In addition, the uptake of manganese in melanin containing tissues in vitro and in vivo was studied by whole body autoradiography. Manganese was bound to beef eye, human hair and synthetic dopamine melanin, and was taken up in pigmented tissues in mice and a monkey. Long-time exposure to manganese may cause a chronic extrapyramidal disorder. It is suggested that manganese, due to its neurotoxicity, may cause lesions in pigment containing neurones in the substantia nigra secondary to its accumulation on the neuromelanin.

A mutation influencing the transportation of manganese, L-dopa, and L-tryptophan

Transportation of manganese, L-dopa, and L-tryptophan was slower through the tissues, of intact pallid mice than through those of black C57Bl/6J mice, presumably because of the influence of the gene pallid.