DOPA Melanin Research Articles

Inhibition of l-tyrosine decarboxylase activity by l-dopa melanin in vitro

1. 1. l-Tyrosine decarboxylase apoenzyme of Streptococcus faecalis can easily decar boxy late dihydroxyphenylalanine and this aromatic l-amino acid decarboxylase is a pyridoxal phosphate-dependent enzyme. 2. 2. The activity of this enzyme involved in the decarboxylation of dihydroxyphenylalanine is continuously inhibited by dihydroxyphenylalanine melanin.

Characterization of "Pyomelanin"-Producing Strains of Pseudomonas aeruginosa

Forty strains of Pseudomonas aeruginosa (Schroeter) Migula isolated from clinical specimens and producing a water-soluble brown pigment on peptone media were compared with the neotype and nine other non-brown strains of P. aeruginosa. The brown pigment is formed from either tyrosine or phenylalanine through the accumulation of homogentisic acid and has no identity with DOPA melanin. A new term, “pyomelanin,” is proposed for this brown pigment synthesized by certain strains of P. aeruginosa. Although the pyomelanin strains share the attributes of the species, some of them have a more limited capacity with regard to certain biochemical reactions than the neotype strain of P. aeruginosa. A careful examination is required to identify these aberrant strains. Actively growing cells of pyomelanin strains converted tyrosine to pyomelanin, but tyrosine itself does not appear to serve as a sole source of carbon and nitrogen for growth. A simple method of differentiating pyomelanin-producing strains of P. aeruginosa from pyorubin-producing strains is presented. To recognize properly the species, the attributes of pyomelanin strains should be included in descriptions of P. aeruginosa.

Latency of the dopa reaction in mast cells.

Light microscopic observations of normal rat peritoneal mast cells and ultrastructural observations of human mast cells from lesions of nodular mastocytosis indicated that structural damage results in a pronounced increase in percentage of cells with a positive dopa reaction. Recent studies have indicated that the dopa reaction in mast cells is peroxidase-dependent. Enhancement of the dopa reaction by structural damage (latency) is probably related to increased substrate-enzyme interaction. Ultrastructural localization of dopa melanin to mast cell granules and the high percentage of mast cells showing a positive dopa reaction after structural damage is evidence against the possibility that dopa melanin is formed in mast cells by phagocytized enzyme.

The structure of melanins and melanogenesis—II

A comparative study on sepiomelanin and three synthetic pigments, namely dopa melanin, 5,6-dihydroxyindole melanin and pyrrole black, has been carried out. Sepiomelanin is structurally related to 5,6-dihydroxyindole and dopa melanins. The only observed differences are the absence of carboxyl groups in position 2 of the indole nuclei in the synthetic pigments (this result, obvious for 5,6-dihydroxyindole melanin, is rather surprising for dopa melanin) and the presence of α-amino acid chains in dopa melanin. Some carboxyl groups found in dopa and 5,6-dihydroxyindole melanins are located on pyrrole units. Considerable differences arise when comparing pyrrole black with sepiomelanin.

New Technic for Comprehensive Demonstration of Melanin, Premelanin, and Tyrosinase Sites: Combined Dopa-Premelanin Reaction

Bizzozero (1) (1908) pointed out that melanin granules reduce silver nitrate to the metallic state and become very dark. Miescher (2) (1922) described that the dark substance resulting from the silver reaction is a compound of silver and melanin. Reduction to organic silver compounds needs a much longer time than does the silver reaction for darkening melanin granules. Masson (3) (1911) introduced ammoniacal (reduced) silver nitrate, which proves to be more sensitive, and blackens all premelanin and melanin granules and those only. Bloch (4) (1917) found that the pigment-forming cells are darkened in their entirety when frozen sections of fresh tissue are placed in 3,4-dihydroxyphenylalanine. This darkening is due to the presence of dopa melanin which has been formed by the action of a tyrosinase on the dopa. Becker, Sr. (5, 6) (1935) discovered that the paraffin section method of performing the dopa reaction permits a more exact cytologic study than Bloch's frozen section method.