Inability of murine melanoma melanosomal "tyrosinase" (L-dopa oxidase) to oxidize tyrosine to melanin in polyacrylamide gel systems.

Melanosomal "tyrosinase" (L-dopa) was isolated from trypsin digest of B-16 mouse melanoma melanosomes, using polyacrylamide gel disc electrophoresis. The enzyme was represented by a single band, having characteristics similar to the T1 dopa-positive band observed when using supernatants of crude melanoma homogenates as the source. When gels with this band were incubated in solutions containing tyrosine and dopa in varying ratios , there was no enhancement of melanin formation by tyrosine when compared with incubations in corresponding concentrations of dopa alone. These data further support previous studies in our laboratory demonstrating an inability of so-called mamalian "tyrosinase" to convert tyrosine to melanin; since this enzyme readily converts L-dopa to melanin, it seems more reasonable to term this enzyme an L-dopa oxidase.

Inability to demonstrate hydroxylation of tyrosine by murine melanoma "tyrosinase" (L-DOPA oxidase), using the tritiated water assay technique.

Validity of the tritiated water assay technique for tyrosine hydroxylase activity as a qualitative method was demonstrated with mushroom tyrosinase. Using this method, isolated murine melanoma "tyrosinase" (L-dopa oxidase) showed no tyrosine hydroxylase activity. This finding supports previous studies in our laboratory which used a variety of histochemical and biochemical methods. The nonenzymatic production of tritiated water caused by tritium exchange with hydrogen peroxide complicates the use of the tritiated water assay technique with crude systems, since hydrogen peroxide is generated by a variety of oxidase reactions. For this reason, previous studies using the tritiated water assay technique with crude systems are ambiguous.

Melanoma resembling spindle and epithelioid cell nevus.

Three cases of malignant melanoma resembling spindle and epithelioid cell nevus histologically are presented. Lesions having histologic features of spindle and epithelioid cell nevus (Spitz nevus or juvenile melanoma) at or after puberty should be regarded with caution, particularly when they are heavily melanized. Such lesions are in a histologic "gray zone" and may be malignant.

Silhouette symmetry: an unsupportable histologic criterion for distinguishing Spitz nevi and compound nevi from malignant melanoma.

BACKGROUND: In 1989, Ackerman proposed that pattern analysis can be a more accurate method of evaluating neoplasms than assessment of nuclear morphology. He stated that malignant melanomas tend to be asymmetrical, and benign melanocytic neoplasms tend to be symmetrical. METHODS: We examined a series of typical Spitz nevi, compound nevi, and malignant melanomas with the loupe and with higher magnifications. We also considered pattern analysis from the standpoint of solid geometry. RESULTS: The ratio of symmetrical to asymmetrical lesions (visualized in histologic sections) in malignant melanomas approximated that of Spitz nevi and compound nevi. The configuration seen in sections does not necessarily reflect the three-dimensional configuration of a lesion. Lesions with three-dimensional reflective symmetry cannot be shown to have this property in sections unless cuts are made perpendicular to the plane of symmetry, which is an impossibility. CONCLUSIONS: The presence or absence of symmetry in sections of neoplasms must be considered coincidental or focal. Using symmetry as a criterion to distinguish malignant melanomas from Spitz nevi and compound nevi is without validity on both theoretical and empirical grounds.

The role of peroxidase in neuromelanin synthesis: a review.

There is currently no general agreement on the enzymatic basis of neuromelanin synthesis. It is generally agreed that neuromelanin synthesis must differ from melanin synthesis in skin, since "tyrosinase" (aerobic dopa oxidase) is not present in brain. Various proposals have suggested that monoamine oxidase, prostaglandin H synthetase, or peroxidase may catalyze neuromelanin formation from dopa, dopamine and norepinephrine. It has also been proposed that neuromelanin synthesis may be based on the pseudoperoxidase activity of metal ions. Our group was the first to demonstrate enzymatic peroxidase activity in brain, which was subsequently confirmed by three other laboratories. Our group was also the first to demonstrate the melanogenic activity of brain peroxidase (subsequently confirmed by another laboratory) and to propose that peroxidase is the key enzyme in neuromelanin synthesis because of this potential and because of its presence in lysosomes, the subcellular site of neuromalin.