Incorporation of L-3,4-dihydroxy-[2-14C]phenylalanine into hamster melanoma melanosomes

Melanosome degradation: fact or fiction

Our mini review summarizes what is known about the (bio)degradation of melanosomes. Unlike melanosome biogenesis where our knowledge enables us to explain it in molecular terms posing many interesting questions on the relation between lysosomes and melanosomes, melanosome degradation has remained 'terra incognita'. Observations at optical and ultrastructural levels describe the disintegration of melanosomes in the lysosomal compartment (in auto- and heterophagosomes). Histochemical studies suggest the participation of acid hydrolases in the process of melanosome degradation. Biochemical data confirm the ability of lysosomal hydrolases to degrade melanosome constituents except the melanin moiety. The similarity of melanin structure to that of polycyclic aromatic hydrocarbons suggests that melanin should be sensitive mainly, if not exclusively, to oxidative breakdown. In vitro melanin can indeed be decomposed by an oxidative attack and the degradation is accompanied by fluorescence and decreasing absorbance. From enzymes engaged in the biotransformation of polycyclic hydrocarbons only phagosomal NADPH oxidase meets the criteria (particularly as for compartmental and catalytic properties) to be involved in melanin biodegradation. The in vivo biodegradation of melanin has so far been clearly demonstrated in Aspergillus and fungi melanins.

An estimate of melanosome concentration in pigment tissues

Concentration of melanosomes in various tissues has been unknown because of the impracticability of their direct quantification. Using an indirect approach comprising the estimation of melanin both in freeze-dried tissue samples and in isolated melanosomes, we obtained data on the amount of melanosomes in various pigment tissues. The concentrations of melanosomes found in the tissues were relatively high, not only reflecting the dark color of pigment tissues but also explaining their capacity to perform various functions ascribed to the presence of melanin.

The natural history of a family of transplantable melanomas in hamsters

We have characterized a family of transplantable melanomas in Syrian (golden) hamsters, which originated in 1959 as a spontaneous melanoma of hamster skin, and which has been maintained since then by serial passage. Emphasis has been placed on using the same method of transplantation, keeping strict records on all passages, and applying the same investigative techniques, in order to trace tumor behavior over long periods of time. This tumor family consists of five variants linked by common origin, but which differ with respect to differentiation level, malignancy, intermediary metabolism, chromosome number, and cell surface properties. Once established, these melanomas possessed a considerable degree of phenotypic stability over decades of passaging. One tumor line in this family is emphasized. The Ab amelanotic melanoma lost its differentiated functions (the ability to synthetize melanin) a quarter of a century ago, and since then has remained dedifferentiated in serial passage in hamsters. After transfer to primary cell culture, the Ab melanoma cells differentiate readily and lose much of their proliferative potential. This process is reversible by reimplantation of the cells into a hamster. Inasmuch as this hamster melanoma system meets many of the conditions required for an experimental tumor model, five melanoma variants are characterized concisely and compared to other melanomas in humans and animals. Mechanisms by which new melanoma variants arise are discussed and compared to some phenomena in the evolution of the species.

Tyrosinase of hamster melanoma: its purification and estimation by radioimmunoassay

1. Tyrosinase was purified from melanosomal fraction of hamster melanoma. 2. A radioimmunoassay was developed to quantitate the tyrosinase protein in hamster serum and hamster melanoma tissue using polyclonal anti-tyrosinase antibodies and 125I-labeled enzyme. 3. The serum tyrosinase levels were found to be about 0.24 micrograms and 1.14-4.48 micrograms/ml in normal hamsters and melanoma-bearing hamsters, respectively. 4. Tyrosinase protein in serum correlated significantly with the enzyme activity in hamsters with melanoma (r = 0.733). 5. In the cytosol fraction of hamster melanoma, a level of 2.2 micrograms of tyrosinase/mg protein was determined. 6. The usefulness and possible applications of the tyrosinase radioimmunoassay are discussed.

An animal model for the survival of tyrosinase isozymes in serum

Two tyrosinase isozymes were purified from pigmented hamster melanoma and injected i.v. into rats. It was shown that while T1 (sialylated) isozyme survived in the circulation, native asialo (T2) isozyme and neuraminidase-desialylated T1 isozyme disappeared from the circulation in a few minutes. Desialylated fetuin had a marked inhibitory effect on the removal of asialo-T1 tyrosinase. These data indicate that the enzyme tyrosinase shares the common pattern of clearance from circulation known for the majority of serum glycoproteins. The electrophoretic pattern of tyrosinase isozymes partially purified from the sera of melanoma-bearing animals were compared with those from the soluble fraction of the tumors. In hamsters, melanoma tissue revealed both T1 and T2 isozymes while serum exhibited T1 and very weak T2, supporting the mechanism of clearance demonstrated in rats. In mice bearing Cloudman S-91 or B-16 melanomas, only T1 isozyme was seen in sera and in tumors.