Identification of two new metabolites in melanoma urine: 5-hydroxy-6-methoxyindole-2-carboxylic and 5-methoxy-6-hydroxyindole-2-carboxylic acids

A Qualitative Gas Chromatographic Analysis of Substituted 5,6-Dihydroxyindoles from the Urine of Patients with Melanoma

A qualitative gas chromatographic analysis of trimethylsilylated ethyl acetate extracts of melanotic urine revealed 5 indolic compounds, which have been identified as substituted 5,6-dihydroxyindoles. Ethyl acetate extracts of melanotic urines at pH 2.0 contained isomeric 5-hydroxy-6-methoxy and 6-hydroxy-5-methoxy-indolyl-2-carboxylic acids which were not separable under the conditions used. A careful hydrolysis of melanotic urine with a Helix pomatia preparation followed by extraction at pH 6.5 in a nitrogen atmosphere released 3 additional indolic compounds from their conjugated form. Using gas chromatographic-mass spectrometric analysis they were identified as 5-hydroxy-6-methoxy, 6-hydroxy-5-methoxyindole and 5,6-dihydroxyindole.

Involvement of photooxidation of melanogenic precursors in prolonged pigmentation induced by ultraviolet A

Melanin is usually produced in melanocytes; however, our histologic research indicates that brownish black persistent pigmentation occurs at the basal layer and, partially, at the suprabasal layer of the epidermis, outside of melanocytes, in skin exposed to high doses of ultraviolet (UV) A radiation. This pigmentation remains for several weeks. We examined whether colorless melanogenic precursors participate in this UVA-induced persistent pigmentation. Among them, 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and its O-methyl metabolite, 6-hydroxy-5-methoxyindole-2-carboxylic acid (6H5MICA), can change into dark-colored pigment upon exposure to UVA, but not UVB, radiation, producing irreversible brownish black pigmentation. These hydroxyindole derivatives readily changed into dark-colored pigment upon UVA irradiation in the presence of hydrogen peroxide in vitro. Histochemical and biochemical studies demonstrated that 6H5MICA accumulates in excised skin specimens and supernatant of cultured human melanocytes. These results suggest that dark-colored pigment is generated directly, outside of melanocytes, in response to UVA exposure, by photooxidation of O-methyl metabolites of DHICA, such as 6H5MICA, at least. We propose that the pigmentation derived from this compound at the basal and suprabasal layers of the epidermis is involved in the mechanism of skin persistent pigmentation without reddening, observed after exposure to sunlight.

Cysteine and indole derivatives as markers for malignant melanoma

Malignant melanoma is a skin tumour, which carries a very unfavourable prognosis. The early detection of a melanoma and even more its metastasis is of decisive importance for the survival prognosis of the patients. So there is always a desire for simple, economical and meaningful serological markers. From the cysteine- and indole-related derivatives, 5-S-cysteinyldopa (5-SCD) and 6-hydroxy-5-methoxy-indole-2-carboxylic acid (6H5MI2C) are the most important substances for this purpose. For 5-SCD, the sample pretreatment was carried out either by a manual extraction onto alumina, by an automated method onto boronic acid affinity gels or by an automated solid-phase extraction. For 6H5MI2C, liquid-liquid extractions or direct injection techniques were applied. The chromatographic analyses in the early years were mostly performed with GC-MS. Today HPLC is the nearly exclusively used separation technique. For HPLC, standard RP18 separating columns and usual compositions of eluents were applied. As detectors both the ECD and the FD showed a sufficient sensitivity and selectivity. 5-SCD and 6H5MI2C are very sensitive to light and oxidation. These properties must be taken into account in the complete analysis procedure, including the sample collection, otherwise false low values will result especially for plasma samples. For a critical discussion of the analytical methods and still more for the interpretation of the obtained results, the detailed analytical procedures must be considered. 5-SCD in plasma is one of the best markers of malignant melanoma. It shows an excellent specificity and also an adequate sensitivity in the metastatic melanoma stages. For the detection of primary melanomas and for urine instead of plasma samples, the sensitivity of 5-SCD is generally lower. Altogether, the sensitivity of this parameter is not yet sufficient. 6H5MI2C and other indole derivatives have been investigated far less than 5-SCD. 6H5MI2C correlates less clearly with the different stages of the melanoma and is therefore a less suitable marker. To improve the sensitivity of the findings, in future the investigations should be performed as multi-marker analysis with the simultaneous measurements of more than one marker substance in a given patient sample. Not only one measurement should be carried out per patient, it would be more meaningful to observe the patients with laboratory diagnostics in the follow-up.

Some aspects of melanin formation of melanocytes cultured on collagen-coated microcarrier beads

Melanocytes cultured on collagen-coated Cytodex 3 microcarrier Sephadex beads caused remarkable pigmentation of the beads during the period of culture when optimal density was reached. Electron microscopy of melanocytes on the microcarriers revealed that the cells and their dendrites invaginate into the microcarrier surface layer. Removal of the cells by trypsinization showed that some pigment granules were left on the carrier surface and within the cavities present on the microcarrier surface. In order to investigate whether the pigmentation of the microcarriers could be a result of indole intermediates of melanogenesis present in the culture medium, extracts were studied by gas chromatography/mass spectrometry for the presence of these compounds. Two compounds (5,6-dihydroxyindole-2-carboxylic acid and 6-hydroxy-5-methoxyindole-2-carboxylic acid) so far have been identified in the medium extracts. Results indicate that microcarrier culture of melanocytes can serve as an interesting model for electron microscopy studies of melanocytes with regard to pigmentation and cell attachment.

O-methylation of L-dopa in melanin metabolism and the presence of catechol-O-methyltransferase in melanocytes

O-Methylation of L-dopa was investigated as a possible regulatory mechanism in melanin metabolism. The methylation product of L-dopa, 3-O-methoxytyrosine was detected in extracts of cultured human melanocytes. The enzyme catechol-O-methyltransferase is responsible for this O-methylation and that of the dihydroxyindolic intermediates of melanogenesis. The enzyme is present in melanocytes in its soluble and membrane-bound isoforms. Immuno-electron microscopy suggests the presence of the membrane-bound enzyme in the endoplasmic reticulum. This localization may indicate a role of catechol-O-methyltransferase in protecting the melanocyte against reactive dihydroxyphenolic intermediates of melanogenesis leaking from the melanogenic compartments. On the other hand, the O-methylation of L-dopa may serve as a regulatory point in melanogenesis during early stage of tyrosinase processing in the endoplasmic reticulum.

Indolic urinary melanogens: separation and identification by gas chromatography with selected-ion monitoring mass spectrometry of 5-hydroxy-6-methoxyindole-2-carboxylic and 5-methoxy-6-hydroxyindole-2- carboxylic acids

Two isomeric urinary melanogens, 5-hydroxy-6-methoxyindole-2-carboxylic acid and 5-methoxy-6-hydroxyindole-2-carboxylic acid, have been separated by gas chromatography with selected-ion monitoring mass spectrometry. After chemical synthesis of one of these two isomers, 5-methoxy-6-hydroxyindole-2-carboxylic acid, and the establishment of the mass spectrum of its trimethylsilylated derivative, a 30-ml sample of a melanotic 24-h urine was adjusted to pH 1 and extracted twice with 10 ml of ethyl acetate. The extract was evaporated to dryness and the residue derivatized with methyl-8, followed by Tri-Sil/TBT. Silylated derivatives were analysed by gas chromatography with selected-ion monitoring mass spectrometry. The mass spectrum of the 5-methoxy-6-hydroxyindole-2-carboxylic acid allowed the determination of the retention times of both isomers.

After dopachrome?

Dopachrome, an intermediate in melanin biosynthesis, exhibits some unusual properties. At physiologic pH (e.g., pH 6-8) it is unstable and spontaneously loses its carboxyl group to form 5,6-dihydroxyindole (DHI) and CO2. However, over this same pH range, if various metals or a melanocyte-specific enzyme are present, it rapidly rearranges to its isomer form--5,6-dihydroxyindole-2-carboxylic acid (DHICA)--which is far more stable than dopachrome in its ability to retain the carboxyl group. Whether or not the carboxyl group is retained could have important implications for the regulation of melanogenesis, since in the presence of oxygen DHI spontaneously forms a black precipitate, whereas DHICA forms a golden-brown solution. The solubility of "DHICA-melanin" is due to the presence of carboxyl groups, which provide negative charges and hydrophilicity. Thus, in vivo, the extent to which dopachrome is converted to DHI or DHICA may well influence the solubility and color of the melanin formed. The purpose of this article is to review recent findings in these areas and to discuss the possible significance of dopachrome conversion in the regulation of melanogenesis and color formation.

Identification of ester glucuronide and sulfate conjugates of 5-hydroxy-6-methoxyindole-2-carboxylic acid and 6-hydroxy-5-methoxyindole-2-carboxylic acid in melanoma urine

HPLC analysis of urine sample from a patient with wide-spread melanoma revealed the presence of unknown indolic compounds at extraordinarily high levels, detectable with electrochemical and/or fluorescent detectors. By enzymic and chemical hydrolyses, they were identified as ester glucuronide and sulfate conjugates of 5-hydroxy-6-methoxyindole-2-carboxylic acid and 6-hydroxy-5-methoxyindole-2-carboxylic acid. Urine samples from B16 melanoma-bearing mice contained the sulfate conjugates but not the ester glucuronide conjugates.

Chromatography of urinary indole derivatives

Latest strategies are discussed for the routine chromatographic analysis of clinically important indole derivatives in urine. Analysis of 5-hydroxyindoleacetic acid and, perhaps more importantly, serotonin and 5-hydroxytryptophan remains attractive in the screening for carcinoid tumours and their differentiation. Analyses of two precursors of the skin pigment eumelanin seem to be promising in the monitoring of treatment of malignant melanoma and screening for pigmentation disorders and gallstone formation. Studies on the clinical relevance of the determination of tetrahydro-beta-carbolines and melatonin-related indoles await routine application of chromatographic methods designed to take into consideration the relative instability of these compounds. Application of GC-MS, although confined to larger and/or governmental laboratories remains attractive as a way of improving the specificity of analyses and in establishing reference methods. As for HPLC, the recent development of chromatographic and detection methods for the concurrent determination of different clinically important and metabolically related compounds from the same sample, preferably by direct injection techniques, seems to be fruitful and should be continued.

Sunlight, melanogenesis and radicals in the skin

Melanocytes are cells of neural crest origin residing at the dermal-epidermal juncture. They produce specialized organelles called melanosomes within which the biochemical processes of melanization occurs. UV radiation is capable of inducing melanogenesis and, during the biosynthesis of melanins, several of the putative precursors "leak out" of the melanosome and can be detected in the skin, serum and urine of individuals undergoing active melanogenesis. Most notable are the cysteinyldopas (formed by nucleophilic addition of cysteine to dopaquinone) and several dihydroxyindoles (formed by intramolecular cyclization of dopaquinone). These catechols often are methylated in the melanocyte to afford a mixture of the monomethoxy derivatives and, in some cases, the dimethoxy species. Recent investigations in our laboratories have demonstrated that the cysteinyldopas, dihydroxyindoles, and their various methylated derivatives are photochemically unstable. Irradiation with biologically relevant ultraviolet radiation (i.e. wavelengths greater than 300 nm) results in the rapid destruction of the precursors/metabolites and the production of a variety of free radical species. The photochemistry and potential photobiological significance of melanogenic intermediates is discussed.

Photogeneration of free radicals from eumelanogenic intermediates and metabolites

We have recently demonstrated that cysteinyldopas, pheomelanogenic precursors and excreted eumelanogenic metabolites, are photolabile and initiate DNA damage in vitro. In this study we have extended our photochemical investigations to eumelanogenic indole intermediates and metabolites. Continuous-wave photolysis of 5,6-dihydroxyindole (DHI), 5,6-dihydroxyindole-2-carboxylic acid (DHICA), or its 5-methoxylated metabolite (HMICA) with biologically relevant ultraviolet radiation (i.e., wavelengths greater than 300 nm) resulted in rapid destruction of starting material. Using ESR spin-trapping techniques we observed the initial production of free radical species; prolonged photolysis resulted in the formation of polymeric photoproducts. Radicals were trapped by the nitrone spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and characterized by their ESR spectra as hydrated electrons and hydrogen atoms. Experiments further demonstrated that while DHI photoionizes, the two indole-2-carboxylic acid derivatives do not ionize appreciably upon irradiation, rather homolysis of X-H bonds appears to be a significant photochemical pathway. The potential photobiological significance of melanogenic indole intermediates is discussed.

Eumelanin-related indolic compounds in the urine of treated melanoma patients

A method was developed for the quantitation of six eumelanin-related indolic compounds in urine. The procedure employs methane ionization of the hexafluoroisopropyl and/or pentafluoropropionyl derivatives of 5,6-dihydroxyindole, 5-hydroxy-6-methoxyindole, 6-hydroxy-5-methoxyindole, 5-hydroxy-6-methoxyindolyl-2-carboxylic acid, 6-hydroxy-5-methoxyindolyl-2-carboxylic acid and 5,6-dimethoxyindolyl-2-carboxylic acid together with their deuterated analogues, and selected ion-monitoring. Deuterated analogues are added as internal standards to enzymatically hydrolysed urine before diethyl ether extraction. The method was used for monitoring urinary excretion in patients with malignant melanoma treated with cytostatics. In several cases, a decrease of the excretion of the eumelanin-related substances was recorded after the therapy. The usefulness of the individual indolic compounds for such follow-up studies is discussed.

Synthesis in vitro of 5,6-dihydroxyindole-2-carboxylic acid by dopachrome conversion factor from Cloudman S91 melanoma cells

The oxidation of dopachrome to melanin occurs spontaneously in vitro but has been shown to be under enzymatic control in vivo. Dopachrome conversion factor, purified from Cloudman S91 melanoma cells, converts dopachrome, which is orange, to a colorless compound. We have purified this colorless compound and identified it as 5,6-dihydroxyindole-2-carboxylic acid. This compound is generated enzymatically from a solution containing dopa and dopachrome and, under these conditions, is not readily oxidized to melanin in solution at pH 6.8. The synthesis in vitro of 5,6-dihydroxyindole-2-carboxylic acid suggests that this compound may be an intermediate in the synthesis of melanin.

Identification of three indolic compounds in a pigmented-melanoma cell-culture supernatant by gas chromatography-mass spectrometry

In the supernatant of melanoma cell culture SK mel 25 three indolic compounds - 5-hydroxy-6-methoxyindole, 5-hydroxy-6-methoxyindolyl-2-carboxylic, and 6-hydroxy-5-methoxyindolyl-2-carboxylic acids - have been identified. The supernatants were extracted with ethyl acetate, derivatized, and analyzed by gas chromatography-mass spectrometry. The significance of this finding is discussed.

Identification of two Thormählen-positive compounds from melanotic urine by gas chromatography-mass spectrometry

The isolation of two Thormählen-positive compounds from the urine of a patient with malignant melanoma and the elucidation of their structure by gas chromatography-mass spectometry is described. The compounds were isolated using a poly-N-vinylpyrrolidone column and separated by preparative thin-layer chromatography. After elution they were analyzed by gas chromatography and gas chromatography-mass spectrometry as their trimethylsilyl derivatives and after hydrolysis also as their tert.-butyldimethylsilyl derivatives. The results showed the main Thormählen-positive compound A to be the glucuronide of 5-hydroxy-6-methoxyindole, whereas the minor compound AX appeared to be the glucuronide of its isomer 6-hydroxy-5-methoxyindole.

Simple and sensitive test for the determination of phenolic compounds in urine and its application to melanoma

A simple procedure is outlined for the determination of phenolic compounds in urine. The method involves the diazotization of p-nitroaniline and subsequent coupling of the diazonium chloride with phenolic compounds. The diazo compound formed is determined by measuring the absorbance at 400 nm. The extinction coefficients for several compounds have been determined. Application of this method to determine the levels of phenolic compounds in urine in patients with melanoma showed that the assay may be of diagnostic value. The amounts of phenols in 24-hr urine samples from patients with melanoma, patients with various other diseases, and healthy individuals were determined. A statistical analysis of these results showed that the results for patients with melanoma were higher than those for healthy individuals and patients with other diseases at 1 percent probability level. A comparison of this test with three other methods (Thormählen test, ferric chloride test, and ferrocitrate test) for determination of melanogens showed that the diazo test is the most sensitive one for detecting melanoma.