Determination of catechol O-methyltransferase activity in relation to melanin metabolism using high-performance liquid chromatography with fluorimetric detection

Optical substrates for drug-metabolizing enzymes: Recent advances and future perspectives

Drug-metabolizing enzymes (DMEs), a diverse group of enzymes responsible for the metabolic elimination of drugs and other xenobiotics, have been recognized as the critical determinants to drug safety and efficacy. Deciphering and understanding the key roles of individual DMEs in drug metabolism and toxicity, as well as characterizing the interactions of central DMEs with xenobiotics require reliable, practical and highly specific tools for sensing the activities of these enzymes in biological systems. In the last few decades, the scientists have developed a variety of optical substrates for sensing human DMEs, parts of them have been successfully used for studying target enzyme(s) in tissue preparations and living systems. Herein, molecular design principals and recent advances in the development and applications of optical substrates for human DMEs have been reviewed systematically. Furthermore, the challenges and future perspectives in this field are also highlighted. The presented information offers a group of practical approaches and imaging tools for sensing DMEs activities in complex biological systems, which strongly facilitates high-throughput screening the modulators of target DMEs and studies on drug/herb‒drug interactions, as well as promotes the fundamental researches for exploring the relevance of DMEs to human diseases and drug treatment outcomes.

Analytical methodologies for sensing catechol-O-methyltransferase activity and their applications

Mammalian catechol-O-methyltransferases (COMT) are an important class of conjugative enzymes, which play a key role in the metabolism and inactivation of catechol neurotransmitters, catechol estrogens and a wide range of endobiotics and xenobiotics that bear the catechol group. Currently, COMT inhibitors are used in combination with levodopa for the treatment of Parkinson's disease in clinical practice. The crucial role of COMT in human health has raised great interest in the development of more practical assays for highly selective and sensitive detection of COMT activity in real samples, as well as for rapid screening and characterization of COMT inhibitors as drug candidates. This review summarizes recent advances in analytical methodologies for sensing COMT activity and their applications. Several lists of biochemical assays for measuring COMT activity, including the probe substrates, along with their analytical conditions and kinetic parameters, are presented. Finally, the challenges and future perspectives in the field, such as visualization of COMT activity in vivo and in situ, are highlighted. Collectively, this review article overviews the practical assays for measuring COMT activities in complex biological samples, which will strongly facilitate the investigations on the relevance of COMT to human diseases and promote the discovery of COMT inhibitors via high-throughput screening.

Gossypol exhibits a strong influence towards UDP-glucuronosyltransferase (UGT) 1A1, 1A9 and 2B7-mediated metabolism of xenobiotics and endogenous substances

Gossypol, the polyphenolic constituent isolated from cottonseeds, has been used as a male antifertility drug for a long time, and has been demonstrated to exhibit excellent anti-tumor activity towards multiple cancer types. The toxic effects of gossypol limit its clinical utilization, and enzyme inhibition is an important facet of this. In the present study, in vitro human liver microsomal incubation system supplemented with UDPGA was used to investigate the inhibition of gossypol towards UGT1A1, 1A9 and 2B7-mediated metabolism of xenobiotics and endogenous substances. Estradiol, the probe substrate of UGT1A1, was selected as representative endogenous substance. Propofol (a probe substrate of UGT1A9) and 3'-azido-3'-deoxythimidine (AZT, a probe substrate of UGT2B7) were employed as representative xenobiotics. The results showed that gossypol noncompetitively inhibits UGT-mediated estradiol-3-glucuronidation and propofol O-glucuronidation, and the inhibition kinetic parameters (K_i) were calculated to be 34.2 and 16.4 μM, respectively. Gossypol was demonstrated to exhibit competitive inhibition towards UGT-mediated AZT glucuronidation, and the inhibition kinetic parameter (K_i) was determined to be 14.0 μM. All these results indicated that gossypol might induce metabolic disorders of endogenous substances and alteration of metabolic behaviour of co-administered xenobiotics through inhibition of UGTs’ activity.

A universal competitive fluorescence polarization activity assay for S-adenosylmethionine utilizing methyltransferases

A high-throughput, competitive fluorescence polarization immunoassay has been developed for the detection of methyltransferase activity. The assay was designed to detect S-adenosylhomocysteine (AdoHcy), a product of all S-adenosylmethionine (AdoMet)-utilizing methyltransferase reactions. We employed commercially available anti-AdoHcy antibody and fluorescein-AdoHcy conjugate tracer to measure AdoHcy generated as a result of methyltransferase activity. AdoHcy competes with tracer in the antibody/tracer complex. The release of tracer results in a decrease in fluorescence polarization. Under optimized conditions, AdoHcy and AdoMet titrations demonstrated that the antibody had more than a 150-fold preference for binding AdoHcy relative to AdoMet. Mock methyltransferase reactions using both AdoHcy and AdoMet indicated that the assay tolerated 1 to 3 microM AdoMet. The limit of detection was approximately 5 nM (0.15 pmol) AdoHcy in the presence of 3 muM AdoMet. To validate the assay's ability to quantitate methyltransferase activity, the methyltransferase catechol-O-methyltransferase (COMT) and a known selective inhibitor of COMT activity were used in proof-of-principle experiments. A time- and enzyme concentration-dependent decrease in fluorescence polarization was observed in the COMT assay that was developed. The IC(50) value obtained using a selective COMT inhibitor was consistent with previously published data. Thus, this sensitive and homogeneous assay is amenable for screening compounds for inhibitors of methyltransferase activity.

Investigations on gossypol: past and present developments

Gossypol has received significant attention as a result of its potential therapeutic application as a male antifertility agent. Furthermore, recent research examining the biological activity of gossypol has revealed a number of other promising lines of enquiry. These have focused on the antitumour, antiviral and antioxidant actions of the compound in various disease states. This article provides an overview of the studies on the biological activity of gossypol, with particular attention paid to the mechanisms of its activity and its prospect as a medicinal product.

A refined HPLC method to measure catecholamine-o-methyltransferase activity in selected brain regions

An improved HPLC method, with fluorogenic detection, for the determination of catecholamine-o-methyltransferase (COMT) activity in the brain has been developed. A catechol compound, 3,4-dihydroxybenzoic acid (3,4DOBA), was used as a highly fluorogenic substrate for COMT. The meta- and para-methylated products formed enzymatically from the substrate, after incubation with a brain region homogenate, were separated and measured using HPLC with fluorescence detection. This described method resulted in a more definitive enzyme product quantification with shorter analysis time than that previously described in the literature. This approach was used successfully to study COMT activity in vitro from small discreet brain regions of individual rats.

Polymorphism in candidate genes: implications for the risk and treatment of idiopathic Parkinson's disease

Idiopathic Parkinson's disease (IPD) is a progressive neurodegenerative disorder for which no restorative or neuroprotective therapy is available. Interest has recently been directed to association studies on polymorphisms of various genes, mainly those related to dopamine metabolism and transport, and their effect on response to PD, which includes primarily levodopa and dopaminomimetics. Approximately 15-20% of patients with PD do not respond to levodopa, and the majority of those who do respond develop adverse fluctuations in motor response, primarily levodopa-induced dyskinesias. This review summarizes the influence of polymorphisms in various genes on the relative risk of IPD and on levodopa efficacy. It focuses on the importance of well-designed polymorphism studies that include large samples of patients with IPD and tightly matched controls and use identical methodologies. Valid data on such polymorphisms might increase the efficacy of levodopa, decrease its side effects, and reduce the occurrence of levodopa-induced dyskinesias. They might also provide a novel diagnostic tool for PD.

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.

Separation methods for catechol O-methyltransferase activity assay: physiological and pathophysiological relevance

Catechol O-methyltransferase (COMT) transfers a methyl group from S-adenosyl-L-methionine to the catechol substrate in the presence of magnesium. After the characterisation of COMT more than four decades ago, a wide variety of COMT enzyme assays have been introduced. COMT activity analysis usually consists of the handling of the sample and incubation followed by separation and detection of the reaction products. Several of these assays are validated, reliable and sensitive. Besides the studies of the basic properties of COMT, the activity assay has also been applied to explore the relation of COMT to various disease states or disorders. In addition, COMT activity analysis has been applied clinically since COMT inhibitors have been introduced as adjuvant drugs in the treatment of Parkinson's disease.

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.

Genotypes of catechol-O-methyltransferase and response to levodopa treatment in patients with Parkinson's disease

A single nucleotide polymorphism at the nucleotide 1947 in the catechol-O-methyltransferase (COMT) gene encodes the high and low activity forms of the enzyme. We investigated COMT genotypes of 73 Korean patients with Parkinson's disease (PD), 29 with multiple system atrophy (MSA), and 49 controls, and analyzed the response to levodopa challenge in the PD patients. We found no significant difference in the distribution of the COMT genotypes among the three groups. The frequencies of the G- and A-alleles in the total population were 75 and 25%, respectively. The levodopa response was determined by a single oral levodopa challenge test with Sinemet (25/250 mg) in the patients with PD. The motor response evaluated by the time to peak response, the duration and magnitude of the response in the motor part of the Unified Parkinson's Disease Rating Scale; tapping or walking times showed no significant difference between the genotypes. Thus, pharmacokinetic or pharmacodynamic factors other than the investigated genetic variant of the COMT enzyme seem to determine the response to levodopa in PD.

Neuroendocrinology of the skin

The classical observations of the skin as a target for melanotropins have been complemented by the discovery of their actual production at the local level. In fact, all of the elements controlling the activity of the hypothalamus-pituitary-adrenal axis are expressed in the skin including CRH, urocortin, and POMC, with its products ACTH, alpha-MSH, and beta-endorphin. Demonstration of the corresponding receptors in the same cells suggests para- or autocrine mechanisms of action. These findings, together with the demonstration of cutaneous production of numerous other hormones including vitamin D3, PTH-related protein (PTHrP), catecholamines, and acetylcholine that share regulation by environmental stressors such as UV light, underlie a role for these agents in the skin response to stress. The endocrine mediators with their receptors are organized into dermal and epidermal units that allow precise control of their activity in a field-restricted manner. The skin neuroendocrine system communicates with itself and with the systemic level through humoral and neural pathways to induce vascular, immune, or pigmentary changes, to directly buffer noxious agents or neutralize the elicited local reactions. Therefore, we suggest that the skin neuroendocrine system acts by preserving and maintaining the skin structural and functional integrity and, by inference, systemic homeostasis.

Radiochemical high-performance liquid chromatographic assay for the determination of catechol O-methyltransferase activity towards various substrates

A new chromatographic catechol O-methyltransferase (COMT) assay based on S-adenosyl-L-[methyl-14C]methionine and on-line radioactivity detection was developed. With minor modifications in the mobile phase composition the methylation velocities for 30 structurally diverse compounds including simple catechols, neurotransmitters, catecholestrogens and catecholic drugs could be measured using human and rat recombinant soluble COMT. The enzymes showed very similar substrate selectivities. The radiochemical method was validated using 3,4-dihydroxybenzoic acid as a model substrate and it was shown that accurate and reproducible methylation velocity values could be achieved for both of the catecholic hydroxyls. The method proved to be suited for determining the enzyme kinetic parameters and can probably be further used for gathering enzyme kinetic data on differentially substituted catechols in order to construct proper structure-activity relationships for COMT.

Determination of differential activities of soluble and membrane-bound catechol-O-methyltransferase in tissues and erythrocytes

Catechol-O-methyltransferase (COMT) exists as two isoenzymes, a membrane-bound form (MB-COMT) and a soluble form (S-COMT), with different roles in the metabolism of catecholamines and other catechol compounds. This report documents an HPLC assay for separate estimation of S-COMT and MB-COMT activity and examines activities of the two isoenzymes among different rat tissues and in human and rat erythrocytes. Activities of MB-COMT and S-COMT varied widely among tissues. There were higher activities of S-COMT than MB-COMT in all tissues except the adrenal medulla where MB-COMT was the predominant isoenzyme, consistent with the importance of this tissue and MB-COMT for the O-methylation of catecholamines. MB-COMT and S-COMT in rat and human erythrocytes showed divergent levels and patterns of activity. The assay represents a rapid and accurate method for quantifying MB-COMT and S-COMT in various tissues and examining the relative roles of COMT isoenzymes in the metabolism of catechol compounds in health and disease.

Evidence of the indirect formation of the catecholic intermediate substrate responsible for the autoactivation kinetics of tyrosinase

Tyrosinase (EC 1.14.18.1) exhibits unusual kinetic properties in the oxidation of monohydric phenol substrates consisting of a lag period that increases with increasing substrate concentration. The cause of this is an autocatalytic process dependent on the generation of a dihydric phenol substrate, which acts as an activator of the enzyme. Experiments with N-substituted dihydric phenol substrates (N-methyldopamine, N-acetyldopamine) demonstrate that oxygen consumption is retarded in the N-acetyl substituted material due to a diminished rate of cyclization. The oxygen uptake exhibited a similar pattern when N-acetyltyramine was oxidized, and this was reflected by a prolongation of the lag period. N,N-Dipropyldopamine was oxidized with normal kinetics but with an oxygen stoichiometry of 0.5 mol of oxygen/mol of substrate. We show that this is the result of the formation of a stable indoliumolate product with oxidation-reduction properties that prevent the formation of dopaminochrome, thus blocking further stages in the tyrosinase-catalyzed oxidation. Evidence that the indoliumolate product is formed by cyclization of the ortho-quinone is presented by pulse radiolysis studies, which demonstrate the formation of the ortho-quinone (by disproportionation of the corresponding semiquinones), which cyclizes to give the indoliumolate. The rate constant for cyclization was shown to be 48 s-1 (at pH 6.0). Tyrosinase-catalyzed oxidation of the monohydric phenol analogue, N, N-dimethyltyramine, was shown to require the addition of a dihydric phenol. Oxygen utilization then exhibited a stoichiometry of 1.0, indicating that the reactions proceed only as far as the cyclization. The analogous stable cyclic indoliumolate product was shown to be formed, with UV absorption and NMR spectra closely similar to the indoliumolate derived from N,N-dipropyldopamine. This material was methylated by catechol O-methyltransferase but was unreactive to redox reagents. The formation of the cyclic product accounts for the indefinite lag when N,N-dimethyltyramine is used as the substrate for tyrosinase in the absence of a dihydric phenol cofactor.

Purification methods of mammalian catechol-O-methyltransferases

The protein purification strategies used for obtaining homogeneous rat and human soluble catechol-O-methyltransferase (S-COMT) polypeptides are reviewed. Expression and purification of recombinant rat and human S-COMT in Escherichia coli and for human S-COMT in baculevirus-infected insect cells made it possible to elucidate the S-COMT polypeptides in more detail. The application of these purification methods has allowed the crystallization of the rat S-COMT protein and the analysis of the kinetic properties of the enzyme in great detail. The availability of the pure S-COMT protein together with the structural data has also greatly enhanced the development of more potent COMT inhibitors.

Oxidative damage caused by free radicals produced during catecholamine autoxidation: protective effects of O-methylation and melatonin

Catecholamine autoxidation produces reactive oxygen species that have been implicated in the loss of dopaminergic neurons in the nigrostriatal region of the brain that occurs during normal aging and in Parkinson's disease. In the present study, the potential protective effects of catecholamine O-methylation and of melatonin against catecholamine autoxidation-induced protein damage were assessed in vitro using the oxygen radical absorbance capacity (ORAC) assay. The rate of oxidation of the fluorescent protein porphyridium cruentum beta-phycoerythrin (beta-PE) caused by the oxidizing agent CuSO4 was shown to be accelerated by addition of the catecholamines dopamine and L-dopa. Replacement of dopamine and L-dopa in the assay with their O-methylated metabolites 3-O-methyldopamine and 3-O-methyldopa significantly decreased the rate of beta-PE oxidation. When melatonin was added to the ORAC assay in combination with dopamine or L-dopa, the rate of beta-PE oxidation was decreased as well. These findings were consistent with the following interpretations: (1) O-methylated catecholamines are less susceptible to autoxidation than their nonmethylated precursors, and (2) melatonin, which has recently been shown to be a powerful antioxidant, is capable of scavenging free radicals produced during catecholamine autoxidation. These findings suggest that O-methylation and melatonin may be important components of the brain's antioxidant defenses against catecholamine autoxidation and may protect against consequent dopaminergic neurodegeneration.

Seasonal variation in serum concentration of 5-S-cysteinyldopa and 6-hydroxy-5-methoxyindole-2-carboxylic acid in healthy Japanese

Serum concentrations of 5-S-cysteinyldopa (5-S-CD) and 6-hydroxy-5-methoxyindole-2-carboxylic acid (6H5MI2C) have been used as biochemical markers of melanoma progression. We examined the effect of solar radiation on serum levels of 5-S-CD and 6H5MI2C in 10 healthy Japanese by measuring these markers every month during a period of 2 years. 5-S-CD levels were higher in early summer and lower in early winter. The difference in the average levels was approximately twofold, but among the 240 samples, no individual values exceeded the upper limit of normal value, 10 nmol/L. A significant correlation (P < 0.02) was observed between 5-S-CD level and solar radiation. 6H5MI2C levels showed a smaller variation than 5-S-CD. No correlation was observed between 6H5MI2C level and solar radiation. This study showed that serum 5-S-CD and 6H5MI2C in healthy Japanese did not exceed the upper limit of normal values even in sunny season.

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.

The DHICA oxidase activity of the melanosomal tyrosinases LEMT and HEMT

Although melanins can be formed in vitro by the unique action of tyrosinase on L-tyrosine, it is now well accepted that other enzymes termed tyrosinase-related proteins are involved in mammalian melanogenesis. However, some aspects of their roles in the regulation of the pathway are still unknown. The action of dopachrome tautomerase on L-dopachrome yields DHICA, a stable dihydroxyindole with a low rate of spontaneous oxidation. However, DHICA is efficiently incorporated to the pigment, as judged by the high content of carboxylated indole units in natural melanins. Therefore, the fate of this melanogenic intermediate and the mechanisms of its incorporation to the melanin polymer are major issues in the study of melanogenesis. We have recently shown that mouse melanosomes contain two electrophoretically distinguishable tyrosinase isoenzymes, LEMT and HEMT, that can be purified and completely resolved (Jiménez-Cervantes et al., 1993a). Herein, we have compared the ability of these tyrosinases to catalyze DHICA oxidation. Although highly purified LEMT shows a very low specific activity for dopa oxidation in comparison to HEMT, it is able to catalyze DHICA oxidation. However, the DHICA oxidase activity of HEMT was very low, if significant. The ability of purified LEMT to catalyze DHICA oxidation was abolished by heat, trypsin, or phenylthiourea treatments. LEMT acting on DHICA caused the formation of a brownish soluble color similar to DHICA-melanin. Immunoprecipitation of the DHICA oxidase activity of LEMT by specific antibodies suggests that this activity corresponds to TRP1. These results indicate that LEMT, most probably identical to the product of the b locus, is a tyrosinase having a specific DHICA oxidase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical study of catechol-O-methyltransferase in the human mesostriatal system

The cellular localization of catechol-O-methyltransferase was analysed in the mesostriatal system of human brain post mortem by means of immunohistochemistry. In the human nigral complex, catechol-O-methyltransferase immunostaining was not detected in melanized dopaminergic neurons, except in the ventral tegmental area and substantia nigra pars lateralis, where few neurons displayed intense immunolabelling. In the striatum, catechol-O-methyltransferase immunostaining was found in numerous cell bodies and in the neuropile. Observations at the electron microscope level revealed that catechol-O-methyltransferase immunoreactivity was present in the cell bodies of neurons and their processes, including the dendritic spines. No catechol-O-methyltransferase immunolabelling was observed in striatal nerve terminals in contact with dendritic spines, indicating that dopaminergic nerve terminals do not exhibit catechol-O-methyltransferase immunoreactivity. Catechol-O-methyltransferase-immunoreactive cell bodies and processes of glial cells were also detected in the striatum. The data suggest that catechol-O-methyltransferase is either not expressed or only slightly expressed by the dopaminergic nigrostriatal neurons, whereas it is clearly present in striatal neurons and glial cells. Thus, the catabolic degradation of striatal released dopamine by its O-methylation may involve postsynaptic neurons rather than dopaminergic presynaptic neurons. The presence of catechol-O-methyltransferase in some dopaminergic neurons of the ventral tegmental area and substantia nigra pars lateralis suggests that methylation of dopamine may occur in these neurons, which may consequently be better protected against dopamine auto-oxidation than those of the substantia nigra pars compacta.

Catechol-O-methyltransferase as a target for melanoma destruction?

Catechols may interfere in melanogenesis by causing increased levels of toxic quinones. Several catechols and known inhibitors of the enzyme catechol-O-methyltransferase (COMT) were therefore tested for their toxicity towards a pigmented melanoma cell line, UCLA-SO-(M14). The inhibition of thymidine incorporation as a result of exposure to the compounds was measured. All agents were compared to 4-hydroxyanisole (4HA), a depigmenting agent extensively studied as an antimelanoma drug. The compounds were also tested on the epithelial cell line, CNCM-I-(221) in the presence and absence of tyrosinase. All the compounds were more effective than 4HA towards the M14-cells at either 10(-4) M or 10(-5) M. The toxicity of 4HA towards the 221-cells was shown to be completely dependent on the presence of tyrosinase. Effects of the test agents on the 221-cells were also observed in the absence of tyrosinase. Although some of them were shown to be good substrates for tyrosinase only small changes in toxicity were observed as a result of the presence of the enzyme in comparison with 4HA. No direct correlation of the toxicity of the agents and COMT inhibition was observed. The possible mode of action of the compounds through inhibition of COMT and interference in melanogenesis is discussed together with other possibilities and factors involved.

High plasma level of a eumelanin precursor, 6-hydroxy-5-methoxyindole-2-carboxylic acid as a prognostic marker for malignant melanoma

Melanin synthesis is a biologic property unique to the melanocyte. It is highly elevated in malignant melanoma with the production of both eumelanin (brown/black pigment) and pheomelanin (yellow/red pigment), dihydroxyindole (DHI) and cysteinyldopa (CD), respectively, being major precursors. Melanin metabolites are often released in the urine of patients with disseminated melanoma metastasis (melanuria). To establish a better method for the detection of occult melanoma this study compares the plasma levels of a pheomelanin metabolite, 5-S-CD, and a eumelanin metabolite, 6-hydroxy-5-methoxyindole-2-carboxylic acid (6H5MI2C), in melanoma and non-melanoma patients and correlates them with tumor thickness and melanoma metastasis. We found a) that the normal plasma levels of 5-S-CD and 6H5M12C are less than 2.22 ng/ml and 1.04 ng/ml, respectively; b) that the group with the normal 6H5MI2C plasma level does not have any metastasis, whereas a normal 5-S-CD level is seen in both non-melanoma and melanoma patients with and without metastasis; c) that a high plasma 6H5MI2C level is seen in all melanoma patients with tumor thickness more than 3.0 mm regardless of the presence or absence of metastasis, whereas in thinner melanoma patients this is seen only in positive metastasis group; and d) that all melanoma patients with positive metastases showed a high plasma 6H5MI2C level (more than 1.75 ng/ml). We conclude that the measurement of plasma levels of melanin metabolites provides a method for detecting occult melanoma metastasis and estimating the prognosis of melanoma patients, plasma 6H5MI2C level being more sensitive and reliable than that of 5-S-CD, and its increased level being a high risk factor.

Catechol-O-methyltransferase in vitiligo

Catechol-O-methyltransferase (COMT) is involved in the metabolism of neurotransmitters such as epinephrine, norepinephrine and dopamine. For melanocytes, the enzyme is of particular importance in preventing the formation of toxic o-quinones during melanin synthesis. It has been suggested that COMT plays a regulatory role in melanin synthesis. Indeed, when the melanin precursor molecule DHI(2C) is methylated by COMT it is no longer available for incorporation into melanin. Auto-destruction by intermediates of melanin metabolism has been implicated in the aetiology of vitiligo. Therefore enzyme activities in vitiligo patients and in healthy controls were compared. Systemic COMT activities were measured using red blood cells (RBC) as starting material. However, as local alterations in COMT activity may be specifically involved in vitiligo, the enzyme activity was also measured in epidermal homogenates. Finally, to ascribe epidermal COMT activity to the responsible cell type(s), enzyme activity was measured in cultured vitiligo non-lesional melanocytes and melanocytes from healthy controls as well as in cultured keratinocytes from lesional skin and in purified keratinocytes from control skin. It was found that epidermal homogenates from vitiligo patients expressed higher levels of COMT activity than homogenates from healthy controls. Such differences were not found at the systemic level (i.e. in RBC) nor could they be explained by measurements on separately cultured epidermal cell types, indicating that the COMT activity was induced at the tissue level by extracellular factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of melanin-related metabolites as markers of melanoma progression

BACKGROUND

Urinary excretion of 5-S-cysteinyldopa (5-S-CD) has been used as a biochemical marker of melanoma progression. Melanomas produce not only 5-S-CD but also 5,6-dihydroxyindole-2-carboxylic acid (5,6DHI2C) as major intermediates in melanin formation. 5,6DHI2C is then metabolized to the two O-methyl derivatives, 5H6MI2C and 6H5MI2C. The aim of this study was to determine which marker in serum and urine most sensitively reflected the progression of melanoma.

METHODS

Serum and 24-hour urine samples were collected and assayed serially by high-performance liquid chromatography every 1 to 4 months in 28 patients with primary or recurrent melanomas, for up to 48 months.

RESULTS

Serum concentration and urinary excretion of 5-S-CD and 6H5MI2C in patients with melanoma without metastases were close to those obtained from normal subjects. Metastases developed in 9 of the 28 patients. In seven of these nine patients, serum or urinary 5-S-CD values were elevated before or at the time of clinical detection of visceral metastases. However, serum 5-S-CD was elevated significantly earlier and reflected melanoma progression better than the physical examination and/or laboratory tests, such as scintigraphy and echography. Serum 6H5MI2C values exceeded the normal range shortly before death in three patients, and urinary 6H5MI2C did not increase at any stage in most patients, therefore these metabolites did not reflect progression of disease.

CONCLUSIONS

Among the four markers, serum 5-S-CD appears to be the best biochemical marker for the detection of progression of melanotic melanoma, a value of more than 10 nmol/l suggesting the presence of metastasis.

Mechanistic aspects of the control of tyrosinase activity

There is now much evidence suggesting that there are multiple control points in the process of melanin production. The most fundamental process of melanogenesis is centered on the oxidative activity of the enzyme tyrosinase. Tyrosinase is a highly unusual enzyme in that it apparently catalyses two processes, i.e., the oxidation of tyrosine and the dehydrogenation of dihydroxyphenylalanine (Dopa), at the same active site. The reactions involved account for the unusual kinetics of tyrosine oxidation and suggest biochemical mechanisms whereby the activity of the enzyme and the process of melanogenesis may be modified. It is proposed that the oxidative engine of melanogenesis resides in an oxidation/reduction cycle involving Dopa and dopaquinone and that this can be modified by processes that result in the removal of dopaquinone or Dopa from the reaction system.

Expression of different immunological markers by cultured human melanocytes

The expression of different immunological markers by cultured human melanocytes (MC) in relation to immune phenomena, were investigated on ten different MC cell lines from early (1st) to late (22nd) passage. Four melanocyte lines (MC-a) which had undergone changes in growth behaviour during prolonged culture were included in the study, together with two melanoma lines. Cytospin preparations of the cells were stained for the presence of a set of different immunological markers and a melanoma-associated antigen (MAA). All MC lines, including the MC-a and the melanoma lines, showed expression of MHC class I, IL-1, IL-2, ICAM-1 and the MAA, NKI-Beteb, during all passages tested. Interestingly, four of the MC lines showed staining for the Fc receptor. A tendency towards a stronger expression of ICAM-1 on a higher percentage of cells was observed on MC with increasing passage number, the MC-a and the melanoma lines. Expression of the MAA was strongly reduced for the MC-a lines in comparison with the MC and the M14 melanoma lines. Positive staining for the HLA class II molecules was obtained on MC of intermediate and late passages, and on the MC-a and the melanoma lines in the decreasing order HLA-DR, DP and DQ. Additionally, we carried out a preliminary study showing that cultured MC also produce IL-1 and IL-6. However, we were not able to show the production of biologically active IL-2 testing several cultured MC lines. Nevertheless, the overall results taken together suggest that MC are immunologically important cells that are susceptible to changes during long-term culture.

Preparation of monoclonal mouse antibodies against two specific eu-melanin related compounds

Two eu-melanin precursors, 6-hydroxy-5-methoxyindole-2-carboxylic acid (HMI2C) and 5,6-dihydroxyindole-2-carboxylic acid (DHI2C) were synthesized and coupled to bovine serum albumin, hemocyanin and polylysine by the combined action of carbodiimide and succinimide. These indole-carrier conjugates served as antigens for the production of specific antibodies against DHI2C and HMI2C in BALB/c mice. The specificity of these antibodies was tested using a combination of affinity chromatography and ELISA procedures. Polyclonal mouse antibodies reacted with the indole-carrier conjugates, but not with the unbound indole compounds. Monoclonal antibodies from two hybridoma cell lines were obtained from a HMI2C-immunized mouse after a fusion with four subclonings. They reacted with free HMI2C and to a lesser extent with unbound DHI2C. One monoclonal showed 50% inhibition in the ELISA test at concentrations of 0.6 mumol.l-1 and 5 mumol.l-1 for HMI2C and DHI2C, respectively. These antibodies did not show any cross-reactivity with nine structurally related compounds and should be valuable reagents for the detection and quantification of HMI2C and other eu-melanin related compounds.

Regulation of the final phase of mammalian melanogenesis. The role of dopachrome tautomerase and the ratio between 5,6-dihydroxyindole-2-carboxylic acid and 5,6-dihydroxyindole

The regulation of the final steps of the melanogenesis pathway, after L-2-carboxy-2,3-dihydroindole-5,6-quinone (dopachrome) formation, is studied. It is shown that both tyrosinase and dopachrome tautomerase are involved in the process. In vivo, it seems that tyrosinase is involved in the regulation of the amount of melanin formed, whereas dopachrome tautomerase is mainly involved in the size, structure and composition of melanin, by regulating to the incorporation of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) into the polymer. Moreover, using L-3,4-dihydroxyphenylalanine (dopa) and related compounds, it was shown that the presence of dopachrome tautomerase mediates an initial acceleration of melanogenesis since L-dopachrome is rapidly transformed to DHICA, but that melanin formation is inhibited because of the stability of this carboxylated indole compared to 5,6-dihydroxyindole (DHI), its decarboxylated counterpart obtained by spontaneous decarboxylation of L-dopachrome. Using L-dopa methyl ester as a precursor of melanogenesis, it is shown that this carboxylated indole does not polymerize in the absence of DHI, even in the presence of tyrosinase. However, it is incorporated into the polymer in the presence of both tyrosinase and DHI. Thus, this study suggests that DHI is essential for melanin formation, and the rate of polymerization depends on the ratio between DHICA and DHI in the medium. In the melanosome, this ratio should be regulated by the ratio between the activities of dopachrome tautomerase and tyrosinase.

Melanogenesis: a realistic target for antimelanoma therapy?

Melanin is a widely-distributed pigment in the biosphere. In the human adult, the enzymatically-catalysed process of melanin generation is the exclusive prerogative of melanocytes. Melanogenesis generates a number of reactive intermediates including orthoquinones and has been recognised as a potential hazard to melanocytes. Amplification of this cytotoxic hazard to selectively damage malignant melanogenic cells has been investigated as a rational therapeutic strategy for melanoma. A number of surrogate substrates for tyrosinase have been studied, including a range of phenols and catechols. Initial attempts to use these agents for the treatment of disseminated melanoma have foundered on problems due to unfavourable pharmacokinetics, primary toxicity or pharmacological actions of the analogue substrates, and the toxicity of hepatic metabolites. Successful exploitation of the undoubted potential of the metabolic targeting strategy presented by the subversion of melanogenesis depends on the development of prodrugs with minimal primary toxicity and improved pharmacokinetics. The range of possible novel approaches is being extended by the emergent understanding of the complexities of melanogenesis which are outlined.