Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

---

Query: EC:1.10.3.1 (tyrosinase)
9,065 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The effect of DOPA and glutathione (GSH) on enzyme systems for 5-S-cysteinyl-DOPA (5SCD) genesis in murine melanoma cells cultured in tyrosine- and cystine-free medium were studied. DOPA at its optimum concentration (10(-5) M) when added alone did not alter tyrosinase, glutathione-S-transferase or gamma-glutamyl transpeptidase activities. In the presence of GSH at its optimum concentration (10(-5) M), DOPA loading did not cause any significant changes in tyrosinase or glutathione-S-transferase (GST) activities. This indicates that the higher 5SCD levels observed in the medium because of DOPA loading in the GSH dependent system results from increased substrate availability rather than the increased enzyme activity. An acute drop in 5SCD at DOPA concentrations above 10(-5) M observed in the GSH dependent system may be due to the inhibition of tyrosinase at high substrate concentrations (10(-4) M). Conversely, in the presence of DOPA, when GSH was increased, the resultant higher production of 5SCD could be explained by the increased activity of GST. When added alone, GSH (10(-5) M) caused a significant increase in GST (approximately 125%) and gamma-GTP (approximately 50%) activities. A drop in 5SCD in the medium when GSH was added beyond its optimum concentration (10(-5) M) in the DOPA-dependent system could be due to competitive inhibition of gamma-GTP by GSH. The data demonstrate that 5SCD genesis may be enhanced due to the accumulation of cytotoxic melanin precursors such as DOPA/DOPA quinone. The relative quantities of GSH at the sites of DOPA quinone formation and the levels of its metabolising enzymes can influence the type of product formed.(ABSTRACT TRUNCATED AT 250 WORDS)
...
PMID:Effect of dopa-loading on glutathione metabolising enzymes and tyrosinase in relation to 5-S-cysteinyl-dopa genesis in cultured B-16 melanoma cells. 168 90

The cytotoxic and growth-inhibitory effect of levodopa methylester (LDME) in murine B16BL6 (BL6) melanoma cells after glutathione (GSH) depletion was studied in vitro. Pretreatment of BL6 cells with 50 microM buthionine sulfoximine (BSO) depleted GSH content by nearly 90% and enhanced the growth-inhibitory effect of even a minimally cytotoxic concentration of LDME. Radiothymidine incorporation into BL6 cells significantly increased compared to untreated controls during the first 4 h of exposure to 0.2 mM LDME. However, pretreatment with BSO prevented this LDME-induced increase in radiothymidine incorporation. Because the percentage of cells in S-phase of the cell cycle was not altered, these results suggest that BSO exposure may be inhibiting unscheduled DNA synthesis, which could contribute to the cytotoxic effects of LDME. In addition, spectrophotometric studies indicated that in a cell-free system, GSH scavenged dopaquinone produced by the tyrosinase-mediated oxidation of LDME, presumably by formation of glutathionyldopa. Thus, enhancement of LDME cytotoxicity by BSO may also involve depleting the amount of GSH available for the nucleophilic addition to the quinone.
...
PMID:Effect of L-dopa methylester and glutathione depletion on murine B16BL6 melanoma growth in vitro. 174 17

Rat liver microsomes contain a membrane-bound GSH S-transferase (GSH-tr), an enzyme that is involved in the detoxication of xenobiotics. Also located on rat liver microsomes is the cytochrome P450 system, an enzyme complex that catalyzes the conversion of several xenobiotics into reactive intermediates. In this study, it was demonstrated that reactive products from alpha-methyldopa formed by the cytochrome P450 system are able to stimulate microsomal GSH-tr. Also, products formed from alpha-methyldopa that are generated by H2O2-horseradish peroxidase and tyrosinase are able to stimulate the activity of microsomal GSH-tr. GSH was able to prevent the activation of microsomal GSH-tr. Our results indicate that the ortho-quinone or semi-ortho-quinone radical of alpha-methyldopa is responsible for the stimulation of microsomal GSH-tr, probably via arylation of the free sulfhydryl group of microsomal GSH-tr. This conclusion was supported by the observation that 4-methyl-ortho-quinone itself was able to stimulate microsomal GSH-tr via sulfhydryl arylation. Our results are in conformity with the hypothesis that reactive products formed by the cytochrome P450 complex are able to stimulate microsomal GSH-tr and possibly in this way enhance their detoxication.
...
PMID:Activation of the microsomal glutathione S-transferase by metabolites of alpha-methyldopa. 189 94

The in vitro conversion of (+)-3,4-methylenedioxymethamphetamine and (-)-3,4-methylenedioxymethamphetamine to the corresponding catecholamine, 3,4-dihydroxymethamphetamine (N-methyl-alpha-methyldopamine), by rat liver microsomes was examined. Metabolite formation was monitored after short-term incubations using high-performance liquid chromatography-electrochemical detection to determine concentrations of the catecholamine. The formation of N-methyl-alpha-methyldopamine exhibited enantioselectivity and levels were significantly higher after incubation of the (+)-isomer. The reaction appears to be cytochrome P-450 dependent as it was sensitive to SKF 525A and carbon monoxide. The catecholamine was unstable and was metabolized rapidly to a compound capable of forming an adduct with glutathione (GSH) and other thiol compounds. This second oxidation did not appear to be cytochrome P-450-dependent but required NADPH and microsomal protein. Catecholamine oxidation was inhibited by superoxide dismutase and by reducing agents. The same catecholamine oxidation product, characterized as the GSH adduct, could be generated by a xanthine-xanthine oxidase mixture and by tyrosinase. Mass spectral data showed that it was a 1:1 amine GSH adduct. These results indicate that MDMA is oxidized by cytochrome P-450 to the catechol and the catecholamine oxidized by superoxide to a quinone to which GSH or other thiol functions add. The formation of this quinone and its thiol adducts may account for some of the irreversible actions of this compound on serotonergic neurons.
...
PMID:Metabolism of methylenedioxymethamphetamine: formation of dihydroxymethamphetamine and a quinone identified as its glutathione adduct. 197 41

The actions of glutathione S-transferase and tyrosinase on the in vitro production of glutathionyl-3,4-dihydroxyphenylalanine and the dopachrome level in the presence of GSH and L-3,4-dihydroxyphenylalanine were studied. No clear evidence of complementarity between tyrosinase and glutathione S-transferase was observed; on the contrary, in the presence of glutathione S-transferase the glutathionyl-3,4-dihydroxyphenylalanine yield was lower than with tyrosinase only, as measured by HPLC. It is concluded that the spontaneous conjugation of GSH with dopaquinone should probably be high enough to scavenge the toxic quinone and to produce precursors for phaeomelanogenesis.
...
PMID:A study on the in vitro interaction between tyrosinase and glutathione S-transferase. 310 90

1. Cataracts were developed by incubating rabbit lenses for 22hr. at 37 degrees in a culture medium containing tyrosine and tyrosinase (EC 1.10.3.1). 2. A 45% diminution in the content of GSH and significant inhibition of glucose 6-phosphate dehydrogenase (EC 1.1.1.49) activity were observed in the cataractous lenses compared with controls. 3. GSSG accumulated in both cataractous and control lenses. Significant amounts of GSSG were transported outward from the cataractous lenses and small amounts from control lenses. 4. Transport of GSSG from rabbit lens incubated in a diffusate of plasma from a naphthalene-fed rabbit was also observed. 5. GSSG was found in the aqueous humour obtained between 2 and 24hr. after feeding of naphthalene to rabbits; subsequently the GSSG in the aqueous humour decreased to almost undetectable amounts in 48hr.; in controls, GSSG was not detectable. 6. A possible mechanism of formation of experimental and senile cataract is briefly discussed.
...
PMID:Cataract produced by tyrosinase and tyrosine systems in rabbitens in vitro. 497 83

Phenolic compounds can act as radical scavengers due to their ability to donate a mobile hydrogen to peroxyl radicals producing a phenoxyl radical if the phenoxyl radical formed in the radical scavenging reaction efficiently interacts with vitally important biomolecules, then this interaction may result in cytotoxic effects rather than in antioxidant protection. In the present work we have chosen two model compounds--a phenolic antitumor drug, VP-16, known to be highly cytotoxic, and a homolog of vitamin E, 2,2,5,7,8-pentamethyl-6-hydroxychromane (PMC)--as typical representatives of phenoxyl radicals to study interactions of their phenoxyl radicals with intracellular thiols. Using a water-soluble source of peroxyl radicals, the azo-initiator 2,2'-azobis(2-aminodinopropane) (AAPH), we found that both PMC and VP-16 are very efficient scavengers of peroxyl radicals as evidenced by their ability to inhibit AAPH-induced chemiluminescence of luminol and oxidation of PnA incorporated into DOPC liposomes. Both PMC and VP-16 were also able to protect against AAPH-induced oxidative degradation of DNA in nuclei from human leukemic K562 cells. In contrast, there was a dramatic difference in the ability of VP-16 and PMC to protect GSH against AAPH-induced oxidation: while PMC inhibited AAPH-induced oxidation of GSH in a concentration-dependent manner, VP-16 did not protect GSH against oxidation. We hypothesized that this was due to different reactivities of the phenoxyl radicals formed by AAPH-derived peroxyl radicals from VP-16 and PMC toward GSH. To substantiate this hypothesis, we compared interactions of the phenoxyl radicals generated from VP-16 and PMC with intracellular thiols in K562 cell homogenates. While the PMC phenoxyl radicals were only slightly affected by thiols, the VP-16 phenoxyl radicals were reduced by thiols. This is evidenced by (i) a significant inhibition of the tyrosinase-induced VP-16 consumption upon addition of K562 cell homogenates, (ii) a depletion of endogenous thiols in K562 cell homogenates induced by VP-16+tyrosinase, (iii) a transient disappearance of the VP-16 phenoxyl radical signal from the ESR spectra and its reappearance after depletion of endogenous thiols, and (iv) elimination of the lag period for the appearance of the VP-16 phenoxyl radical ESR signal subsequent to depletion of thiols by mersalyl acid. To evaluate the contribution of GSH and protein thiols to reduction of the VP-GSH-peroxidase + cumeme hydroperoxide to specifically deplete endogenous GSH.(ABSTRACT TRUNCATED AT 400 WORDS)
...
PMID:Phenoxyl radicals of etoposide (VP-16) can directly oxidize intracellular thiols: protective versus damaging effects of phenolic antioxidants. 771 69

Mechanisms of phenoxyl radical-induced generation of oxygen radicals potentially involved in toxicity of benzene were studied. We hypothesized that phenoxyl radical intermediates formed from phenolic metabolites of benzene by oxidative enzymes (e.g., peroxidases, tyrosinase) are able to damage biomolecules via (i) oxidation of low-molecular-weight thiols and protein thiols and (ii) thiol-dependent generation of oxygen radicals and subsequent oxidation of DNA. Phenoxyl radicals were generated by the oxidation of phenol by myeloperoxidase+H2O2, horseradish peroxidase+H2O2, or tyrosinase. The reaction of phenolphenoxyl radicals with GSH and dihydrolipoic acid was studied. Our HPLC measurements showed that both thiols reduced the phenoxyl radical back to phenol. This reaction was accompanied by the formation of thiyl radicals (detected by ESR as 5,5-dimethyl-1-pyrroline-N-oxide/glutathione thiyl radical spin adducts) and of superoxide radicals (measured by their chemiluminescence response in the presence of lucigenin). Hydroxylation of 2'-deoxyguanosine to 8-oxo-7,8-dihydro-2'-deoxyguanosine was demonstrated in the course of the tyrosinase-catalyzed oxidation of phenol in the presence of dihydrolipoic acid and Fe(III)-EDTA. Redox-cycling of phenoxyl radicals by thiols produces oxygen radicals which can be responsible for the oxidative damage of DNA by radical intermediates of benzene metabolism.
...
PMID:Phenoxyl radical-induced thiol-dependent generation of reactive oxygen species: implications for benzene toxicity. 789 44

Phenoxyl radicals are intermediates in the oxidation of phenolic compounds to quinoid derivatives (quinones, quinone methides), which are known to act as ultimate mutagenic, carcinogenic, and cytotoxic agents by directly interacting with macromolecular targets or by generating toxic reactive oxygen species. One-electron reduction of phenoxyl radicals may reverse oxidative activation of phenolic compounds to quinoids, thus preventing their cytotoxic effects. In the present work, we studied interactions of ascorbate, thiols (glutathione, dihydrolipoic acid, and metallothioneins), and combinations thereof with the phenoxyl radical generated by tyrosinase-catalyzed oxidation of VP-16 [etoposide, 4'-demethylepipodophyllotoxin-9-(4,6-O-ethylidene-beta-D-glucop yra noside)], a hindered phenol widely used as an antitumor drug. We found by liquid chromatography-ionspray mass spectrometry and electron spin resonance (ESR) that tyrosinase caused oxidation of VP-16 to its o-quinone and aromatized derivative via intermediate formation of the phenoxyl radical. Both ascorbate and thiols (GSH, dihydrolipoic acid, and metallothioneins) were able to directly reduce the VP-16 phenoxyl radical and prevent its oxidation. The characteristic ESR signal of the VP-16 phenoxyl radical was quenched by the reductants. The semidehydroascorbyl radical ESR signal was detected in the presence of ascorbate; thiols did not produce signals in the ESR spectra. In combinations, ascorbate plus GSH and ascorbate plus metallothionein acted independently and additively in reducing the VP-16 phenoxyl radical. Ascorbate was more reactive: the VP-16-dependent oxidation of GSH or metallothionein commenced only after complete oxidation of ascorbate. The semidehydroascorbyl radical ESR signal preceded the quenching of the VP-16 phenoxyl radical by GSH and metallothionein. In the presence of ascorbate plus dihydrolipoic acid, ascorbate was also more reactive toward the VP-16 phenoxyl radical than dihydrolipoic acid, but the ascorbate concentration was maintained at the expense of its regeneration from dehydroascorbate by dihydrolipoic acid. In ESR spectra, the semidehydroascorbyl radical ESR signal was continuously detected and then was abruptly substituted by the VP-16 phenoxyl radical signal. When VP-16 and tyrosinase were incubated in the presence of retina or hepatocyte homogenates, a two-phase lag period was observed by ESR for the appearance of the VP-16 radical signal: an ascorbate-dependent part (semidehydroascorbyl radical observable, sensitive to ascorbate oxidase) and thiol-dependent part (no radical signals in the spectra, sensitive to mersalyl acid). About 50% of the thiol-dependent part of the lag period could be accounted for by endogenous GSH (as revealed by treatment with GSH peroxidase+cumene hydroperoxide).(ABSTRACT TRUNCATED AT 400 WORDS)
...
PMID:Ascorbate is the primary reductant of the phenoxyl radical of etoposide in the presence of thiols both in cell homogenates and in model systems. 806 42

Etoposide (VP-16) is an antitumor drug currently in use for the treatment of a number of human cancers. Mechanisms of VP-16 cytotoxicity involve DNA breakage secondary to inhibition of DNA topoisomerase II and/or direct drug-induced DNA strand cleavage. The VP-16 molecule contains a hindered phenolic group which is crucial for its antitumor activity because its oxidation yields reactive metabolites (quinones) capable of irreversible binding to macromolecular targets. VP-16 phenoxyl radical is an essential intermediate in VP-16 oxidative activation and can be either converted to oxidation products or reduced by intracellular reductants to its initial phenolic form. In the present paper we demonstrate that the tyrosinase-induced VP-16 phenoxyl radical could be reduced by ascorbate, glutathione (GSH) and dihydrolipoic acid. These reductants caused a transient disappearance of a characteristic VP-16 phenoxyl radical ESR signal which reappeared after depletion of the reductant. The reductants completely prevented VP-16 oxidation by tyrosinase during the lag-period as measured by high performance liquid chromatography; after the lag-period VP-16 oxidation proceeded with the rate observed in the absence of reductants. In homogenates of human K562 leukemic cells, the tyrosinase-induced VP-16 phenoxyl radical ESR signal could be observed only after a lag-period whose duration was dependent on cell concentration; VP-16 oxidation proceeded in cell homogenates after this lag-period. In homogenates of isolated nuclei, the VP-16 phenoxyl radical and VP-16 oxidation were also detected after a lag-period, which was significantly shorter than that observed for an equivalent amount of cells. In both cell homogenates and in nuclear homogenates, the duration of the lag period could be increased by exogenously added reductants. The duration of the lag-period for the appearance of the VP-16 phenoxyl radical signal in the ESR spectrum can be used as a convenient measure of cellular reductive capacity. Interaction of the VP-16 phenoxyl radical with intracellular reductants may be critical for its metabolic activation and cytotoxic effects.
...
PMID:Tyrosinase-induced phenoxyl radicals of etoposide (VP-16): interaction with reductants in model systems, K562 leukemic cell and nuclear homogenates. 816 27


1 2 3 4 5 6 7 8 9 10 Next >>

---