Synthesis and characterization of copper(II) complexes of cysteinyldopa and benzothiazine model ligands related to pheomelanin.

A new cysteinyldopa model ligand Cydo {3-[(2-aminoethyl)sulfanyl]-4,6-di-tert-butylbenzene-1,2-diol} was prepared and its reactivity with Cu(II) explored. Under anaerobic conditions, tetranuclear [Cu4(Cydo)4] is isolated, but in the presence of O2, a benzothiazine intermediate accumulates that is trapped as the Cu(II) complex [Cu(zine)2]. Under slightly different reaction conditions, the benzothiazine further oxidizes to benzothiazole (zole). All three compounds were characterized by X-ray crystallography, and the reactions were monitored spectrophotometrically.

5-S-cysteinyldopa, a diffusible product of melanocyte activity, is an efficient inhibitor of hydroxylation/oxidation reactions induced by the Fenton system.

Interest in 5-S-cysteinyldopa (5-S-CD), a major excretion product of normal and malignant melanocytes, has traditionally concentrated on its significance as a biosynthetic precursor of pheomelanins, the characteristic pigments of red hair, and as a specific biochemical marker for monitoring melanoma progression. The present study shows that 5-S-CD is a potent inhibitor of hydroxylation/oxidation reactions mediated by hydrogen peroxide and the Fe2+/EDTA complex under both aerobic and anaerobic conditions. The inhibitory effect of 5-S-CD, as determined by the deoxyribose and salicylic acid assays in phosphate buffer (pH 7.4), is much stronger than that of dopa, acetylsalicylic acid and mannitol, increases with increasing ligand-to-metal ratio, and is inversely proportional to the concentration of EDTA present in the Fenton system. Spectrophotometric evidence and competition experiments indicate that 5-S-CD forms a chelate complex with ferric ions (lambda max = 500 nm at pH 7.4), which may account for both an altered production of hydroxyl radicals by the Fenton reagent and a site-specific localization of oxidative damage on the chelate complex itself.

Synthesis of 5-S-L-cysteinyl-glycine-L-dopa, a natural substrate for serum and melanocyte dipeptidase.

A method for synthesis of the phaeomelanin pigment intermediate compound 5-S-L-cysteinyl-glycine-L-dopa is presented. This thioether has been suggested as a precursor to 5-S-L-cysteinyl-L-dopa, the key intermediate compound in phaeomelanin pigment formation. 5-S-Glutathione-L-dopa is first synthesized by the tyrosinase-catalyzed reaction between L-dopa and glutathione. The 5-S-glutathione-L-dopa is then converted to 5-S-L-cysteinyl-glycine-L-dopa using the enzyme gamma-glutamyl transpeptidase. The compound thus obtained was suitable as a substrate for melanoma cell and serum dipeptidase which converts the compound into 5-S-L-cysteinyl-L-dopa and glycine. The optimum pH for the dipeptidase from melanoma cells was 7.5 and the Km was 1.2 mM.

Tyrosinase-catalyzed conjugation of dopa with glutathione.

A convenient method is described for the preparation of 5-S- and 2-S-glutathionyldopa, based on tyrosinase oxidation of dopa in the presence of glutathione. The yields of 5-S, 2-S, and 6-S isomers produced were about 76, 12, and 5%, respectively.

Determination of urinary 5-S-cysteinyldopa by high-performance liquid chromatography.

A high-performance ion-pair liquid chromatographic method with electrochemical detection is described, which is suitable for routine determination of urinary 5-S-cysteinyldopa. The clean-up procedure includes a first purification step on the cation exchanger AG 50 W (H +). After desorption from the resin at moderately raised pH the catecholic amino acid is adsorbed on alumina at pH 8.6, washed and finally desorbed by elution with perchloric acid. By the combined clean-up procedures, easily oxidized compounds are eliminated, which otherwise cause a number of interfering peaks in the chromatography. The synthesis of 5-S-cysteinyl-L-3,4-dihydroxyphenyl [2,3-3H]alanine is described, and this tritium-labelled 5-S-cysteinyldopa is used to determine the recovery in the sample. The precision (C.V. = 5.7% at low and C.V. = 4.9% at high 5-S-cysteinyldopa concentration) and recovery (105.0 +/- 8.6%) were satisfactory. The mean urinary excretion was 0.34 +/- 0.13 (S.D.) mumol per 24 h (range 0.02-0.58 mumol per 24 h) in healthy subjects (n = 24) and in patients with melanoma metastates (n = 13) the excretion ranged from 0.9 to 4.8 mumol per 24 h.

The effect of cysteine on oxidation of tyrosine, dopa, and cysteinyldopas.

The influence of cysteine on the oxidation of tyrosine, dopa, and monocysteinyldopas by mushroom tyrosinase was reexamined. During oxidation of tyrosine in the presence of cysteine the concentration of dopa increased slowly, whereas the concentration of cysteinyldopas increased more rapidly. When the concentration of cysteine decreased the cysteinyldopas were rapidly consumed and dopa concentrations increased sharply. Experiments on the oxidation of dopa by tyrosinase in the presence of cysteine showed that this thiol does not inhibit the oxidation. Dopa concentrations decreased more rapidly in the presence of cysteine because cysteine addition to dopaquinone prevented reformation of dopa from dopaquinone. Both 2-S-cysteinyldopa and 5-S-cysteinyldopa are substrates for tyrosinase. The oxidation of cysteinyldopas was inhibited at high cysteine concentrations. The greater part of 2,5-S,S-dicysteinyldopa formed during the oxidation of monocysteinyldopas in the presence of cysteine is derived from 5-S-cysteinyldopa, which is a better substrate for tyrosinase than 2-S-cysteinyldopa. The fact that cysteine binds more rapidly to 5-S-cysteinyldopaquinone than to 2-S-cysteinyldopaquinone further stresses the importance of 5-S-cysteinyldopa in the formation of 2,5-S,S-dicysteinyldopa. Oxidation of dopa in the presence of cysteine and glutathione or methionine showed that glutathione is added to dopaquinone but less rapidly than cysteine. Methionine showed insignificant addition to dopaquinone. When dopa or 5-OH-dopa is added to an incubate of cysteinyldopa and tyrosinase the oxidation of cysteinyldopa is accelerated owing to oxidation of cysteinyldopa by dopaquinone or 5-OH-dopaquinone.

Synthesis and antitumor activity of cysteinyl-3,4-dihydroxyphenylalanines and related compounds.

The natural catecholic amino acid 5-S-cysteinyl-3,4-dihydroxyphenylalanine (1) was selectively toxic to a variety of human tumor cell lines in culture and exhibited antitumor activity against L1210 leukemia and B-16 melanoma in mice at doses which were not toxic to the host. Structural analogues of 5-S-cysteinyl-3,4-dihydroxyphenylalanine including several new compounds, were synthesized and tested for growth inhibition of cultured cells of human neuroblastoma YT-nu and Chinese hamster fibroblasts Don-6. Some were also examined for antitumor activity against L1210 and B-16 in vivo. 4-S-Cysteinylcatechols and 2- and 4-S-cyteinylphenols, which cannot be prepared by conventional methods, were synthesized by the reaction of catechols and phenols with cystine and boiling aqueous HBr. 5-S-Cysteinyl- and 2-S-Cysteinyl-3,4-dihyroxyphenylalanine (1 and 2), L-3,4-dihydroxyphenylalanine (L-Dopa), and 2- and 4-S-cysteinylphenol (14 and 15) were toxic to the YT-nu cell line only, while 4-S-cysteinylcatechol (6), 3-S-cysteinyl-5-methylcatechol (8), 5-S-cysteaminyldopamine (9), and 4-methylcatechol were strongly toxic to both cell lines. Compound I (1000 mg/kg), 6 (500 mg/kg), and 8 (400 mg/kg) increased the life span of L1210-bearing mice by 50, 50, and 43%, respectively, and compounds 1 and 8 were marginally effective against B-16 melanoma as well. Compound 9 was too toxic to show any activity. There was a good correlation between the cytotoxicity and the in vivo activity.

A facile one-step synthesis of cysteinyldopas using mushroom tyrosinase.

A convenient one-step procedure, based upon the tyrosinase co-oxidation of dopa and cysteine, is reported for the synthesis of 5-S-cysteinyldopa (I) in 74% yield. Secondary products of the reaction turned out to be 2-S-cysteinyldopa (II, 14%), 2,5-S, S-dicysteinyldopa (iv, 5%), and the hitherto unknown 6-S-cysteinyldopa (III, approximately 1%).