Mechanisms of 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide (Dacarbazine) cytotoxicity toward Chinese hamster ovary cells in vitro are dictated by incubation conditions.

Decomposition of the antitumor agent 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide (DTIC, Dacarbazine) produces several potentially toxic compounds, the concentration of which depend on incubation parameters such as pH, temperature and illumination. The action of DTIC on chinese hamster ovary (CHO) cell clone formation in the dark (7-8-day incubation) reflects the slow formation of 2-azahypoxanthine. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT, EC 2.4.2.8)-deficient cells are resistant to DTIC under these conditions, reflecting their inability to utilize 2-azahypoxanthine. The toxicity of DTIC in conventional survival experiments (1-2-h exposure to drug) is dependent upon illumination and is highly influenced by the pH of the medium. Toxicity of DTIC in these experiments appears to reflect rapid accumulation of the immediate photodecomposition product of the drug, 4-diazoimidazole-5-carboxamide (DZC), since HGPRT-deficient cells are not resistant to DTIC under these conditions. The biologically initiated pathway of DTIC action (enzymatic hydroxylation) has little, if any, role in the action of this agent toward cultured CHO cells.

The enzymatic synthesis of 5-amino-4-imidazolecarboxamide riboside triphosphate (ZTP).

5-Amino-4-imidazolecarboxamide riboside triphosphate (ZTP) is thought to play a regulatory role in cellular metabolism. Unlike other nucleoside triphosphates, ZTP is synthesized in a one-step reaction in which the pyrophosphate group of 5-phosphoribosyl-l-pyrophosphate is transferred to the riboside monophosphate (ZMP) in a reaction catalyzed by 5-phosphoribosyl-l-pyrophosphate synthetase; reversal of this reaction leads to dephosphorylation of ZTP to ZMP. This unusual route of synthesis (and catabolism) of ZTP may be important in defining its metabolic effects in the cell.

1-thiocarbamoyl-2-imidazolidinone, a metabolite of niridazole in Schistosoma mansoni.

Niridazole, a nitro heterocyclic antischistosomal drug, is extensively metabolized to unknown metabolites by Schistosoma mansoni. We report that 1-thiocarbamoyl-2-imidazolidinone was isolated by high pressure liquid chromatography and identified by high resolution electron impact mass spectroscopy as a niridazole metabolite in schistosomes. After a 20-h in vitro incubation in 30 ml of medium containing 10 micrograms ml-1 [14C]niridazole (5.2 Ci mol-1), 100 S. mansoni worm pairs contained approximately 275 ng of 1-thiocarbamoyl-2-imidazolidinone. This amount represented 4% of the total metabolized fraction of niridazole in the parasite. Incubation of schistosomes with 1-thiocarbamoyl-2-[2 14C]imidazolidinone (2.7 Ci mol-1) indicated that this metabolite was not taken up. However, schistosomes released an average of 44 ng ml-1 or 1% of the total 1-thiocarbamoyl-2-imidazolidinone found in the worm back into 1 ml of medium during incubation. No host oxidative metabolites of niridazole were found in the parasites.

[Enzymatic conversion of L-histidine to urocanic acid using immobilized histidase from the rat liver]. / Fermentativnoe prevrashchenie L-gistidina v urokaninovuiu kisloty s pomoshch'iu immobilizovannoi gistidazy iz pecheni krysy.

A procedure is described for immobilization of partially purified histidase by means of covalent binding of the enzyme with amino ethyl cellulose activated by glutaraldehyde. The preparation obtained was stable within 6 months at 5 degrees and within 40 days at 39 degrees. The preparation of immobilized histidase might be used for enzymatic synthesis of urocanic acid from histidine.

Factors controlling the expressed activity of histidine ammonia-lyase in the epidermis and the resulting accumulation of urocanic acid.

The synthesis of urocanic acid by histidine ammonia-lyase in guinea-pig epidermis was investigated in various ways. 1. In epidermal homogenates the enzyme obeys Michaelis-Menten kinetics and shows marked dependence of its activity of pH, such that below pH 6 it is inactive. 2. Part-thickness skin samples cultured with radioactive histidine do not accumulate detectable radioactive urocanic acid during 3 days in culture. 3. Very little histidine ammonia-lyase activity can be detected in the living cells of the epidermis. The enzyme is almost completely restricted to the dead cells of the stratum corneum. 4. Radioactive histidine injected into living animals does not result immediately in the accumulation of radioactive urocanic acid. By 3 days after the injection, however, both radioactive urocanic acid and histidine appear, apparently at the expense of radioactive proteins, 5. In isolated stratum corneum, the residual histidine can be converted into urocanic acid by the histidine ammonia-lyase in the tissue only if the natural acidity of the tissue is neutralized. It is concluded from these observations that the biosynthesis of urocanic acid occurs naturally only in the stratum corneum, which contains only dead cells. The amount of urocanic acid accumulated is limited by the availability of free histidine produced by proteolysis of residual protein in these cells and by the penetration into the stratum corneum of the 'acid mantle' of the skin.

The enzymatic conversion of L-histidine to urocanic acid by whole cells of Micrococcus luteus immobilized on carbodiimide activated carboxymethylcellulose.

Whole cells of Micrococcus luteus (formerly Sarcina lutea ATCC 9341) have been covalently linked to a carboxymethylcellulose support system, with the retention of histidine ammonia-lyase activity. The dependence of the rate of urocanic acid formation on pH, temperature, and added surfactant concentration was similar for the free and the immobilized cells. The immobilization procedure used is based on the carbodiimide activation of carboxymethylcellulose and has been optimized for the histidine ammonia-lyase activity of the immobilized cells on a given weight of cellulose. In a column reactor at 23 degrees C and superficial velocity of 0.044 cm/min, 5 g of cellulose with bound cells gave a 35% conversion of an L-histidine solution (0.25M, pH 9.0) to urocanic acid for 16 days of continuous operation. The scope of this carbodiimide assisted immobilization procedure has been investigated for a series of microorganisms and a variety of carboxylate functionalized supports.

Enzymatic production of urocanic acid by Achromobacter liquidum.

To develop an efficient method for the production of urocanic acid, optimal conditions for the production of microbial L-histidine ammonia lyase and for the conversion of L-histidine to urocanic acid by this enzyme were studied. A number of microorganisms were screened to test their ability to form and accumulate urocanic acid from L-histidine. Achromobacter liquidum was selected as the best organism. With this organism, enzyme activity as high as 2.0 units/ml could be produced by a shaking culture at 30 C in a medium containing glucose, urea, potassium phosphate, L-histidine, yeast extract, peptone, and inorganic salts. Appropriate addition of a surface-active agent to the reaction mixture shortened the time required for the conversion. A large amount of L-histidine was converted stoichiometrically to urocanic acid in 48 h at 40 C. Accumulated urocanic acid was readily isolated in pure form by ordinary procedures with isoelectric precipitation. Yields of isolated urocanic acid of over 92% from L-histidine were easily attainable. When the culture of Achromobacter liquidum was added to DL-histidine, D-histidine and urocanic acid were simultaneously obtained in high yields.

The degradation of L-histidine in the rat. The formation of imidazolylpyruvate, imidazolyl-lactate and imidazolylpropionate.

1. Soluble and mitochondrial forms of histidine-pyruvate aminotransferase were separated from rat liver preparations by chromatography on DEAE-cellulose. 2. These enzymes were characterized with respect to substrate specificity, substrate affinity, pH optimum, stability and molecular weight by chromatography on Sephadex G-200. 3. Each enzyme has a relatively broad specificity showing significant activity towards l-phenylalanine and l-tyrosine and catalysing transamination with a number of monocarboxylic 2-oxo acids. 2-Oxoglutarate is not a substrate for either enzyme. 4. The molecular weights of the two enzymes, by chromatography on Sephadex G-200, are in the range 130000-150000. 5. The formation in vitro of imidazolyl-lactate from imidazolylpyruvate and NADH was demonstrated by using liver preparations. 6. From a study of imidazolyl-lactate-NAD(+) oxidoreductase activity after electrophoresis of liver preparations on polyacrylamide gel, and from an examination of the activity of l-lactate-NAD(+) oxidoreductase (EC 1.1.1.27) towards imidazolylpyruvate, it is concluded that this latter enzyme is responsible for the formation of imidazolyl-lactate in the liver. 7. Preparations of bacteria obtained from rat faeces form imidazolylpropionate from l-histidine and urocanate without further subculture. The amount of imidazolylpropionate formed is increased under anaerobic conditions and more so in an atmosphere of H(2). It is suggested that the gut flora of the rat contribute largely, if not exclusively, to the formation of imidazolylpropionate normally found in the urine.

Oxidative N-dealkylation: a mannich intermediate in the formation of a new metabolite of lidocaine in man.

Evidence is presented for a new metabolite of lidocaine. Its structure, N(1)-ethyl-2-methyl-N(3)-(2,6-dimethylphenyl)-4-imidazolidinone, suggests reactive electrophilic intermediates for the oxidative removal of N-alkyl groups in general.