S-Adenosyl-L-methionine (AdoMet):arsenic(III) methyltransferase, purified from liver cytosol of adult male Fischer 344 rats, catalyzes transfer of a methyl group from AdoMet to trivalent arsenicals producing methylated and dimethylated arsenicals. The kinetics of production of methylated arsenicals in reaction mixtures containing enzyme, AdoMet, dithiothreitol, glutathione (GSH), and arsenite are consistent with a scheme in which monomethylated arsenical produced from arsenite is the substrate for a second methylation reaction that yields dimethylated arsenical. The mRNA for this protein predicts a 369-amino acid residue protein (molecular mass 41056) that contains common methyltransferase sequence motifs. Its sequence is similar to Cyt19, a putative methyltransferase, expressed in human and mouse tissues. Reverse transcription-polymerase chain reaction detects S-adenosyl-L-methionine:arsenic(III) methyltransferase mRNA in rat tissues and in HepG2 cells, a human cell line that methylates arsenite and methylarsonous acid. S-Adenosyl-L-methionine:arsenic-(III) methyltransferase mRNA is not detected in UROtsa cells, an immortalized human urothelial cell line that does not methylate arsenite. Because methylation of arsenic is a critical feature of its metabolism, characterization of this enzyme will improve our understanding of this metalloid's metabolism and its actions as a toxin and a carcinogen.In many species, including humans, exposure to inorganic arsenic results in urinary excretion of methylated and dimethylated arsenicals (1-3). Cullen and co-workers (4) summarized the conversion of inorganic arsenic into these methylated products in a reaction scheme which incorporates oxidative methylation and the cycling of arsenic between the pentavalent (As V ) 1 and trivalent (As III ) oxidation states,Because reduction of arsenic to trivalency is a prerequisite for its oxidative methylation, pentavalent arsenicals are reduced by endogenous thiols such as glutathione (GSH) (5, 6) or by As V reductases (7-9). A protein has been purified from rabbit liver cytosol that catalyzes the methylation of both arsenite and methylarsonous acid (10, 11); however, this protein has not been sequenced. These activities are designated arsenite methyltransferase (EC 2.1.1.137) and methylarsonite methyltransferase (EC 2.1.1.138), respectively. This protein (estimated molecular mass 60 kDa) uses S-adenosyl-L-methionine (AdoMet) as the methyl group donor. The methylation of arsenite by this protein is stimulated by a monothiol (GSH) and the methylation of methylarsonous acid is highly stimulated by a dithiol, dithiothreitol (DTT). The methylation of arsenic has been commonly regarded as a mechanism for its detoxification (12). However, recent research has shown that methylated arsenicals that contain As III are important intermediates in the metabolism of inorganic arsenic. Methylated arsenicals that contain As III are found in the urine of individuals who chronically consume drinking water that contains inorganic arsenic and in cells cu...

A new method for detecting trace vapors of NO2-containing compounds near atmospheric conditions has been demonstrated with the use of one-color-laser photofragmentation/ionization spectrometry. An ArF laser is employed to both photolytically fragment the target molecules in a collision-free environment and ionize the characteristic NO fragments. The production of NO is hypothesized to result from a combination of two NO2 unimolecular fragmentation pathways, one yielding NO in its X2II electronic ground state and the other in its A2Σ+ excited state. Ionization of ground-state NO molecules is accomplished by resonance-enhanced multiphoton ionization processes via its A2Σ+ ← X2II (3, 0), B2II ← X2II (7, 0) and/or D2Σ+ ← X2II (0, 1) bands at 193 nm. The analytical utility of this method is demonstrated in a molecular beam time-of-flight apparatus. Limits of detection range from the parts-per-million (ppm) to parts-per-billion (ppb) level for NO, NO2, CH3NO2, dimethylnitramine (DMNA), ortho- and meta-nitrotoluene, nitrobenzene, and trinitrotoluene (TNT). Under effusive beam experimental conditions, discrimination between structural isomers, ortho-nitrotoluene and meta-nitrotoluene, has been demonstrated with the use of their characteristic photofragmentation/ionization mass spectra.

The development of a novel technique for sensing trace vapors of NOr-containlng compounds Is reported. This technique Is based on the use of one laser operating at 226 nm to both photofragment the target molecule and detect the characteristic NO fragment, formed from a rapid predissociation of N02, by resonance-enhanced multiphoton Ionization (REMPI) and/or laser-induced fluorescence (LIF) via Its A* 12 32+-X2II (0,0) band origin. The analytical utility of this technique Is demonstrated on a number of compounds, Including dbnethylnltramlne, nltromethane, nitrobenzene, trinitrotoluene (TNT), and 1,3,5-trinltrohexahydro-1,3,5-triazlne (RDX), employing molecular beam sampling with tlme-of-fllght mass spectrometrlc analysis of the Jet-cooled analyte seeded In an atmosphere of buffer gas. With the present system, limits of detection of 8 and 24 parts per blWon (ppb) are obtained for RDX and TNT, respectively, using only ~100 pj/pulse of laser energy. The limits of detection of the other compounds studied are also presented and discussed.

ArF-laser-produced microplasmas in CO, CO(2), methanol, and chloroform are studied by time-resolved emission measurements of the plasma decay. Electron densities are deduced from Stark broadening of the line profiles of atomic H, C, O, and Cl. Plasma ionization and excitation temperatures are determined from measurements of relative populations of ionic and neutral species produced in the plasmas. A discussion of the thermodynamic equilibrium status of ArF-laser microplasmas is presented. In general, the ArF-laser-produced microplasma environment is found to be similar in all the gases studied, in terms of both temperature (15,000-20,000 K) and electron density (10(17) cm(-3)-10(18) cm(-3)), despite the considerable differences observed in the breakdown thresholds and relative energies deposited in the various gases.