Trans-sulphuration

Homocysteine (Hey) metabolism is closely linked to that of the essential amino acid methionine and thus plays a central role in several vital biological processes. Methionine itself is needed for protein synthesis and donates methyl groups for the synthesis of a broad range of vital methylated compounds. It is also a main source of sulphur and acts as the precursor for several other sulphur-containing amino acids such as cystathionine, cysteine and taurine. In addition, it donates the carbon skeleton for polyamine synthesis [1,2]. Hey is also important in the metabolism of folate and in the breakdown of choline. Hey levels are determined by its synthesis from methionine, which involves several enzymes, its remethylation to methionine and its breakdown by trans-sulphuration. 

As well as remethylation, Hey can be degraded in the trans-sulphuration pathway, which first involves condensation of Hey with serine forming cystathionine, then breakdown of this compound to cysteine and a-oxo-butyrate. These reactions... 

The metabolic control of methionine metabolism is complex and involves, for example, changes of enzyme levels in particular tissues, mechanisms linked to the kinetic properties of the various enzymes and their interaction with metabolic effectors [6, 7]. A particularly important metabolic effector is AdoMet. This inhibits the low Km isoenzymes of MAT, and MTHF reductase, inactivates betaine methyltransferase, but activates MAT III (the high-Km isoenzyme) and cystathionine /1-synthase. Therefore, high methionine intake and thus higher AdoMet levels favour trans-sulphuration, and when levels are low methionine is conserved. AdoHcy potently inhibits AdoMet-dependent methyltransferases and both Hey remethylating enzymes. Another important control mechanism is the export of Hey from cells into the extracellular space and plasma, which occurs as soon as intracellular levels increase [8]. 

L-Methionine (Met) conversion to Cys is impaired under stress, such as in premature infants or AIDS patients. Thus, the metabolic flow through the trans-sulphuration pathway may be inadequate to meet the Cys demand under critical conditions. 

Epithio compounds are obteuned from epoxides by reaction with sulphur-containing compounds such as thiourea, thiocyanogen, thiocyanates, potassium methyl xanthate and base, or 3-methylbenzothiazol-2-one and trifluoroacetic acid. 9,10-Epithiostearic acid melts at 58 °C (cis) and 64 °C (trans). Sulphur-containing compounds can also be made by radical addition (usually photolytic) of hydrogen... 

Cysteine Serine (a) Trans-sulphuration from homo-... 

Louie, D.K., 2008. Storage/loading/unloading—transportation—sulfur. Downloaded from http //www.sulphuric-acid.com/techmanual/storage/trans sulphur.htm on May 15, 2012. Outotec, 2008. Sulphuric acid plants (sulphur combustion section). Company brochure. Google outotec sulphuric acid plants. 

If a molecule has a centre of inversion (or centre of symmetry), i, reflection of each nucleus through the centre of the molecule to an equal distance on the opposite side of the centre produces a configuration indistinguishable from the initial one. Figure 4.4 shows s-trans-buta-1,3-diene (the x refers to trans about a nominally single bond) and sulphur hexafluoride, both of which have inversion centres. 

H. Rieschel and K. Zech, Mufbereitungs-Technik, 22(9), 475 (1981). English trans.. Phosphorus and Potassium, British Sulphur Corp. Ltd., London, Sept.—Oct. 1981. 

Cid etal. studied the corrosion resistance of Ni, 5% Fe-Ni and 10% Fe-Ni alloys in the trans-passive region in sulphuric acid. For a given acid concentration the addition of iron reduced the corrosion rate. It was concluded that the addition of small percentages of Fe was doubly beneficial, decreasing both general and intergranular corrosion. 

In many cases, it has been found that 7r-bonding ligands favour S-bonding. In a complex with both N- and S-bonded thiocyanate (Figure 3.74) the N-bonded group is trans to P while the sulphur-bonded thiocyanate is trans to the harder nitrogen ( anti-symbiosis ). 

Allen PL, Hickling A (1957) Electrochemistry of sulphur. Part 1 - Overpotential in the discharge of the sulphide ion. Trans Faraday Soc 53 1626-1635... 

Jorgensen BB. 1982. Ecology of the bacteria of the sulphur cycle with special reference to anoxic-oxic interface environments. Philos Trans R Soc Lond B Biol Sci 298 543-561. 

The BASF route started from hydroquinone, which was converted to 2,5-dihydroterephthalic acid by a Kolbe-Schmitt reaction. One mole of this acid was treated with two moles of an arylamine, both components being in the form of a suspension in aqueous methanol. This was added to a small amount of a solution of vanadium(III) chloride and sodium chlorate. Gentle heating gave a 95% yield of 2,5-bis(arylamino)benzo-l,4-quinone-3,6-dicarboxylic acid. Ring closure to the trans-quinacridonequinone took place in the presence of concentrated sulphuric acid at 60-80 °C. This was then reduced to the required crude pigment by zinc or aluminium powder in caustic soda under pressure,in an aluminium chloride/urea melt or by the use of a sulphuric acid/polyphosphoric acid mixture. 

Kelly DP. 1982. Biochemistry of the chemolithotrophic oxidation of inorganic sulphur. Phil Trans Roy Soc Lond Ser B 298 499-528. 

Cox, R. A., and S. A. Penkett. Aerosol formation from sulphur dioxide in the presence of ozone and olefink hydrocarbons. J. Chem. Soc. Faraday Trans. 1. 68 1735-1753, 1972. 

Dainton, F. S., and K. J. Ivin. The photochemical formation of sulphuric acids from sulphur dioxide and hydrocarbons. Trans. Faraday Soc. 46 374-381, 1950. 

James, F. C., J. A. Kerr, and J. P. Simons. Direct measurement of the rate of reaction of the method radical with sulphur dioxide. J. Chem. Soc. Faraday Trans. I. 69 2124-2129, 1973. 

Electrochemical reduction of a,a -dibromoketones affords the unstable cyclo-propanone, which is in equlibrium with the dipolar intermediate 22. The cyclopro-panone hemiacetal is isolated in yields of 40 - 85 % from reaction in acetonitrile and methanol at -20 °C [99], The dipolar form can be trapped in a cycloaddition process with furan [100], Reaction with acetic acid leads to the a-acetoxy-ketone.[101]. Unstable three membered heterocyclic rings are intermediate in the reduction of sulphur and phosphorus linked dibromo compounds 23. In these reactions, the heteroatom is extruded leaving ci - and trans-stilbenes as the isolated products [102,103],... 

N-, 2- and 2,3- alkylsubstituted indoles are reduced in acid solution to give good yields of the dihydroindolc [63]. Indole itself is polymerised under the acid conditions. Reaction was originally carried out in a medium of 60 % sulphuric acid at a lead cathode with no attempt made to find the optimum acid concentration. More recently, 20 % sulphuric acid has been proposed as the solvent in these reactions [64], Reduction of tetrahydrocarbazole 17 gives predominately cis-hexahydro-carbazole, the thermodynamically favoured isomer, along with 1 -2 % of the trans-isomer [65], 2,3-Dimethyl-indole yields a mixture of the cis- and trans-dihydro-indole isomers [63,66],... 

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