| hydrogen cyanide | <chemical> Hydrogen cyanide (hcn). A toxic liquid or colourless gas. It is found in the smoke of various tobacco products and released by combustion of nitrogen-containing organic materials. Pharmacological action: poisons. Chemical name: Hydrocyanic acid (12 Dec 1998) |
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| hydrogen dehydrogenase | <enzyme> Hoxf, hoxh, hoxu, and hoxy from rhodococcus opacus encode the alpha-, beta-, gamma-, and delta-subunits, respectively Registry number: EC 1.12.1.2 Synonym: hydrogen-nad+ oxidoreductase, nad-reducing hydrogenase, hoxf gene product, hoxu gene product, hoxy gene product, hoxh gene product (26 Jun 1999) |
| hydrogen dioxide | <chemical> Hydrogen peroxide is produced by vertebrate phagocytes and is used in bacterial killing (the myeloperoxidase halide system). (05 May 1997) |
| hydrogen donor | A metabolite from which hydrogen is removed (by a dehydrogenase system) and transferred by a hydrogen carrier to another metabolite, which is thus reduced. (05 Mar 2000) |
| hydrogen electrode | The ultimate standard of reference in all pH determinations, limited and technically difficult to use, consisting of a piece of spongy platinum black partly immersed in a solution in a small glass tube; the tube above the solution is filled with hydrogen gas that is bubbled through the solution and absorbed by the platinum; the electrode thus measures the potential between H2 and H+, the "standard" potential of which (1 atmosphere, 1 molar) is taken as zero; hence, the hydrogen electrode potential measures [H+] or pH. (05 Mar 2000) |
| hydrogen embrittlement | <physics> A decrease in the fracture strength of metals (embrittlement) due to the incorporation of hydrogen within the metal lattice. (09 Oct 1997) |
| hydrogen exponent | The logarithm of the hydrogen ion concentration in blood or other fluid; its negative is the pH of that fluid. (05 Mar 2000) |
| hydrogen ion | A hydrogen atom minus its electron and therefore carrying a unit positive charge (i.e., a proton); in water, it combines with a water molecule to form hydronium ion, H3O+. (05 Mar 2000) |
| hydrogen number | The quantity of hydrogen that 1 g of fat will absorb; it is a measurement of the amount of unsaturated fatty acids in the fat. See: iodine number. (05 Mar 2000) |
| hydrogen peroxide | <chemical> Hydrogen peroxide is produced by vertebrate phagocytes and is used in bacterial killing (the myeloperoxidase halide system). (05 May 1997) |
| hydrogen phosphide | <chemistry> A colourless gas, PH3, analogous to ammonia, and having a disagreeable odour resembling that of garlic. Synonym: hydrogen phosphide, and formerly, phosphureted hydrogen. It is the most important compound of phosphorus and hydrogen, and is produced by the action of caustic potash on phosphorus. It is spontaneously inflammable, owing to impurities, and in burning produces peculiar vortical rings of smoke. Source: Websters Dictionary (01 Mar 1998) |
| hydrogen-potassium-exchanging atpase | <enzyme> An enzyme isolated from the gastric mucosa that catalyses the hydrolysis of ATP coupled with the exchange of hydrogen and potassium ions across the cell wall. Chemical name: ATP phosphohydrolase (H+/K+-transporting) Registry number: EC 3.6.1.36 (12 Dec 1998) |
| hydrogen pump | Molecular mechanism for acid secretion from gastric parietal cells based on the activity of a H+-K+-ATPase. (05 Mar 2000) |
| hydrogen sulfide | <chemical> Hydrogen sulfide (h2s). A flammable, poisonous gas with a characteristic odour of rotten eggs. It is used in the manufacture of chemicals, in metallurgy, and as an analytical reagent. Chemical name: Hydrogen sulfide (H2S) (12 Dec 1998) |
| hydrogen transport | The transfer of hydrogen from one metabolite (hydrogen donor) to another (hydrogen acceptor) through the action of an enzyme system; the donor is thus oxidised and the acceptor reduced. (05 Mar 2000) |