| histic epipedon | An 8- to 16-inch soil layer at or near the surface that is saturated for 30 consecutive days or more during the growing season in most years and contains a minimum of 20 percent organic matter when no clay is present or a minimum of 30 percent organic matter when 60 percent or more clay is present, generally a thin horizon of peat or muck if the soil has not been plowed. (09 Oct 1997) |
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| histidase | <enzyme> An enzyme of the lyase class that catalyses the reaction of l-histidine to form urocanate and ammonia. The reaction is the initial step of histidine catabolism. Genetic deficiency of the enzyme, transmitted as an autosomal recessive trait, causes histidinaemia. Chemical name: L-Histidine ammonia-lyase Registry number: EC 4.3.1.3 (12 Dec 1998) |
| histidinaemia | <biochemistry> A medical condition in which the amount of the amino acid histidine in the blood is abnormally high.Also, histidine is present in the urine and the amount of the enzyme histidase (which breaks down histidine) is abnormally low. Mild retardation and speech or reading disorders often occur along with histidinaemia. (11 Jan 1998) |
| histidinal | The aldehyde analogue of histidine (-CHO replacing -COOH). (05 Mar 2000) |
| histidinase | <enzyme> An enzyme of the lyase class that catalyses the reaction of l-histidine to form urocanate and ammonia. The reaction is the initial step of histidine catabolism. Genetic deficiency of the enzyme, transmitted as an autosomal recessive trait, causes histidinaemia. Chemical name: L-Histidine ammonia-lyase Registry number: EC 4.3.1.3 (12 Dec 1998) |
| histidine | <amino acid> An amino acid with an imidazole side chain with a pKa of 6-7. Acts as a proton donor or acceptor and has high potential reactivity and diversity of chemical function. Forms part of the catalytic site of many enzymes. (18 Nov 1997) |
| histidine ammonia-lyase | <enzyme> An enzyme of the lyase class that catalyses the reaction of l-histidine to form urocanate and ammonia. The reaction is the initial step of histidine catabolism. Genetic deficiency of the enzyme, transmitted as an autosomal recessive trait, causes histidinaemia. Chemical name: L-Histidine ammonia-lyase Registry number: EC 4.3.1.3 (12 Dec 1998) |
| histidine deaminase | <enzyme> An enzyme of the lyase class that catalyses the reaction of l-histidine to form urocanate and ammonia. The reaction is the initial step of histidine catabolism. Genetic deficiency of the enzyme, transmitted as an autosomal recessive trait, causes histidinaemia. Chemical name: L-Histidine ammonia-lyase Registry number: EC 4.3.1.3 (12 Dec 1998) |
| histidine decarboxylase | <enzyme> An enzyme that catalyses the decarboxylation of histidine to histamine and carbon dioxide. It requires pyridoxal phosphate in animal tissues, but not in microorganisms. Chemical name: L-Histidine carboxy-lyase Registry number: EC 4.1.1.22 (12 Dec 1998) |
| histidine N-acetyltransferase | <enzyme> Synthesises nalpha-acetyl-l-histidine from l-histidine and acetylcoenzyme a Registry number: EC 2.3.1.33 Synonym: acetyl-CoA - l-histidine n-acetyltransferase, histidine acetyltransferase, coasac - l-histidine n-acetyltransferase, n-acetylhistidine synthetase (26 Jun 1999) |
| histidine permease | <chemical> Hisp, hism and hisq are membrane proteins found in histidine permease (the hisqmp2 complex); also contains the histidine-binding protein hisj Chemical name: permease, histidine Synonym: hisp protein, hism protein, hisq protein, hutm gene product, ee57d gene product (26 Jun 1999) |
| histidine-trna ligase | <enzyme> An enzyme that activates histidine with its specific transfer RNA. Chemical name: L-Histidine:tRNA(His) ligase (AMP-forming) Registry number: EC 6.1.1.21 (12 Dec 1998) |
| histidino | The radical of histidine produced by removal of a hydrogen from a nitrogen atom; prefixed by Na, Nτ, or Nπ. (05 Mar 2000) |
| histidinol | <chemical> Beta-amino-1h-imidazole-4-propanol. The penultimate step in the pathway of histidine biosynthesis. Oxidation of the alcohol group on the side chain gives the acid group forming histidine. Histidinol has also been used as an inhibitor of protein synthesis. Chemical name: 1H-Imidazole-4-propanol, beta-amino- (12 Dec 1998) |
| histidinol-phosphatase | <enzyme> An enzyme that catalyses the hydrolysis of histidinol-phosphate to histidinol. One of the regulatory enzymes in histidine biosynthesis. Chemical name: L-Histidinol-phosphate phosphohydrolase Registry number: EC 3.1.3.15 (12 Dec 1998) |