Enzyme degradation half-lives

Litde is known about metaboHc inactivation of ( -endorphin and the dynorphins. NEP, and to a lesser extent APN, are only weaMy active against P-endorphin (183). Enzymes are known which degrade P-endorphin in vitro under nonphysiological conditions (202) or which inactivate P-endorphin by N-acetjlation (203). Alack of specific degradative enzymes for these peptides may account for their relatively long half-life in vivo though this has not been definitively estabUshed. 

Degradation of HMG-CoA reductase. This enzyme has a half-life of only three hours, and the half-life itself depends on cholesterol levels high [cholesterol] means a short half-life for HMG-CoA reductase. 

The susceptibility of a protein to degradation is expressed as its half-life (t,/2)> the time required to lower its concentration to half the initial value. Half-lives of hver proteins range from under 30 minutes to over 150 hours. Typical housekeeping enzymes have tj/2 values of over 100 hours. By contrast, many key regulatory en-... 

HA turnover is rapid in the brain, with a half-life of about 30 min. This can change very quickly depending on neuronal activity. There is no high-affinity uptake system for HA once released, HA is inactivated by catabolism. In the brain, released HA is methylated almost exclusively by the enzyme histamine-N-methyltransferase (E.C. 2.1.1.8). The tele-methyl-HA is subsequently degraded by monoamine oxidase-B (MAO-B) and aldehyde dehydrogenase to produce tele-methylimidazoleacetic acid (Brown et ah, 2001). 

The hydrolysis half-lives at pH 7.4 and 20 and 37.5 °C were 130 and 27 d, respectively. At pH 6.1 and 20 °C, the hydrolysis half-life was 170 d (Freed et al, 1979). When equilibrated with a prereduced pokkali soil (acid sulfate), parathion instantaneously degraded to aminoparathion. The quick rate of reaction was reportedly due to soil enzymes and/or other heat labile substances. Desethyl aminoparathion was also identified as a metabolite in two separate studies (Wahid and Sethunathan, 1979 Wahid et al., 1980). The half-lives for the degradation of parathion in a silty clay (pH 5.5) and sandy clay (pH 6.9) were 23 and 22 d, respectively (Sattar, 1990). 

Greenhouse soil was treated at 500, 1000, 2000 and 5000 ppm with Diazinon 4E. Parathion hydrolase was added to the soil to determine the efficacy of the enzyme to rapidly degrade diazinon during a spill situation. The half-life of the diazinon in the 500 ppm treatment without enzyme present was 9.4 days while the half-life of diazinon in the 500 ppm treatment with enzyme present was one hour. The half-lives of diazinon in the 1000, 2000 and 5000 ppm treatments with enzyme present were 1.2, 5.6 and 128 hours (5.3 days), respectively. These data indicate that parathion hydrolase can be used effective " to rapidly reduce large concentrations of diazinon in soil. 

The enzyme parathion hydrolase is active enough to be used effectively to degrade high concentrations of diazinon in soil. The half-life of diazinon in soil treated at 2000 ppm was 5.6 hours. 

Fig. 8. Variation of the skin wound half-life with degradation rate R (in collagenase) of collagen-GAG matrix. The half-life is the time required for a wound to contract to 50% of the original area. The degradation is in empirical units, which are defined in terms of an in vitro assay. A somewhat arbitrary broken vertical line is drawn near R = 140 enzyme units. This line shows the level of degradation rate above which the half-life of matrices rapidly drops to the level of the ungrafted wound. The horizontal scale is logarithmic [79]...

Glucagon is extensively degraded in the liver and kidney as well as in plasma and at its tissue receptor sites. Because of its rapid inactivation by plasma, chilling of the collecting tubes and addition of inhibitors of proteolytic enzymes are necessary when samples of blood are collected for immunoassay of circulating glucagon. Its half-life in plasma is between 3 and 6 minutes, which is similar to that of insulin. 

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