Enzyme Half-Lives

Little work has been done on the effect of anabolic inhibitors on cellular heat dissipation probably because there is empirical evidence that anabolic processes do not contribute significantly to it (see p. 312). Loike et al. (1981) found that 0.07 mmol dm-3 cycloheximide, an antibiotic inhibitor of protein synthesis, had no effect in 30 minutes on the heat production of murine macrophages. On the other hand, Krakauer and Krakauer (1976) showed that long-term exposure of lymphocytes from immunized horses to 1 mg dm-3 cycloheximide considerably reduced heat production. This was likely to be due to a secondary effect of the antibiotic arresting catabolism by inhibiting the turnover of short half-life enzymes. 

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. 

Plasma levels of 3—5 p.g/mL are obtained two hours after adraiinistration of 200 mg ketoconazole. No accumulation in the bloodstream was noted after a 30-wk treatment with this dose. The half-life is approximately eight hours. When ketoconazole is taken with meals, higher plasma levels are obtained. Distribution studies using radioactive ketoconazole in rats show radioactivity mainly in the Hver and the connective tissue. Radioactivity is also present in the subcutaneous tissue and the sebaceous glands. After one dose of 200 mg in humans, ketoconazole is found in urine, saUva, sebum, and cenimen. Like miconazole, the mode of action is based on inhibition of the cytochrome P-450 dependent biosynthesis of ergosterol. This results in disturbed membrane permeabiUty and membrane-bound enzymes (8,10,23,25). 

Streptokinase has an initial plasma half-life (/ 2 of 18 min, and a P half-life of 83 min (73) it is well recognized that the thrombolytic efficacy of the enzyme decreases as the age of the thrombus increases thus, thrombolysis is significantly decreased when therapy is initiated more than three hours after an occlusion (74). 

Anistreplase has a considerably longer a half-life than streptokinase, ie, 90 min compared to 20 min (87,88). Moreover, it does not require prolonged infusion to achieve its thrombolytic effects. Anistreplase was found to be highly effective after a single intravenous dose of 30 units over a 5-min period compared to a 60-min infusion of 1.5 million units of streptokinase (89—94). In direct comparative studies, anistreplase was as effective as intracoronary (95,96) and intravenously (96—100) adrninistered streptokinase. In a randomized, double-blind, placebo-controUed study (AIMS trial) with 1004 patients given this modified enzyme, the 30-day mortaUty rate was 12.2% for patients receiving placebo, compared to 6.4% for patients who received 30 units of anistreplase intravenously within six hours of the onset of symptoms (101). 

Poly(ethylene glycol) (PEG) molecules attached to adenosine deaminase (ADA) have been used in patients exhibiting symptoms of the severe combined immunodeficiency syndrome (SCID) caused by ADA deficiency. The modified enzyme has a plasma half-life of weeks as compared to the unmodified enzyme (minutes) (248). PEG-L-asparaginase has induced remissions in patients with non-Hodgkin s lymphoma (248). However, one disadvantage of PEG-enzyme treatment is its expense, ie, a year s treatment costs about 60,000 (248). 

The enzyme catalyzes the hydrolysis of an amide bond linkage with water via a covalent enzyme-inhibitor adduct. Benzoxazinones such as 2-ethoxy-4H-3,l-benzoxazin-4-one [41470-88-6] (23) have been shown to completely inactivate the enzyme in a competitive and stoichiometric fashion (Eigure 5). The intermediate (25) is relatively stable compared to the enzyme-substrate adduct due to the electron-donating properties of the ortho substituents. The complex (25) has a half-life of reactivation of 11 hours. 

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. 

Almost all types of cell can be used to convert an added compound into another compound, involving many forms of enzymatic reaction including dehydration, oxidation, hydroxyla-tion, animation, isomerisation, etc. These types of conversion have advantages over chemical processes in that the reaction can be very specific, and produced at moderate temperatures. Examples of transformations using enzymes include the production of steroids, conversion of antibiotics and prostaglandins. Industrial transformation requires the production of large quantities of enzyme, but the half-life of enzymes can be improved by immobilisation and extraction simplified by the use of whole cells. 

Intestinal absorption of digoxin is less complete compared to digitoxin. In order to improve absorption, acetylated- and methylated-digoxin derivates were developed. Digitoxin is metabolised in hepatic microsomal enzymes and can be cleared independently from renal function. The therapeutical serum level of digoxin is 0.5-2.0 ng/ml and 10-35 ng/ml of digitoxin. Steady state plateau of therapeutic plasma concentrations is reached after 4-5 half-life-times using standard daily doses [5]. 

The active metabolite of leflunomide, the ring-opened drug A771726, inhibits dihydroorotate dehydrogenase (DHOD) which is the key enzyme of the de novo pyrimidine synthesis. Inhibition of synthesis stops proliferation of activated lymphocytes. The leflunomide derivative FK778 which shows similar therapeutic efficacy but shorter half-life is investigated in clinical trials. 

Acetylsalicylic acid irreversibly inhibits both COX-1 and COX-2 by acetylating the enzymes. Since mature platelets lack a nucleus, they are unable to synthesise new enzyme. The anti-platelet effects of acetylsalicylic acid persist therefore throughout the lifetime of the platelet and the half-life of this effect is thus being much longer than the elimination half-life of acetylsalicylic acid (15 min). Since new platelets are continuously launched into the circulation, the clinically relevant anti-platelet effect of aspirin lasts for up to five days. This is the reason why low doses of acetylsalicylic acid (ca. 100 mg per day) are sufficient in the prophylaxis of heart attacks. 

After oral administration, acetylsalicylic acid is rapidly and almost completely absorbed but in the intestinal mucosa it is partly deacetylated to salicylic acid, which also exhibits analgesic activity. The plasma half-life of acetylsalicylic acid is 15 min whereas that of salicylic acid, at low dosages of acetylsalicylic acid, is 2-3 h. Salicylic acid is eliminated more slowly when acetylsalicylic acid is administered at high dose rates because of saturation of the liver enzymes. The metabolites are mainly excreted via the kidney. 

Thyroxine (3, 5, 3,5-L-teraiodothyronine, T4) is a thyroid hormone, which is transformed in peripheral tissues by the enzyme 5 -monodeiodinase to triiodothyronine. T4 is 3-8 times less active than triiodothyronine. T4 circulates in plasma bound to plasma proteins (T4-binding globulin, T4-binding prealbumin and albumin). It is effective in its free non-protein-bound form, which accounts for less than 1%. Its half-life is about 190 h. 

The regulation of receptor synthesis is a second component of receptor downregulation. It involves processes that reduce gene transcription, mRNA stability, and receptor half-life time. It should be noted that mechanisms in addition to the regulation of the receptor number may account for tolerance development. Second messenger levels and enzyme activities that participate in the signaling of a given receptor are... 

Complex inactivation kinetics caused by enzyme-catalyzed decomposition of epoxide kinetic constants calculated from initial rates of inactivation. Approximate value calculated from half-life in the presence of 50 mAf inhibitor. 

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-... 

ALASl. This repression-derepression mechanism is depicted diagrammatically in Figure 32-9. Thus, the rate of synthesis of ALASl increases greatly in the absence of heme and is diminished in its ptesence. The turnover rate of ALASl in rat liver is normally rapid (half-life about 1 hour), a common feature of an enzyme catalyzing a rate-limiting reaction. Heme also affects translation of the enzyme and its transfer from the cytosol to the mitochondrion. 

It is relevant also to compare the results in Fig.5 with previously published data for PAC production under similar environmental conditions, where with higher concentrations of initial benzaldehyde (600 mM), pyruvate (400 mM) and PDC activity (8.4 U ml ) a similar maximum concentration of PAC of 330 mM was produced [6]. PDC stability was similar in both processes with half life values of approximately 27h. However, PAC production was much faster in the benzaldehyde emulsion system, presumably due to higher initial enzyme concentration. 

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