Factor half-life

Indications Severe oral mucositis in cancer patients Category Keratinocyte growth factor Half-life 4.5 hours... 

In selection of a radioactive nuclide as a tracer, attention must be paid to the following factors half-life, radiation emitted, specific activity, chemical form, and purity along with license of use and availability. In the case of a non-iso topic tracer, the element of the tracer must be chosen after due consideration as stated above. In any case, radiation safety and legal regulations have to be carefully followed throughout acquisition, storage, use, and disposal of a radioactive tracer. 

Concentration factor Half-life in tissues (days)... 

Initiators. The degree of polymerization is controlled by the addition rate of initiator(s). Initiators (qv) are chosen primarily on the basis of half-life, the time required for one-half of the initiator to decay at a specified temperature. In general, initiators of longer half-Hves are chosen as the desired reaction temperature increases they must be well dispersed in the reactor prior to the time any substantial reaction takes place. When choosing an initiator, several factors must be considered. For the autoclave reactor, these factors include the time permitted for completion of reaction in each zone, how well the reactor is stirred, the desired reaction temperature, initiator solubiUty in the carrier, and the cost of initiator in terms of active oxygen content. For the tubular reactors, an additional factor to take into account is the position of the peak temperature along the length of the tube (9). 

Excretion factors are often related to lipophilicity. More lipophilic compounds tend to be excreted by the Hver into the bile, resulting in elimination ultimately in the feces. As this is a relatively slow process, much of the radioactivity having a shorter half-life decays before being eliminated. Polar compounds are more likely to be excreted by the kidneys. 

PET imaging systems are somewhat more complex, and therefore more expensive than are SPECT systems, and the price factor is generally between two and three. The primary cost premium associated with these systems, however, is the need for a cyclotron and its attendant staff combined with the relative complexity of radiopharmaceutical preparation for short half-life isotopes. As of 1996, there are considerable hurdles blocking widespread regulatory approval and full reimbursement of PET studies. 

Approximately 0.05 to 0.2% of vitamin > 2 stores are turned over daily, amounting to 0.5—8.0 )J.g, depending on the body pool size. The half-life of the body pool is estimated to be between 480 and 1360 days with a daily loss of vitamin > 2 of about 1 )J.g. Consequentiy, the daily minimum requirement for vitamin B22 is 1 fig. Three micrograms (3.0 J.g) vitamin B22 are excreted in the bile each day, but an efficient enterohepatic circulation salvages the vitamin from the bile and other intestinal secretions. This effective recycling of the vitamin contributes to the long half-life. Absence of the intrinsic factor intermpts the enterohepatic circulation. Vitamin > 2 is not catabolized by the body and is, therefore, excreted unchanged. About one-half of the vitamin is excreted in the urine and the other half in the bile. 

Factor Synthesis site Biologica I half-life, h Volume of distribution, MPV" Hemostasis concentratio 0/ n, % Per population Inheritanc d e pattern Chromoso me... 

Discrimination between exposed and unexposed areas in this process requires the selection of thia zolidine compounds that do not readily undergo alkaline hydrolysis in the absence of silver ions. In a study of model compounds, the rates of hydrolysis of model /V-methyl thia zolidine and A/-octadecyl thiazolidine compounds were compared (47). An alkaline hydrolysis half-life of 33 min was reported for the /V-methyl compound, a half-life of 5525 min (3.8 days) was reported for the corresponding V/-octadecyl compound. Other factors affecting the kinetics include the particular silver ligand chosen and its concentration (48). Polaroid Spectra film introduced silver-assisted thiazolidine cleavage to produce the yellow dye image (49), a system subsequentiy used in 600 Plus and Polacolor Pro 100 films. 

Viscosity Increase. The flocculation rate of an emulsion is iaversely proportional to the viscosity of the continuous phase and an iacrease of the viscosity from 1 mPa-s (=cP) (water at room temperature) to a value of 10 Pa-s (100 P) (waxy Hquid) reduces the flocculation rate by a factor of 10,000. Such a change would give a half-life of an unprotected emulsion of a few hours, which is of Httle practical use. 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

Typical precautions with trichloroethylene are summarized in Table 5.52. An important factor is that the vapours are much heavier than air they will therefore spread and may accumulate at low levels, particularly in undisturbed areas. Because of its volatility, releases to the environment usually reach the atmosphere. Here it reacts with hydroxyl or other radicals (estimated half-life for reaction with hydroxyl radicals is less than a week) and is not therefore expected to diffuse to the stratosphere to any significant extent. There is some evidence for both aerobic and anaerobic biodegradation of trichloroethylene. 

Ta 1.5 X 10 2, K3 2.1 X 10 and 2.4 x and the corresponding negative logarithms are pA" 1.0, pA"2 1.8, pA"3 6.57 and pA"4 9.62. The P—O—P linkage is kinetically stable towards hydrolysis in dilute neutral solutions at room temperature and the reaction half-life can be of the order of years. Such hydrolytic breakdown of polyphosphate is of considerable importance in certain biological systems and has been much studied. Some factors which affect the rate of degradation of polyphosphates are shown in Table 12.10. 

Coagulation factors are glycoproteins named by roman numbers (the numbers being ascribed at the time of the components definition, not sequence of activation) (Table 1). Besides von Willebrand factor (vWF), the coagulation factors are synthesized in the liver. They have very different half-lifes and different concentrations in the plasma. Several coagulation factors are stored in platelets and endothelial cells and can be released during activation of these cells, which can result in a much higher local concentration of the respective factor (e.g., vWF). 

Endosulfan does not bioaccumulate to high concentrations in terrestrial or aquatic ecosystems. In aquatic ecosystems, residue levels in fish generally peak within 7 days to 2 weeks of continuous exposure to endosulfan. Maximum bioconcentration factors (BCFs) are usually less than 3,000, and residues are eliminated within 2 weeks of transfer to clean water (NRCC 1975). A maximum BCE of 600 was reported for a-endosulfan in mussel tissue (Ernst 1977). In a similar study, endosulfan, isomers not specified, had a measured BCE of 22.5 in mussel tissue (Roberts 1972). Tissue concentrations of a-endosulfan fell rapidly upon transfer of the organisms to fresh seawater for example, a depuration half-life of 34 hours (Ernst 1977). Higher BCFs were reported for whole-body and edible tissues of striped mullet (maximum BCF=2,755) after 28 days of exposure to endosulfan in seawater (Schimmel et al. 1977). However, tissue concentrations decreased to undetectable levels 48 hours after the organisms were transferred to uncontaminated seawater. Similarly, a BCE of 2,650 was obtained for zebra fish exposed to 0.3 pg/L of endosulfan for 21 days in a flow-through aquarium (Toledo and Jonsson 1992). It was noted that endosulfan depuration by fish was rapid, with approximately 81% total endosulfan eliminated within 120 hours when the fish were placed in a tank of water containing no endosulfan. 

Table II gives published ( ) half-life data for the two initiators along with values calculated from the optimized values of Yl and Y2. In each case, solvent C data were used to calculate the base activation energies and frequency factors, and the equality of half-life values at Tb illustrates the anchoring of the rate constant for each initiator. Except for initiator 1 at the low temperature, the differences between the optimized and published values are within the range of the differences reported for differing solvents.

In this paper we formulated and solved the time optimal problem for a batch reactor in its final stage for isothermal and nonisothermal policies. The effect of initiator concentration, initiator half-life and activation energy on optimum temperature and optimum time was studied. It was shown that the optimum isothermal policy was influenced by two factors the equilibrium monomer concentration, and the dead end polymerization caused by the depletion of the initiator. When values determine optimum temperature, a faster initiator or higher initiator concentration should be used to reduce reaction time. 

The rpoH gene is expressed from four different promoters. Under normal physiological conditions, the PI promoter is responsible for most of the rpoH transcription, while P2 and P4 promoters contribute varying minor amounts. The P3 promoter is under the control of the Eo holoenzyme (see below) and becomes induced at temperatures above 45 °C. The rpoH gene is expressed at all temperatures, and after a heat shock its transcription is increased by a factor of 1.5 only, but there is a large transient increase in intracellular levels. Two factors contribute significantly to this increase an enhanced rate of translation of the rpoH mRNA, and a transient stabilization in the half-life of... 

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