Saturation activity

X. Liu, and L. Z. Benet. Contributions of saturable active secretion, passive transcellular, and paracellular diffusion to the overall transport of furosemide across adenocarcinoma (Caco-2) cells,/. Pharm. Sci. 2002,... 

Intestinal absorption of Ca is via two distinct mechanistic routes which involve the (i) transcellular pathway, a saturable active transfer process that is unidirectional (i.e., mucosal-to-serosal), and (ii) the paracellular... 

Thus, a large dose may be ineffectively distributed and remain at the site of administration as a depot. A large dose of a compound given orally, for instance, may not be all absorbed, depending on the rates of absorption and transit time within the gut. Saturable active absorption processes would be particularly prone to dose effects, which could result in unexpected dose-response relationships. 

Note that in the limit of infinitely long irradiation, e kt —> 0, and thus the activity present is no Ax 4>, which is termed the saturation activity. Note also that for very... 

Thus, the activity increases linearly with time. In general, we note that we achieve one-half the saturation activity after an irradiation of one half-life, three-fourths of the saturation activity after irradiating two half-lives, seven-eighths of the saturation activity after irradiating three half-lives, and so forth. Thus, it does not pay to make the irradiation longer than one to two half-lives. (This effect can be used to tune the length of the irradiation to maximize the yield of the product of interest relative to the other reaction products.)... 

Two thin 1-mg samples of dysprosium are irradiated and counted in a similar manner, except for the use of a Cd cover foil on one sample. A Cd ratio of 7 is measured, with the bare foil saturation activity of 1 x 104 dpm. Calculate the thermal neutron flux at the irradiation position in the reactor. 

Recall that the equilibrium activity, also called the saturation activity, occurs when the rate of production is equal to the rate of decay and requires that the sample be irradiated for more than three half-lives, or 22 s for 16N. Also, 1 mb am is 1 x 10 27cm2, thus,... 

If the hradiation is long enough to produce saturation activities in both the target and monitor foils, we have... 

The activity observed at 5.2 days is 61% of saturation activity. The value observed at the 5.2 day count time is divided by 0.61 to obtain value of the saturation activity. 

An interesting concept that must always be taken into account in cyclotron-produced radionuclides is the saturation activity characteristic of each target and each nuclear reaction. The saturation activity is the activity of the radionuclide in which the secular equilibrium is obtained between the activity produced in the target and the disintegration of the radioisotope. The activity produced at a target can be calculated by the equation... 

The influence of irradiation time on activation is illustrated in Fig. 8.8. For production of radionuclides, irradiation times of up to several half-lives are sufficient. About 99% of the saturation activity is obtained after irradiating for seven half-lives. 

Activities at constant neutron flux (saturation activity)... 

A 50.0-liier tank contains an air-carbon tetrachloride mixture at 1 atm absolute, 34°C, and 30.0% relative saturation. Activated carbon is placed in the tank to adsorb CCI4. The temperature of the tank contents is maintained at 34°C, and clean air is continuously supplied to the lank throughout the process to maintain the total pressure at 1.00 atm. The process may be shown schematically as follows ... 

Air-saturated water has a dissolved O2 concentration dependent on temperature, the Henry s law constant and ionic strength. In pure water at 0 °C, O2 saturation is 450 p,M at 25 °C, saturation falls to 270 p,M. Other solutes reduce O2 solubility, such that at normal seawater salinities, O2 saturation is reduced by —25%. Seawater is, of course, rarely at perfect O2 saturation. Active photosynthesis may locally increase O2 production rates, resulting in supersaturation of O2 and degassing to the atmosphere. Alternately, aerobic respiration below the sea surface can consume dissolved O2 and lead to severe 02-depletion or even anoxia. 

Fipire 4- 4 According to the concerted-symmetry model, an allosteric inhibitor binds preferendaliy to the T form. This causes the velocity curve to become more sigmoidal tvith a higher [S]gj. An allosteric activator mimics the substrate by binding preferentially to the R form. As a result, the velocity curve becomes less sigmoidal (hyperbolic at saturating activator) and fS]oi decreases. These observations can also be explained in terms of the sequential interaction model. 

Quite apart from the so-called blood-brain barrier (and where it is lacking, e.g. some parts of the pituitary, hypothalamus and brain stem, i.e. the chemoreceptor trigger zone), there are many other factors which govern equilibration of drug concentration between CSF and the parenchyma of the brain itself. The differential effects of P-glyco-protein saturable active transport can govern the CNS sequestration in a manner that is completely unrelated to relative lipophilicity or ambient drug concentration. 

Uptake and Metabolism. A saturable active transport system in the jejunum provides efficient uptake of the vitamin into the intestinal mucosa cell. Thiamine kinase in the intestinal mucosa cell transfers a pyrophosphate from the ATP to the propyl alcohol at po-... 

Dietary pantothenic acid is consumed as coenzyme A and the intermediates from coenzyme As biosynthesis (Fig. 8.39). These are hydrolyzed to free pantothenic acid. Absorption is by saturable, active transport. 

Saturated renal excretion For example, dicloxa-cillin demonstrates saturable active renal secretion showing decreased renal clearance with increased dose. 

The formation of vinyl chloride from acetylene and hydrogen chloride is catalyzed by activated carbon impregnated with mercuric and mercurous chlorides.54- 5S The catalyst is prepared by saturating activated carbon with mercury vapor at 200° C and then adding hydrogen chloride to form mercuric and mercurous chlorides. 

After a month these approach saturation activity, so that uranium older than this has four-fourteenths the activity of the uranium ore from which it came. The activity of uranium then remains constant for hundreds of years because of the long half-life of Th, the first daugihter of... 

When the half-lives of a fission product and of its decay precursors are short compared to the irradiation time (Fi/j < Tr), the fission-product nuclide reaches saturation prior to the end of the irradiation. Its saturation activity per unit of reactor power is a constant, so that... 

The saturation activity is conveniently expressed in curies per watt of thermal power, or... 

Practical irradiation periods for fuel in power reactors are in the range of about 1 to 4 years. Most of the fission-product nuclides reach saturation in this period. An example is 8.05-day I, which is formed in 2.93 percent of U fissions. Its saturation activity is 0.023 Ci/W. 

Assume that a reactor is operated for a very long time so that all fission products reach saturation activity. On the average there are three radioactive decays in each fission-product decay chain. Calculate the fission-product radioactivity at saturation in units of curies per watt of thermal power from fission. 

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