Pool sizes, estimating

Pool size estimated by isotope dilution formula, using the specific activity of the duodenal sample obtained 3 hr after injection of the labeled bile salt (3). 

The largest potential error in pool fire modeling is introduced by the estimate for siufacc emitted flux. Where predictive formulas arc used (especially Steflm-Boltzmann types) simple checks on ratios of radiated energy to overall combustion energy should be carried out. Pool size estimates arc important, and the potential for dikes or other containment to be overtopped by fluid momentum effects or by foaming should be considered. 

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. 

Another model, first introduced by Moore, et al. (2i), was used to examine the role of terrestrial vegetation and the global carbon cycle, but did not include an ocean component. This model depended on estimates of carbon pool size and rates of CO2 uptake and release. This model has been used to project the effect of forest clearing and land-use change on the global carbon cycle (22, 23, 24). 

In order to estimate the flux through the SMM cycle and to explore its function, a computer model of methionine metabolism in mature Arabidopsis rosette leaves was developed based on data from radiotracer experiments and on metabolite contents. This model suggested that the cycle serves to stop accumulation of AdoMet, rather than to prevent depletion of free methionine, as proposed by Mudd and Datko.54 Because plants lack the AdoMet feedbacks on MTHFR and AdoMet synthetase that regulate AdoMet pool size in other eucaryotes, the SMM cycle may be the main mechanism whereby plants achieve short-term control of AdoMet level. MMT knockouts of maize and Arabidopsis recently became available, and these can now be used to further investigate the role of the SMM cycle, and to test the predictions of the model. 

Pool Sizes Metabolic tracers can in principle be used to estimate the size of various body pools for trace elements It is somewhat analogous to making specific activity measurements in radiotracer experiments ... 

This is illustrated for the non-pregnant subjects in this example in Figure 4 The slope for the first 3 days corresponds to a pool size of 230 ug Se An interesting coincidence is that the plasma selenium concentrations and the estimated plasma volumes (very similar in these subjects) correspond to about 230 ug of circulating plasma selenium This may, of course, be coincidence, but since plasma and urine can exchange things in the kidneys one might speculate that there is perhaps more to it than coincidence ... 

FeigI, B. J., G. P. Sparling, D. J. Ross, and C.C. Ceni. 1995. Soil microbial biomass in Amazonian soils evaluation of methods and estimates of pool sizes. Soil Biology and Biochemistry 27 1467-1472. 

Kinetic parameters were estimated in nonsmokers and smokers to help elucidate the quantitative ascorbate metabolism in humans. This approach allows calculation of turnover rates at different levels of steady state intakes of ascorbate. Metabolic and renal turnovers were calculated separately. At plasma levels above about 0.7 mg/100 mL the renal elimination increased sharply and the metabolic turnover showed a saturation at a plasma level corresponding to a total turnover of about 60 mg/d. At the tested levels of intake of ascorbic acid the calculated total pool size increased to a level reached at a steady state plasma concentration achieved at an intake of about 90 mg/d. At intakes of this magnitude the absorption is substantially less than 100%. A daily intake of 100 mg of ascorbate for larger populations should be attained. Similar experiments with smokers showed an increase in the metabolic turnover corresponding to a demand of 140 mg/d to reach a similar stage. 

Transthyretin levels are often used as an indicator of protein status because of its relatively short half-life, a high tryptophan content, a high proportion of essential-to-nonessential amino acids, and small pool size. However, it is a negative APR. Levels fall in inflammation and malignancy and in cirrhosis of the liver and protein-losing diseases of the gut or kidneys. Therefore a sensitive acute phase reactant, such as CRP, should always be assayed along with TTR if levels are to be used to estimate nutritional status. History and physical examination are also important aspects of such evaluations. ... 

Fig. 2. (A) Fluorescence yield (in arbitrary units) for isolated, dark-adapted chloroplasts as a function of time in the absence (a) and presence (b) of DCMU. (B) Light-induced fluorescence-yield change in lyophilized chloroplasts (control), in hexane-methanol extracted chloroplasts, and in extracted chloroplasts reconstituted with PQ-9. Small upward arrows indicate weak, modulated green monitoring light turned on larger arrows for intense actinic light on (upward arrow) and off (downward arrow). (A) adapted from Zankel and Kok (1972) Estimation of pool size and kinetic constants. In A San Pietro (ed) Methods in Enzymology 24 222 (B) from Liu, Hoff, Gu, Li and Zhou (1991) The relationship between the structure of plastoquinone derivatives and their biological activity in photosystem II of spinach. Photosynthesis Res 30 100.

Stevens CF, Tsujimoto T (1995) Estimates for the pool size of releasable quanta at a single central synapse and for the time required to refill the pool. Proc Natl Acad Sci USA 92 846-849. 

The system 0.002 M NP-5/ 0.1 M AOT/ water / n-heptane thus contains smaller but also more numerous water pools according to sizes estimated by TEM observation of FF obtained through HPF. 

Using this technique, pool sizes of cholic acid and chenodeoxycholic acid have been estimated to be similar and around 1.0 to 1.5 g each in healthy subjects, with the total bile acid pool amounting to 2 to 4 g (H18, LIO, VIO). Cholic acid turnover is more rapid than for chenodeoxycholic acid, and the rate of hepatic synthesis of cholic acid (300 to 400 mg/day) is therefore approximately double that for chenodeoxycholic acid (150 to 200 mg/day) (H18, VIO). In the steady state, total bile acid synthesis by the liver should equal bile acid loss in the feces, which is around 400 mg/day. Some studies have found that estimates of bile acid synthesis by the isotope dilution technique give values that are higher than those obtained by direct chemical measurement of fecal bile salts (S45), but good agreement has recently been claimed between the two methods (DIO). ITie Lindstedt technique for measuring bile acid turnover and pool size has been modified so that only one bile sample need be collected after intravenous administration of the labeled bile acid. These modified methods measure either pool size alone (D9) or pool size and turnover if both and bile acid are administered at an interval of 24 hours apart (V6). 

Spermidine and spermine interact with DNA and also with tRNA. This binding is responsible in part for maintaining the conformation of these molecules (5). The interaction of polyamines with macromolecules, however, makes the estimation of intracellular bound and unbound pool sizes difficult since disruption of cells frequently destroys cell compartments, promoting reassociation of amines. 

While effective doses of 1 to 2 mg seem small, the total body pool of /3C can be estimated at 15 to 20 mg, with a total plasma pool of ateut 0.5 to 1 mg. In comparison with these pool sizes, a 2-mg effective dose is relatively large. In addition, the average daily effective dose of jSC from food sources is likely to be less than 1 mg, since matrix effects probably impair bioavailability of /3C to a larger extent than with supplements such as that used by van Vliet et al (1995). The minimum effective dose required to yield a significant TGR fraction response, taking into account the error associated with measurement, has not been determined, but is not likely to be much less than 1 mg. Consequently, use of unlabeled jSC, even coupled with use of the TGR fraction, is probably not well suited to study /3C uptake and metabolism at effective doses typically derived from dietary sources. 

An unanswered question in this type of protocol is the bioavailability of the ingested folate. If one assumes a fractional absorption of 0.67 for total ingested folate (i.e., labeled + dietary), this model yields estimates for the masses of pools 1 and 6 of 2.9S and 36.S mg, respectively. Thus, the bioavailability of ingested folate is a critically important factor in controlling the in vivo pool sizes and, thus, the nutritional status of the individual. 

Also, the isotope retention curves and estimates of the rate constants appear to be very sensitive to the dose. It is usually assumed, particularly in radioisotope studies, that the amount of tracer is much smaller than the pool size. Johansson et al. (1966a Johansson and Tiselius, 1973) used doses of 5 to 200 /xg of pyridoxine, which they acknowledged would constitute a significant fraction of the pool (Johansson et al, 1966a). This may partially explain why the rate constant(Ac3) for excretion from the small pool averages 0.60 when calculated from their data for the 20-jug dose compared with 2.3-4.1 based on the 200-/ug dose (Johansson and Tiselius, 1973). 

Equation (6) defines the fractional turnover rate( 2) for the large pool (B) in terms of the exponent (n) for the slow phase of the curve, the small pool (A), the large pool (B), and the intake (/). In the case of normal vitamin B6 intake, IS nmol/g times g body wt usually yields a reasonable estimate of the total pool. Since muscle usually contains 70-80% of the pool, muscle biopsies provide a means of verifying the pool size. The input and/or excretion can usually be measured. The exponent can be obtained by fitting a curve to the data. The size of pool A must be estimated. However, assuming that A is small relative to B and that n is small, variations in A have relatively little effect on the estimates of kz- Therefore, Eq. (6) provides a means for making k2 more consistent with physiological observations. 

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