Protein radioactive decay

Radioactive decay with emission of particles is a random process. It is impossible to predict with certainty when a radioactive event will occur. Therefore, a series of measurements made on a radioactive sample will result in a series of different count rates, but they will be centered around an average or mean value of counts per minute. Table 1.1 contains such a series of count rates obtained with a scintillation counter on a single radioactive sample. A similar table could be prepared for other biochemical measurements, including the rate of an enzyme-catalyzed reaction or the protein concentration of a solution as determined by the Bradford method. The arithmetic average or mean of the numbers is calculated by totaling all the experimental values observed for a sample (the counting rates, the velocity of the reaction, or protein concentration) and dividing the total by the number of times the measurement was made. The mean is defined by Equation 1.1. 

All cases of radioactive decay involve the atom s nucleus. Since these processes do not involve the atom s electrons, they occur regardless of the chemical environment of the nucleus. For example, radioactive hydrogen-3, or tritium, will decay by (1 particle emission whether it is contained in a water molecule or hydrogen gas, or in a complex protein. 

The radioactive decay follows first order kinetics with a half-life of the radionuclide. Theoretically, after one half-life, 50% of the labeled protein should remain intact in the stored sample. In practice, this is not the case. Some radioactive molecule of the protein change their identity and in this manner they are converted to impurities during the preparation of the radioactively labeled protein. Radiochemical conversions f H He, - N, - C1, 1 - Te, - Xe) cause destruc-... 

Molecules of a labeled protein may be also degraded during interactions of radiation emitted by adjacent molecules in the preparation. The interaction of a beta particle or a gamma quantum with a protein molecule produces various ionizations and even disruption of chemical bonds. The number of different fragments generated increases with die complexity of the original molecule, but the concentration of each remains negligible. The loss of the labeled compound due to this process is much lower than that due to the radioactive decay process. 

Transitions occur constantly in nature molecules change from one tautomeric form to another, radioactive nuclei decay to form other nuclei, acids dissociate, proteins alter their shapes, molecules undergo transitions between electronic states, chemicals react to form new species, and so forth. Transition rules allow the simulation of these changes. 

Similar conclusions were reached by Kao et al. (1961) who injected female Wistar rats, 5 weeks, 8 months, and 2 years old with uniformly labeled Ci -lysine. The animals were sacrificed at intervals up to 40 days after injection. Elastin and collagen were isolated from aortas, tendon, uterus, and skin and examined for radioactivity. The results showed that with the exception of the uterus, insoluble collagen and elastin were synthesized at a significantly higher rate by 5-weeks-old rats than at 8 months or 2 years, but at all ages the turnover rate relative to other proteins was low. In agreement with the results of Slack, the elastin of aorta did not decay in activity in any age group above 5 weeks old. 

Cesium, on the other hand, is toxic to plants in anything but trace amounts, whereas indications are that Cs+ ions impair the activity of potassium-binding sites in proteins. Excess cesium can be found in the air and in soils as a by-product of nuclear testing and spent nuclear fuels. Radioactive cesium 137, which results from the fission of uranium 235, decays by emission of a... 

Accelerator mass spectrometry (AMS) is an ultrasensitive analytical method for radioactivity analysis. AMS offers 10 -10 -fold increases in sensitivity over LSC or other decay counting methods so that levels as low as 0.0001 DPM can be detected (Brown et al., 2005, 2006). AMS has been applied to mass balance determination, pharmacokinetic studies of total radioactivity, and measurement of chemically modified DNA and proteins in humans after the administration of a low radioisotope dose (approximately lOnCi/person for mass balance and drug metabolism studies) (Buchholz et al., 1999 Garner, 2000 Garner et al., 2002 Liberman et al., 2004 White and Brown, 2004). In addition, off-line HPLC-AMS has been explored for metabolite profiling after... 

If an essential amino acid such as leucine is used to study protein turnover, the only sources are from dietary intake (I) or protein breakdown (B). Amino adds like leudne can be removed from the plasma, either for protein synthesis (S), oxidation (O), or incorporation into other metaboUc pathways after transamination/deamina-tion. In the case of leucine, this amino acid can also be completely oxidized in the human body to COj, allowing for the measurement of O as well. A simple model in the measurement of protein tumover/flux therefore takes into consideration the above factors so that flux (Q) is equal to I + B, which is in turn equal to S + O (i.e., Q = I + B = S-i- O). Isotope studies are used to derive these variables of protein turnover, since amino acid tracers can be used to measure Q and O, and if intake equals zero, then Q = B, and by subtraction, S = Q - Isotopes are molecules that share the same atomic number (protons), yet have different numbers of neutrons (atomic mass). Stable isotopes occur naturally and do not emit ionizing radiation, whereas radioactive isotopes undergo spontaneous decay. For these reasons, stable isotopes have become very popular in exercise research.Although useful, this model is very simplistic in that there are multiple amino acid pools mining over at very different rates. 

The isotopic label stands out well above natural levels, but decay counting is an intrinsically inefficient method of detection. No matter what type of counter is used, only fourteen C nuclei will decay per hour for every 10 that are present in the sample. In the case of tagged aldolase, the amount of incorporated radioactivity was too low for decay counting to be used for fragments of the tagged enzyme, and AMS presented the only method for analysing content after cleavage of the protein. 

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