Tissue-turnover time measurement

POM at some earlier time in the summer. Exact measures of the fraction of POM in the zooplankton diet or, conversely, measures of tissue-turnover time in the zooplankton are possible only if the 15N pool in POM has come to equilibrium. Even though this equilibrium in the POM was not reached, it is clear that the growth and maintenance of these zooplankton depended... 

The measurement of thiamine levels in human tissue other than blood is limited to biopsy or autopsy samples. This limitation translates into high sample variability due to factors such as age and nutritional, disease and medication status. Another limitation associated with measurement of thiamine in human tissue is the biological instability of thiamine derivatives. Because the turnover time of TTP is approximately one hour, significant hydrolysis can occur during extended delays between death and sample retrieval, preparation, and storage (Bettendorff et al. 1996a). 

With Potter s development of experimental plasmacytoma in mice, it was established that the turnover of paraprotein (N2), or more simply the serum level (08), was directly related to the weight of solid soft-tissue plasmacytoma. In our laboratory an ascitic form of plasmacytoma has been studied, and using isotope dilution it has been possible to estimate the actual total number of plasmacytoma cells in a mouse. At the same time the serum level of paraprotein was measured and a simple correlation was shown (F2). To the best of my knowledge, this was the first time that the serum level of a tumor product had been directly related to the actually counted number of tumor cells. Incidentally it was noted that the paraprotein could be first detected in the serum, when a 23 g mouse had 3 million tumor cells. 

Proteins differ significantly in their turnover rates, which are measured in half-lives. (A half-life is the time required for 50% of a specified amount of a protein to be degraded.) Proteins that play structural roles typically have long half-lives. For example, some connective tissue proteins (e.g., the collagens) often have half-lives that are measured in years. In contrast, the half-lives of regulatory enzymes are typically measured in minutes. Several selected examples are listed in Table 15.1. 

Neurotoxicity. Cells and tissues in the nervous system may be less radiosensitive, due to a lack of cell turnover or cellular regeneration, than faster regenerating cells of the gastrointestinal tract or pulmonary epithelium. Consequently, neuronal impairment as a result of radon alpha emissions is not expected. Therefore, studies which specifically or directly measure either pathological or functional damage to the nervous system following exposure to radon do not appear to be necessary at this time. 

More recently we have measured the turnover rate of lAA in tomato shoots after different incubation times, as part of a study of lAA biosynthesis in that tissue. lAA was supplied to the surface of the youngest leaf, 1 cm or longer, of excised shoots from twenty, 4-week-old tomato plants, as a 10 /xl drop (2 1 1, ethanol propan-2-ol H2O) containing 3711 Bq pH]IAA (1.11 GBq//xmol). Uptake was allowed for 2 h, surface radioactivity was then removed by several washes with 50% aqueous propan-2-ol followed with water, and the plants equilibrated for 1 h. After 0,4,10 or 20 h incubation in continuous light, lAA was extracted, purified, derivatised to form the pentafluorobenzyl ester [10] and the specific activity measured by liquid scintillation counting and GC-ecd. The purity and identity of selected samples was confirmed by combined GC-MS. 

The tissue distribution and levels of RBP in normal and in retinol-deficient rats were measured in order to explore the role of different tissues in the metabolism of RBP (J. E. Smith et al., 1975). The tissues examined included liver, kidney, fat, muscle, brain, eye, salivary gland, thymus, lung, heart, intestine, spleen, adrenal, testes, thyroid, and red blood cells. The RBP levels were low or very low in tissues other than liver, kidney, and serum and varied from 12 p.g/g of tissue for normal spleen to an undectable level in red blood cells. Much of the RBP in the tissues with low levels was most likely due to residual serum in the samples. In general, except for liver, RBP levels were lower in tissues from retinol-deficient rats than in those fixim normal rats. In normal rats, the liver, kidney, and serum levels were 30 4 (mean SEM), 151 22, and 44 3 p.g/g, respectively. In retinol-deficient rats, the liver RBP level was about three times the normal level whereas the kidney and serum levels were about one-fifth the normal values. It was suggested that die levels of RBP in normal as compared to deficient liver, serum, and kidney appear to reflect the relative rates of RBP secretion and turnover (see later discussion). 

Similar studies designed to measure the turnover of cerebrosides of brain tissue have been carried out by a number of investigators. Davison et al. (1959) employed serine-3-to label brain cerebrosides and computed the turnover rate to be near 200 days. On the other hand Hajra and Rad in (1964) administered acetate-and measured the rate of loss of radioactivity from the fatty acid moieties of the cerebrosides. In the latter experiments the half-time was near 42 days. When the hexose of the cerebrosides is labeled by means of glucose-1- C or galactose-1- C, the rate of depletion of the isotope indicated a half-time of near 40 days (Burton, et al, 19 ). These data indicate that cerebrosides turn over slowly in brain, at a rate near 40 days, as measured by the hexose and fatty acid moieties. The data derived from the serine experiment undoubtedly indicates that the sphingosine pool in brain is small and that this compound is reused in the synthesis of the sphingolipids. 

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