Biological half-life measurement

In mammals, the principal route of excretion of cesium is via the urine, and almost 80% of incorporated cesium is excreted by the kidneys. The biological half-life (measured using jjCs) is short, and depends heavily on the intensity of metabolism in the organism. In humans, the biological half-life is about 20 days for children, but up to 60 days for older adults (Burkhardt and Wirth, 1986). 

Superoxide has a chemical half-life measured in microseconds, but in even this short time serious damage can be caused to all types of biological macromolecules. Peroxidation of membrane lipids could cause haemolysis but the oxidation of ferrous (Fe2+) to ferric (Fe3+) iron in haemoglobin due to free radical action is a more immediate cause for concern within the red cell (Figure 5.17). 

Destruction of nitric oxide by superoxide in the buffers is more likely to account for the short half-life of nitric oxide in vitro. Superoxide dismutase (15-100 U/ml) substantially increased the apparent half-life of EDRF, strongly suggesting that superoxide contributes to the short biological half-life of nitric oxide. In the perfusion cascade bioassay system, the buffers are bubbled with 95% oxygen, contain 11 mM glucose as well as trace iron plus copper contamination and are incubated under the weak ultraviolet (UV) radiation of fluorescent lights. These are prime conditions for the autoxidation of glucose to form small amounts of superoxide in sufficient amounts to account for the short half-life of nitric oxide in nanomolar concentrations. The rate of reaction between superoxide and nitric oxide is 6.7 X 10 M sec L The shortest half-life of nitric oxide measured is approximately 6 sec. To achieve a half-life of 6 sec, the steady state concentration of superoxide would only need to be 17 pM, calculated as ln(2)/ (6 sec X 6.7 X 10 M" sec )-... 

Human chorionic gonadotropin is well absorbed after intramuscular administration and has a biologic half-life of 8 hours, compared with 30 minutes for LH. The difference may lie in the high sialic acid content of hCG compared with that of LH. It is apparently modified in the body prior to urinary excretion, because the half-life measured by immunoassay far exceeds that measured by bioassay. 

Heinemann, K. Vogt, K.J. (1980) Measurements of the deposition of iodine onto vegetation and of the biological half-life of iodine on vegetation. Health Physics, 39, 463-74. 

Measurements of activity in blood of seven subjects were continued for 14 d after inhalation and showed that lead was lost from blood with a biological half-life of 18.0 (s.e.) 0.9 d. Because of radioactive decay, it was not possible to continue measurements for a longer period. Rabinowitz et al. (1976) gave oral doses of the stable lead isotope 204Pb to four subjects daily for several months, and used mass spectrometry to measure 204Pb in blood. The decline in 204Pb in blood was found to be exponential over periods of about 100 d from the end of the period of... 

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