Radioactivity and Radiotracers
Graham Lappin, Simon Temple in Radiotracers in Drug Development, 2006
The other method of introducing a radiotracer into a compound is by addition to the molecule. In this case, unlike substitution, the compound is structurally altered. A common example is where therapeutic proteins are under study (1.4). Proteins are obtained from biological sources and since they are not chemically synthesized, substituting a stable isotope with a radioisotope other than 3H is very difficult. The alternative is therefore to iodinate the protein with 125I, or sometimes with 131I, although a range of isotopes have been used from Bromine to Samarium.3 It may be necessary under such circumstances to demonstrate that the iodinated protein is an adequate surrogate for the intended therapeutic product. This is typically undertaken with a protein-receptor binding assay. Historically this method is known as non-isotopic labeling. The terms isotopic and non-isotopic labeling have however, become less common in recent years. Indeed the terminology has become confused as non-isotopic labeling is sometimes used for fluorescent labels, or other methods where isotopes are not relevant.
Formation, Metabolism, and Properties of Pyrrolic Compounds Appearing in the Gut
Karel P. M. Heirwegh, Stanley B. Brown in Bilirubin, 1982
In the healthy adult a major source of bile pigment is the breakdown of hemoglobin derived from senescent red blood cells which normally have a lifespan of about 120 days. Two other sources that have been identified are ineffective erythropoesis and catabolism of hepatic hemes. Most of this information has been derived from isotopic labeling experiments (see Chapter 1, Volume II). An early product of the fission of the heme ring is biliverdin which is rapidly converted to bilirubin by tissue biliverdin reductase. The bilirubin, formed primarily in the liver and the reticuloendothelial system, consists almost entirely of the IXα isomer, and any not formed in the liver itself is bound to albumin for transport to the liver via the systemic circulation (see Chapter 2, Volume II). In the liver this unconjugated bilirubin is conjugated with polar molecules in which form it is secreted into the bile as a labile, water soluble pigment (see Chapter 3, Volume II).
Isotopic Labeling With Carbon-14 and Tritium
Lelio G. Colombetti in Principles of Radiopharmacology, 2019
In isotopic labeling, a compound is labeled with an isotope of an element already present in the compound so that it is identical (apart from the isotope) with the normal unlabeled form. It should be clearly distinguished from nonisotopic, sometimes referred to as nonnuclidic, labeling in which a compound is labeled with an isotope of a “foreign” element not normally present in the unlabeled substance. Examples of the latter type are the numerous peptides and proteins labeled with radioactive iodine isotopes.
Use of proteomics to detect sex-related differences in effects of toxicants: implications for using proteomics in toxicology
Published in Critical Reviews in Toxicology, 2018
Ivonne M. C. M. Rietjens, Jacques Vervoort, Anna Maslowska-Górnicz, Nico Van den Brink, Karsten Beekmann
Table 1 reveals that several approaches have been used to measure differences in protein abundance upon exposure to toxicants. In a number of studies 2D-differential Gel Electrophoresis (2-DIGE or 2-DE) has been applied to the protein mixtures. This 2-DIGE/2-DE technique separates the proteins based on molecular charge in the first dimension (using isoelectro focusing methods) and based on mass in the second dimension using SDS-PAGE principles (Chandramouli and Qian 2009). The 2-DE method for protein identification is a useful method as differences in abundant proteins can be visualized and using specific software tools differences in protein spots can be observed with subsequent identification of the spots using either MALDI-TOF (or MALDI-TOF/TOF) or LC-ESI-MS/MS. It is important to stress that quantification from optical density measurements has to be handled with care because spots or peaks in LC chromatograms may contain more than one protein, hence the optical density or peak intensity may represent a mixture of proteins (Thiede et al. 2013). A solution to this problem is to apply isotopic labeling (Thiede et al. 2013). Then relative quantification can be performed by MS directly with peptides of the interesting protein species and protein species abundance ratios can be determined between different biological situations.
Dietary vitamin K is remodeled by gut microbiota and influences community composition
Published in Gut Microbes, 2021
Jessie L. Ellis, J. Philip Karl, Angela M. Oliverio, Xueyan Fu, Jason W. Soares, Benjamin E. Wolfe, Christopher J. Hernandez, Joel B. Mason, Sarah L. Booth
In Study 2 (in which all diets were vitamin K sufficient), the cecal microbial community composition at sacrifice was significantly different by sex (PERMANOVA r2 = 0.38 and P < .001, Supplemental Figure 3a), but did not differ by diet in either female or male mice (Supplemental Figure 3b and 3c). No differences were observed in composition between unlabeled (control) and stable-isotope labeled diets that would suggest an influence of the stable isotope labeling, though this cannot be entirely ruled out. Shannon diversity was again significantly greater in female mice as compared to males (r2 = 0.49, P < .001), but did not statistically differ by diet group (data not shown).
Proteomic analysis of the cardiac extracellular matrix: clinical research applications
Published in Expert Review of Proteomics, 2018
Merry L. Lindsey, Mira Jung, Michael E. Hall, Kristine Y. DeLeon-Pennell
Isotopic labeling. Another approach for MS quantification involves labeling the samples with isobaric tags, such as those used for relative and absolute quantitation (iTRAQ) or tandem mass tag (TMT) [51]. This allows the mass spectrometer to distinguish between identical proteins in separate samples [52]. While multiplexing can be costly and sample processing time consuming, overall this technique reduces running time and is less affected by experimental bias than label-free quantification. In addition, labeling is introduced after peptide digestion, and therefore, isobaric tags cannot evaluate in vivo or in vitro protein changes but only for quantitative comparisons using tissue samples [11].
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