Radio-labelling dilution

A rapid semiautomated microdilution method for the microbiological assay of the chloroquine has been developed by Desgardins (26). Antimalarial activity of chloroquine may be studied against cultured Plasmodium falciparum, microplates are used to prepare serial dilution of the drug. Parasites obtained from continuous stock cultures are subcultured in the micro-plates for 42 h. Inhibition of uptake of a radio labeled nucleic acid precursor by parasites serves as the indicator of antimicrobial activity. 

Use of internal standards. Mann and Jaworski (31) reported that when the recovery of 1-TI C IAA is monitored during a sample purification procedure, considerable loss of IAA can be detected. Bandurski and Schulze (32) suggested the use of reverse isotope dilution to help quantify the actual loss of IAA during sample analysis. In this procedure, one adds a trace amount of radio-labeled compound which ideally is identical to the compound being monitored. High specific activity is required so that statistically significant amounts of isotope can be detected without having to add an excessive quantity (mass) of internal standard. The amount of internal standard must be less than the amount of PGS. One may then accurately determine the recovery efficiency of the internal standard and thus of the PGS (32). 

In isotope dilution analysis, a known amount of radio-labeled compound with known specific activity is spiked to a known amount of an unknown mixture containing the same compound made up of stable isotopes. Then, the components of the radiotracer-diluted samples are mixed thoroughly to form a homogeneous mixture. This istopically diluted mixture, with known levels of dilution, is then suitably treated to isolate a small amount of the desired constituent. The radioactive isotope content of the isolated portion is determined by measuring its specific activity. From the... 

Parental H-2 alleles are separated by slash. The designation of the recombinant haplotypes are indicated in parentheses t Mean percentage of radio-labelled GLT ligand bound by a 1 5 dilution of antisera obtained seven days after secondary... 

The amount of radio-labelled compound obtained at the end of the experiment may be so little that more cold or unlabelled final compound may have to be added to have enough to manipulate. In the studies of the biosynthesis of cholesterol (Chapter 7) unlabelled cholesterol for dilution is available in abundance. That is rarely so with insect substances, and a sample of the cold target compound may have to be synthesized too. 

In a study of the conversion of phenylalanine to mandelonitrile, benz-aldehyde and HCN, DL-phenylalanine (0.17 pmoles) labelled with C in the phenyl ring (specific activity 82 pCi mmoL ) was injected into each of 10 individual millipedes. The specific activity of the benzaldehyde isolated from all ten at the end of the experiment was 7.39 X 10 pCi mmoL. Allowance must be made for the dilution with endogenous precursors, because 2.3 pmoles of benzaldehyde was isolated while only 0.17 pmoles of phenylalanine was injected. Presuming that only the radioactivity in L-phenylalanine is used for incorporation, what was the %age conversion of radio-labelled phenylalanine to benzaldehyde No radioactivity was found in the HCN. 

In clinical chemistry the determination of stable elements by radiochemical methods offers no outstanding advantages over alternative methods, but the use of radioisotopes for determining organic compounds is developing rapidly. In isotope dilution methods (G6), a pure but radioactive form of the compound to be measured is mixed with the sample, a fraction is isolated, and its activity is determined. In radio-metric or derivative analysis (W14), a radioactive reagent is allowed to react with the analyte the labeled compound is separated and its activity is measured. The isotopes commonly used are C,... 

The turnover of monoamines has been estimated by measuring the rate of decline of specific activity of the amines after introducing small amounts of radio-isotopic amines into the endogenous pools. This technique was first used to measure the rate of turnover of adrenal medullary catecholamines after the administration of radioactive precursors . The problem of the relative impenetrability of the blood-brain barrier to peripherally circulating amine was circumvented by the development of techniques for the administration of radioactive NE into the lateral cerebral ventricle , which allows a direct labelling of brain catecholamines. The principle of this approach is to introduce a "pulse dose" of labelled NE into the endogenous stores of this amine in the brain, and to follow the rate at which the radioactive material is catabolized and becomes diluted with newly S3mthesized NE1 9,132. Similar studies with H -5-HT have also been reported 3. 

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