Liquid scintillation counting of 14C- and 3H-labeled samples on solid supports: a general solution of the problem

The affinity of [3H]quinuclidinyl benzilate and some other radioligands for glass microfiber and cellulose filters in some common liquid scintillation solvents

The optimum conditions for measuring radioactivity in the filtration assay of muscarinic cholinoceptors with tritiated quinuclidinyl benzilate are to use Whatman GF/B filters and to add a simple toluene scintillant to them while they are still damp. Practically all the radioactive material is then slowly extracted into the scintillant and high counting efficiencies are obtained after 24 h. Dried filters, or dry filters in control experiments in the absence of receptor, adsorb much of the radioactivity with a 30% reduction in counting efficiency. Other scintillants were able to extract the radioactive material from dry filters, but were generally not preferable to toluene. The GF/B filters performed better than other glass microfiber and cellulose filters in terms of retention of receptor-bound ligand, rapid filtration rates, and low filter blanks. Toluene is unsuitable as a scintillant with GF/B filters for some other radioligands examined.

Simultaneous determination of total cholesterol concentration and radioactivity in plasma

Total plasma cholesterol concentration and radioactivity were measured simultaneously using a gas chromatograph equipped with a flame ionization detector and an effluent splitter. More than 99% of the recovered radioactivity was in the cholesterol peak. Specific activities were highly correlated with the amounts of labeled cholesterol present in plasma. The recovery of label was quantitative over a wide range of carrier cholesterol concentrations. The method is highly reproducible, accurate, rapid and specific.

Enzymes of phospholipid synthesis: axonal versus Schwann cell distribution

Using quantitative EM autoradiography to localize sites of incorporation of tritiated inositol and choline into mouse sciatic nerve, we observed a substantial axon-based phosphatidylinositol synthesis, but no axonal phosphatidylcholine synthesis. In the present communication we provide biochemical evidence for the axonal transport of CDP-diglyceride:inositol transferase (EC 2.7.8.11), the terminal enzyme in de novo phosphatidylinositol biosynthesis. Axonal transport of 1,2-diacyl-glycerol:CDP-choline choline phosphotransferase (EC 2.7.8.2), required for de novo phosphatidylcholine synthesis, was not apparent in these studies. During subcellular fractionation activities for the synthesis of phosphatidylinositol by inositol transferase (IT) and phosphatidylcholine by choline phosphotransferase (CPT) were recovered in crude microsomal fraction of rat sciatic nerve. However, CPT was much more highly enriched in the microsome fraction than IT, which may be an indication of the different subcellular localizations of these enzymes. Following ligation, we detected localized increases in the activities of both enzymes in 5 (and 3) mm segments taken immediately proximal and distal to the ligature. Both activities increased in a linear fashion in the proximal segments over the ensuing 72 h period. It took about 40 h (IT) and 56 h (CPT) for the activities in the segments proximal to the ligature to double compared to unligated contralateral (control) nerves. The time-dependent accumulation of IT was primarily due to axonal transport, while that of CPT was largely a result of increased enzyme activity in local Schwann cells. Evidence came from double ligation studies, where a proximal ligature, acting to restrict orthograde axonal transport, reduced accumulation in a distal ligature by 80% for IT, but only 28% for CPT. Conversely, blockage of the Schwann cell response with actinomycin D, reduced accumulation of CPT by 83% and IT by only 36%. Finally, light microscopic autoradiography was used to show that in the segment proximal to the ligature, tritiated inositol incorporation into lipid was primarily axonal, whereas that of tritiated choline remained primarily associated with Schwann cells.

Affinity chromatography of thymidine kinase from a rat colon adenocarcinoma

Thymidine kinase from a transplantable colon adenocarcinoma, induced by 1,2-dimethylhydrazine and maintained in CDF rats, was purified by affinity chromatography using thymidine-3'(4-aminophenylphosphate) coupled to carboxyhexyl-Sepharose. Most of the contaminating protein passed through the column; non-specifically adsorbed protein was washed from the column by 0.1 M KC1 in 0.01 M Tris-HC1, 7.5. Thymidine kinase was eluted with 0.1 mM thymidine, 0.1 M KC1 in 0.01 M Tris-HC1, pH 7.5. The purified enzyme accounted for about 26% of the applied activity; the specific activity of the purified material (peak fraction) was 3,500 moles TMP formed per mg protein per 10 min., a 1,800-fold purification of the applied extract. The preparation is free of nucleoside phosphotransferase, but contains other protein impurities. Purification was completed in less than 1 hour, making this a useful procedure for isolation of this unstable enzyme.