How Much Free Arachidonate Is Produced in Cells

Measuring the concentration of free arachidonate in cells raises several technical problems. Methods based on radioisotope 

If quantity is a limiting factor in the analysis, as it is often the case, one may increase the sensitivity of detection by selecting from the mass spectrum of the fatty acid derivative one or two diagnostic ions of reasonable abundance and focus the mass spectrometer on those, a technique called selected ion monitoring (SIM). (For instance, one may choose the ion of m/z 175 from the spectrum of methylarachidonate shown in Fig. 3.2.) Alternatively, detection by flame ionization (FID) may be routinely used when quantities of the analyte are not limiting. 

Although arachidonate and other fatty acids may be analyzed by HPLC without derivatization (for example, by linking the HPLC column to a light-scattering detector), derivatives prepared with either UV-absorbing or fluorescent tags are often preferred because they can be detected with much greater sensitivity. The reader interested in these technical issues is referred to several comprehensive books that have recently been published. 

GC/MS coupled to SIM detection has been used by John Turk and coworkers (at the Washington University School of Medicine in Saint Louis) to quantify the accumulation of free fatty acids which occurs in pancreatic islets, when these are stimulated with concentrations of D-glucose that promote insulin release from P cells. Each unstimulated islet was found to contain approximately 0.5 pmole of free arachidonate, and this level was significantly 

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