Kelleher J K, Masterson T M
Department of Physiology, George Washington University Medical Center, Washington, DC 20037.
Am J Physiol. 1992 Jan;262(1 Pt 1):E118-25. doi: 10.1152/ajpendo.1992.262.1.E118.
Important syntheses in living systems occur by condensation reactions of the type nA----1B (where n is the number of A molecules needed to synthesize 1 molecule of B). Quantitative relationships for estimating the rate of synthesis of B from radioactive and stable isotope tracers are compared. With radioisotope tracers, only a single quantity is detected, the amount of radioactivity in B. In contrast, isotopes of varying mass produce multiple mass isotopomers B that are detected using mass spectrometry. The analysis demonstrates that the rate of synthesis of B is identifiable from stable isotope data but not from radioisotope data. This results because the isotopomer distribution of B at any time after tracer addition is a function of only the multinomial distribution representing the synthesis of B from n molecules of A and two parameters representing the fractional fluxes of isotopically enriched molecules to the sampled compartment of B. The model considers the possibility that the sampled compartment of B may not reach isotopic steady state during the experiment. A graphical method for obtaining initial estimates of the two parameters is presented.
生命系统中的重要合成反应是通过nA→1B类型的缩合反应发生的（其中n是合成1分子B所需的A分子数量）。比较了用于从放射性和稳定同位素示踪剂估算B合成速率的定量关系。对于放射性同位素示踪剂，只能检测到一个量，即B中的放射性量。相比之下，不同质量的同位素会产生多个质量同位素异构体B，这些异构体通过质谱法进行检测。分析表明，B的合成速率可以从稳定同位素数据中确定，但不能从放射性同位素数据中确定。之所以如此，是因为在添加示踪剂后的任何时间，B的同位素异构体分布仅取决于代表从n个A分子合成B的多项分布以及代表同位素富集分子向B的采样隔室的分数通量的两个参数。该模型考虑了在实验过程中B的采样隔室可能未达到同位素稳态的可能性。提出了一种获取这两个参数初始估计值的图形方法。
