Matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) coupled with stable isotope standards (SIS) has been used to quantify native peptides. This peptide quantification by MALDI-TOF approach has difficulties quantifying samples containing peptides with ion currents in overlapping spectra. In these overlapping spectra the currents sum together, which modify the peak heights and make normal SIS estimation problematic. An approach using Gaussian mixtures based on known physical constants to model the isotopic cluster of a known compound is proposed here. The characteristics of this approach are examined for single and overlapping compounds. The approach is compared to two commonly used SIS quantification methods for single compound, namely Peak Intensity method and Riemann sum area under the curve (AUC) method. For studying the characteristics of the Gaussian mixture method, Angiotensin II, Angiotensin-2-10, and Angiotenisn-1-9 and their associated SIS peptides were used. The findings suggest, Gaussian mixture method has similar characteristics as the two methods compared for estimating the quantity of isolated isotopic clusters for single compounds. All three methods were tested using MALDI-TOF mass spectra collected for peptides of the renin-angiotensin system. The Gaussian mixture method accurately estimated the native to labeled ratio of several isolated angiotensin peptides (5.2% error in ratio estimation) with similar estimation errors to those calculated using peak intensity and Riemann sum AUC methods (5.9% and 7.7%, respectively). For overlapping angiotensin peptides, (where the other two methods are not applicable) the estimation error of the Gaussian mixture was 6.8%, which is within the acceptable range. In summary, for single compounds the Gaussian mixture method is equivalent or marginally superior compared to the existing methods of peptide quantification and is capable of quantifying overlapping (convolved) peptides within the acceptable margin of error.
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机译：基质辅助激光解吸/电离飞行时间（MALDI-TOF）与稳定同位素标准品（SIS）结合已用于定量天然肽。这种通过MALDI-TOF方法进行的肽定量分析很难用离子流在重叠光谱中定量分析包含肽的样品。在这些重叠的光谱中，电流加总在一起，从而改变了峰高，并使正常的SIS估计成为问题。在此提出一种使用基于已知物理常数的高斯混合物对已知化合物的同位素簇进行建模的方法。对于单个和重叠的化合物，将检查此方法的特性。将该方法与两种常用的单一化合物的SIS定量方法进行了比较，即峰强度方法和曲线下黎曼和面积（AUC）方法。为了研究高斯混合法的特性，使用了血管紧张素II，血管紧张素-2-10和血管紧张素-1-9及其相关的SIS肽。研究结果表明，高斯混合法与两种方法相比具有相似的特性，可用来估计单个化合物的孤立同位素簇的数量。使用收集的肾素-血管紧张素系统肽的MALDI-TOF质谱测试了这三种方法。高斯混合法准确估计了几种分离的血管紧张素肽的天然/标记比率（比率估计误差为5.2％），其估计误差与使用峰强度和Riemann sum AUC方法计算的估计误差相似（分别为5.9％和7.7％）。对于重叠的血管紧张素肽（在其他两种方法均不适用的情况下），高斯混合物的估计误差为6.8％，在可接受的范围内。总而言之，对于单一化合物，高斯混合法与现有的肽定量方法相比具有同等优势或略胜一筹，并且能够在可接受的误差范围内定量重叠（卷积）的肽。
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