Stable isotope dilution mass spectrometry for membrane transporter quantitation

Limitations of deuterium-labeled internal standards for quantitative electrospray ionization mass spectrometry analysis of fatty acid metabolites

RATIONALE

The electrospray ionization mass spectrometry (ESI-MS) methodology often shows poor ionization reproducibility in the analysis of biological samples. Therefore, normalization of the measured peak intensities is essential. It is believed that quantitative data with high reproducibility can be obtained by adding a constant amount of an internal standard (IS) material labeled with stable isotopes to each sample, thus allowing the correction of the quantitative value of the target compound by that of the IS. We investigated whether the presence or absence of a labeled IS improves the accuracy of these quantitative values.

METHODS

Triple quadrupole MS coupled with liquid chromatography was used to analyze fatty acid metabolites in biological samples as target compounds. Two independent systems were used to provide a measure of reproducibility in two different laboratories.

RESULTS

Data having poor reproducibility in the raw peak areas were efficiently normalized using the IS, but, crucially, the IS method using stable isotopes was not always necessary. In some cases, the reproducibility was relatively good even without using the IS. In a contaminant matrix, the MS response behavior of the target compound and its stable isotope-labeled material was complicated. Since ion suppression by matrix contaminants was dependent on the concentration of the target compound, the added amounts of the ISs were also important, Furthermore, an equivalent normalization effect was obtained by using a pooled quality control sample as an external standard, thus obviating the need for labeled IS samples, which are often expensive and sometimes not commercially available.

CONCLUSIONS

Our results raise the question as to whether the quantitative method using stable-isotope-labeled ISs is always necessary and beneficial. However, the results obtained in this study cannot be generalized because only fatty acid metabolites were examined using ESI-MS and only a highly substituted deuterium-labeled IS was used.

Targeted proteomic quantitation of the absolute expression and turnover of cystic fibrosis transmembrane conductance regulator in the apical plasma membrane

Deficient chloride transport through cystic fibrosis (CF) transmembrane conductance regulator (CFTR) causes lethal complications in CF patients. CF is the most common autosomal recessive genetic disease, which is caused by mutations in the CFTR gene; thus, CFTR mutants can serve as primary targets for drugs to modulate and rescue the ion channel’s function. The first step of drug modulation is to increase the expression of CFTR in the apical plasma membrane (PM); thus, accurate measurement of CFTR in the PM is desired. This work reports a tandem enrichment strategy to prepare PM CFTR and uses a stable isotope labeled CFTR sample as the quantitation reference to measure the absolute amount of apical PM expression of CFTR in CFBE 41o- cells. It was found that CFBE 41o- cells expressing wild-type CFTR (wtCFTR), when cultured on plates, had 2.9 ng of the protein in the apical PM per million cells; this represented 10% of the total CFTR found in the cells. When these cells were polarized on filters, the apical PM expression of CFTR increased to 14%. Turnover of CFTR in the apical PM of baby hamster kidney cells overexpressing wtCFTR (BHK-wtCFTR) was also quantified by targeted proteomics based on multiple reaction monitoring mass spectrometry; wtCFTR had a half-life of 29.0 ± 2.5 h in the apical PM. This represents the first direct measurement of CFTR turnover using stable isotopes. The absolute quantitation and turnover measurements of CFTR in the apical PM can significantly facilitate understanding the disease mechanism of CF and thus the development of new disease-modifying drugs. Absolute CFTR quantitation allows for direct result comparisons among analyses, analysts, and laboratories and will greatly amplify the overall outcome of CF research and therapy.