Samples. 190 plasma samples were collected from breast cancer patients during 2016–2018. These samples were used for establishment of quantitative model of 20 plasma amino acids. The breast cancer patients were recruited from the breast surgeons of affiliated hospital of Guizhou medical university. 80 samples were assigned to the calibration set, 30 samples constituted to the validation set, while the remaining 80 samples were assigned to test. At the same time, 30 plasma samples were collected from benign breast disease and 70 plasma samples were collected from healthy females. All samples were stored at -80◦C until analysis. All patients provided written informed consent, and ethical consent was granted from the Committees for Ethical Review of Research involving the Affiliated Hospital of Guizhou Medical University (Guizhou, China). All procedures were performed in compliance with the Declaration of Helsinki.
Reagents. All reagents used in this study were of analytical reagent grade. 20 amino acids were purchased from Sigma-Aldrich (America), and the concentration for each amino acid was 1000 µg/mL. The 20 amino acids used as standards were aspartic acid (Asp), glutamic acid (Glu), serine (Ser), Asparagine Asn (Asn), glycine(Gly), Glutamine Gln(Gln), Histidine(His), threonine (Thr), alanine (Ala), arginine (Arg), proline (Pro), tyrosine (Tyr), valine(Val), methionine (Met), cysteine(Cys), isoleucine (Ile), leucine (Leu), phenylalanine (Phe), tryptophan (Trp), lysine (Lys).
NIRS analysis. Near-infrared(NIR)spectroscopy together with chemometrics has gained wide acceptance in the fields of food industry and agriculture in recent years, mainly because it is a low-cost, nondestructive method and generally requires minimal sample processing prior to analysis[32].NIR spectroscopy can record the response of chemical bonds in functional groups to the NIR spectrum, which is related to the primary structural components of organic molecules. Quan-titative NIR spectroscopy measurement is based on the correlation between sample composition, as determined by reference methods, and the absorption of NIR radiation by bonds between light atoms at different wavelengths in the NIR region[33].However, the absorption peaks of NIR spectra are broad and overlap, and it is impossible to make direct quantification analysis due to the high dimension and complexity of NIR spectral data. Chemometrics methods, such as principal component analysis(PCA), principal component regression (PCR), and partial least-squares regression (PLSR), are often used to extract spectral features and investigate the correlation between the spectra and component concentrations.Before NIR spectra acquisition, all of the samples were stored in the laboratory at -80℃.All plasma samples were scanned using Antaris II Fourier transform near infrared spectrometer (Thermo Nicolet, USA). Spectra were collected using OMNIC software (Thermo Electron Corp.) and saved in absorbance format. Each sample was fitted in a 1 mm diameter cup that rotated during NIRS scanning. Spectra were the sum of 64 co-added scans across the spectral range of 4000-11,000 cm− 1 with a spectral resolution of 8 cm− 1. Water background was taken each hour.
Calibration models were developed using PLSR, which is the most commonly used multivariate method for the evaluation of NIR spectra.
PLSR analysis was conducted using the spectra from the calibration dataset to develop an empirical equation for predicting the concentrations of total amino acids.
Here, Partial least squares (PLS) models with 1–15 factors were investigated, and the optimum number of factors used in PLSR was determined by the lowest value of the predicted residual error sum of squares (PRESS) to avoid overfitting. According to Williams, a calibration model should contain at least 100 samples, and so, due to our rather small sample set, leave-one-out cross-validation was employed to evaluate the established models[34]. Cross-validation has the advantage that all of the data available can be used to determine the calibration model,because no sample has to be held back in a separate validation set [35] Several studies, including that of Moron and Cozzolino [36], have shown that both procedures provided similar results. The performance of the calibration is assessed by the correlation coefficient in calibration (rcal),root-mean-square error of calibration (RMSEC) and root-mean-square error of cross-validation(RMSECV) [37].In addition, to evaluate the prediction ability of the calibration model, the performance of the validation is assessed by the correlation coefficient (rval) and root-mean-square error of prediction (RMSEP).To develop NIRS calibration models for prediction of amino acids, 80 samples was split up into a calibration set and 30 for a validation set. Three scans were conducted on each sample and the data were averaged before analysis.
Chemometrics and data analysis. In this study, spectra were exported from OMNIC software to TQ analyst software (version 9.7,Thermo Electron Corp.) for spectral pretreatment and chemometrics analysis.Two tailed Student’s t-test was used for comparisons of two independent groups. A P value < 0.05 was considered to indicate statistical significance. Canonical Correlation Analysis (CCA) was used to find the correlation between plasma amino acid metabolism patterns and clinical parameters in breast cancer patients.