Reductive Amination Combining Dimethylation for Quantitative Analysis of Early-Stage Glycated Proteins

Due to the critical role glycation plays in many serious pathological conditions, such as diabetes, it is of great significance to discover protein glycation at an early stage for precaution and prediction of the disease. Here, a method of reductive amination combining dimethylation (RAD) was developed for the quantification of early-stage glycated proteins. The quantitative analysis was first carried out by reducing the samples using NaBH₃CN or NaBD₃CN, resulting in a 1 Da mass shift and the stabilization of early-stage protein glycation. The two samples were then digested and isotopically dimethylated to achieve the mass shift of 4m + 3n (m represents the number of N-termini and Lys residues, and n represents the number of glycated sites) between light- and heavy-labeled glycated peptides for quantification. Consequently, the false positive result can be removed according to the different mass shifts of glycated peptides and non-glycated peptides. In quantification of glycated myoglobin, RAD showed good linearity (R² > 0.99) and reproducibility (CVs ≤ 1.6%) in 2 orders of magnitude (1:10–10:1). RAD was then applied to quantify the endogenous glycated proteins in the serum of diabetic patients, revealing significant differences in the glycation level between the patients with complicated retinal detachment and those without. In conclusion, RAD is an effective method for quantifying endogenous glycated proteins.