Review: Glycation of human serum albumin

Abstract

Glycation involves the non-enzymatic addition of reducing sugars and/or their reactive degradation products to amine groups on proteins. This process is promoted by the presence of elevated blood glucose concentrations in diabetes and occurs with various proteins that include human serum albumin (HSA). This review examines work that has been conducted in the study and analysis of glycated HSA. The general structure and properties of HSA are discussed, along with the reactions that can lead to modification of this protein during glycation. The use of glycated HSA as a short-to-intermediate term marker for glycemic control in diabetes is examined, and approaches that have been utilized for measuring glycated HSA are summarized. Structural studies of glycated HSA are reviewed, as acquired for both in vivo and in vitro glycated HSA, along with data that have been obtained on the rate and thermodynamics of HSA glycation. In addition, this review considers various studies that have investigated the effects of glycation on the binding of HSA with drugs, fatty acids and other solutes and the potential clinical significance of these effects.

Keywords: Diabetes; Drug–protein binding; Glycated albumin; Glycation; Human serum albumin; Protein modification.

Figure 1

The process of glycation, which begins during early stage glycation (a) with the reaction of a reducing sugar such as D-glucose with a free amine group on a protein to form a reversible Schiff base, followed by the formation of a more stable Amadori product (i.e., fructosyl-lysine, or FL, in the case of glucose). The open chain glucose adducts can also convert to the closed forms that are shown above, which represent an N-substituted aldosylamine in the case of the Schiff base or an N-substituted 1-amino-1-deoxyketose in the case of the Amadori product [15]. These early stage glycation reactions can later be followed by additional events during advanced stage glycation (b) that lead to the formation of advanced glycation end-products (AGEs). More details on the structures of FL and common AGEs are provided in Figure 6.

Figure 2

Number of publications that have appeared over the last three decades that are related to the topic of glycation involving albumin or human serum albumin (HSA). These numbers were obtained on SciFinder in February 2013 using the key terms “glycated” or “glycosylated” and “albumin” or “HSA”.

Figure 3

The crystal structure of HSA. The locations of the main drug binding sites in this protein (i.e., Sudlow sites I and II) are shown, as well as the locations for several lysines that have often been reported to take part in glycation. This structure was generated using Protein Data Bank (PDB) file ID: 1AO6.

Figure 4

Use of a column and support containing a boronate ligand (e.g., phenylboronic acid) for the retention of glycated HSA. The active binding agent in this process is the tetrahedral boronate anion, which forms under alkaline conditions.

Figure 5

Number of reports from the literature (see Table 1) that have identified glycation-related modifications at the various lysine or arginine residues in HSA. The three regions of the bar graphs show the number of reports that have involved in vitro samples (bottom section, dark gray; up to 8 total papers), in vivo samples (middle section, light gray; up to 5 total papers) or plasma that was spiked with glucose and incubated under in vitro conditions (top section, intermediate gray; up to 1 paper).

Figure 6

Structures of fructosyl-lysine (FL) and examples of advanced glycation end-products (AGEs) that have been reported for glycated HSA (see summary in Table 1).

Figure 7

Change in (a) the total amount of glycation or (b) the amount of modification at specific residues as a function of time for in vitro glycated HSA. The data in both plots were acquired at pH 7.4 and 37° C for 0.63 mM HSA incubated with 15 mM glucose. In (b) the relative extent of modification is given by the value of the measured glycated/control index, as obtained for residues 1–10 (○), 521–531 (□), 189–208 (△), and 426–442 (◇) and containing potential modification sites such as the N-terminus, R521/K525, K199/K205 and R428/K439. Adapted with permission for Refs. [106,108].