Glycation and phosphorylation of α-lactalbumin by dry heating: Effect on protein structure and physiological functions

Abstract 

α-Lactalbumin (α-LA) was glycated with maltopentaose (MP) through the Maillard reaction (MP-α-LA) and subsequently phosphorylated by dry heating in the presence of pyrophosphate to investigate its structure and physiological functions. Glycation occurred effectively, and the sugar content of α-LA increased by approximately 22.3% through the Maillard reaction. The phosphorylation of MP-α-LA was enhanced with an increase in the dry-heating time from 1 to 5 d, and the phosphorous content of MP-α-LA increased by approximately 1.01% by dry heating at pH 4.0 and 85°C for 5 d in the presence of pyrophosphate. The electrophoretic mobility of α-LA increased with an increase in the phosphorylation level. The circular dichroism spectra showed that the change in the secondary structure of the α-LA molecule by glycation and subsequent phosphorylation was slight. However, the Trp fluorescence intensity was increased by phosphorylation after glycation. In addition, the differential scanning calorimetry thermograms of α-LA showed that the denaturation temperature of MP-α-LA was decreased by phosphorylation. These results indicated that molten (partially unfolded) conformations of α-LA were formed by dry heating in the presence of pyrophosphate after glycation. The anti-α-LA antibody response was significantly reduced by glycation and subsequent phosphorylation. The suppressive effect of α-LA on the production of proinflammatory cytokines such as IL-6 and tumor necrosis factor-α from THP-1 cells after stimulation with lipopolysaccharide was significantly enhanced by glycation with MP and was further enhanced by phosphorylation after glycation. The Ca phosphate-solubilizing ability of α-LA was enhanced by phosphorylation. The apoptotic activity of α-LA was reduced by glycation and subsequent phosphorylation. These results suggest that phosphorylation by dry heating in the presence of pyrophosphate after glycation with MP through the Maillard reaction is a useful method for improvement of the physiological functions of α-LA.

Key words: α-lactalbumin, glycation, phosphorylation, physiological function