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

H. Enomoto^*, Y. Hayashi^*, C. P. Li, S. Ohki, H. Ohtomo, M. Shiokawa and T. Aoki^,1

^* United Chair of Applied Resource Chemistry, Course of Bioresource Science for Processing, United Graduate School of Agricultural Sciences, Kagoshima University, Kagoshima 890-0065, Japan
Department of Food and Pharmacy Engineering, School of Chemistry Science and Technology, Yunnan University, Kunming 650091, China
Food Technology Research Institute, Division of Research and Development, Meiji Dairies Corporation, 540 Naruda, Odawara, Kanagawa 250-0862, Japan
Department of Biochemical Science and Technology, Faculty of Agriculture, Kagoshima University, Kagoshima 890-0065, Japan

¹ Corresponding author: aoki{at}chem.agri.kagoshima-u.ac.jp

-Lactalbumin (-LA) was glycated with maltopentaose (MP) through the Maillard reaction (MP--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 -LA increased by approximately 22.3% through the Maillard reaction. The phosphorylation of MP--LA was enhanced with an increase in the dry-heating time from 1 to 5 d, and the phosphorous content of MP--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 -LA increased with an increase in the phosphorylation level. The circular dichroism spectra showed that the change in the secondary structure of the -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 -LA showed that the denaturation temperature of MP--LA was decreased by phosphorylation. These results indicated that molten (partially unfolded) conformations of -LA were formed by dry heating in the presence of pyrophosphate after glycation. The anti--LA antibody response was significantly reduced by glycation and subsequent phosphorylation. The suppressive effect of -LA on the production of proinflammatory cytokines such as IL-6 and tumor necrosis factor- 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 -LA was enhanced by phosphorylation. The apoptotic activity of -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 -LA.

Key Words: -lactalbumin • glycation • phosphorylation • physiological function