Journal
JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY
Volume 49, Issue 8, Pages 4011-4018Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jf001380a
Keywords
beta-lactoglobulin A; N-ethylmaleimide; heat-induced aggregation; intermolecular disulfide bridge; sulfhydryl-disulfide interchange reaction
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beta -Lactoglobulin A (beta LG A) modified with N-ethylmaleimide (NEM-beta LG A) was purified by ion exchange chromatography, and modification of beta LG A by NEM was confirmed by time of flight mass spectrometry and 5,5 ' -dithiobis(2-nitrobenzoic acid) methods. The fluorescent spectrum of NEM-beta LG A was slightly different from that of native beta LG A. NEM-beta LG A gave no polymerization after heating at 80 degreesC and pH 7.5, as shown by polyacrylamide gel electrophoresis. Conformational change of NEM-beta LG A was observed at 80 degreesC by ultraviolet differential spectra, whereas after cooling it recovered to its original state as before heating, indicating apparent reversible thermal denaturation. Native beta LG A is resistant to pepsin hydrolysis, whereas heated beta LG A was easily hydrolyzed by pepsin. NEM-beta LG A before heating was also resistant to pepsin hydrolysis, and after heating NEM-beta LG A was still resistant to pepsin hydrolysis. These results indicate that NEM-beta LG A maintained a conformation similar to its native form even after heating. Addition of 0.2 M NaCl to the beta LG A heated under salt-free condition induced polymerization of heated beta LG A molecules, but not that of heated NEM-beta LG A. This seemed to indicate that the formation of inter- or intramolecular disulfide linkage made the heat-induced conformational change of beta LG A irreversible.
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