Effect of pressure on the deuterium exchange reaction of α-lactalbumin and β-lactoglobulin

Abstract

The effect of pressure on the deuterium exchange reaction of α-lactalbumin (LA) and β-lactoglobulin (LG) was investigated to determine the structural change in these proteins induced by elevated pressure. LG, one of the main components of milk whey, has been degraded selectively from other milk proteins including LA by protease treatment under high pressure (Hayashi, R., Kawamura, Y. and Kunugi, S. J. Food Sci. 1987; 52: 1107–1108). This was considered to occur because LG lost its native structure under high pressure more remarkably than LA. In the present study, the H/D exchange reaction was carried out under high pressure and the resulting structures were analysed by Fourier-transform infra-red (FTIR) and nuclear magnetic resonance (NMR) spectroscopy, after the release of elevated pressure. The wavenumber of amide I bands in the FTIR spectrum assigned to α-helix and β-sheet structures of the proteins, shifted to lower regions as the H/D exchange of protons proceeded. The integral band area of the amide proton signal in low-field regions of the NMR spectrum is related to the H/D exchange of less stable protons in the protein. H/D exchanges for LA at 200 MPa and LG at 50 MPa were detectable by NMR as a decrease in the amide proton signals, but they were detected less unambiguously by FTIR. This apparent difference may be explained by reference to an intermediary unfolding stage of the protein that is generated under moderately high pressure.