Proteolytic digestion of α-lactalbumin: Physiological implications

Publisher Website
Google Scholar
Abstract
The kinetics of the partial digestion of bovine α-lactalbumin (α-LA) by trypsin, α-chymotrypsin, and pepsin was monitored by lactose synthase activity, HPLC, and difference spectrophotometry. The relative stabilities of the various metal-bound states of α-LA to trypsin and chymotrypsin at 37 and 5°C decrease in the following order: Ca(II)-α-LA>Zn(II), Ca(II)-α-LA>apo-α-LA. The HPLC digestion patterns of Ca(II)-α-LA and Zn(II), Ca(II)-α-LA at 5 and 37°C were similar, while the corresponding digestion patterns for apo-α-LA were quite different, reflecting the existence of the thermally induced denaturation states of apo-α-LA within this temperature region. Occupation of the first Zn(II)-binding site in Ca(II)-loaded α-LA slightly alters the HPLC digestion patterns at both temperatures and accelerates the digestion at 37°C due to Zn(II)-induced shift of the thermal transition of α-LA, exposing some portion of thermally denatured protein. The results suggest that the binding of Zn(II) to the first Zn(II)- (or Cu(II))-specific site does not cause any drastic changes in the overall structure of α-LA. The acidic form of α-LA (atpH 2.2 and 37°C) was digested by pepsin at rates similar to that for the apo- or Cu(II), Ca(II)-loaded forms by trypsin or α-chymotrypsin at neutralpH. Complexation of α-LA with bis-ANS affords protection against pepsin cleavage. It is suggested that the protective effects of similar small lipophilic compounds to α-LA may have physiological significance (e.g., for nutritional transport).