Nuclear magnetic resonance characterization of aromatic residues of alpha-lactalbumins. Laser photo chemically induced dynamic nuclear polarization nuclear magnetic resonance studies of surface exposure.

The alpha-lactalbumins are involved as modifier proteins in lactose biosynthesis. Lactalbumins from different mammalian species are cross-reactive with the galactosyltransferases from others because of their homologous primary and tertiary structure. We have studied the surface exposure of several alpha-lactalbumins (bovine, goat, human, guinea pig, and rabbit) by the laser photo-CIDNP technique, an NMR method which measures the access of a photoexcited flavin dye to surface exposed Tyr, Trp, and His residues. An exposed histidine-68 residue exists in the bovine and goat species but is missing in other species and is replaced with a His-10 in the guinea pig species. The only exposed tryptophan residue was Trp-104, which cross-relaxed to nearby Trp-60. Cross-polarization to the latter residue was proven by taking free induction decays after extremely short light pulses (0.1 s). Furthermore, this cross-polarized Trp-60 resonance was absent in the guinea pig species, which has a substitution at position 60. The chemical shifts of these Trp residues were extremely similar in all species but rabbit, which has substituted His for a Tyr at position 103, situated close to both Trp-104 and -60. Three of the four tyrosines in bovine, goat, human, and guinea pig alpha-lactalbumin were exposed (the latter species has an extra Tyr in its sequence which was apparently not accessible). The rabbit species contains only two Tyr residues, the one which was not accessible (Tyr-50) being common to all species.