Binding of N-Methyl Nicotinamide Chloride by Tryptophan Residues of α-Lactalbumin

Studies of the binding of N -methyl nicotinamide chloride by α-lactalbumin show that complex formation occurs with 1 of 4 tryptophan residues at pH 6 whereas two form complexes at pH 2 as a result of acid denaturation. Alkaline denaturation likewise leads to an increase in exposure of tryptophans, and three of the four groups form complexes at pH 11. From a consideration of previous studies and a recently proposed molecular model for α-lactalbumin, the residues involved in binding N -methyl nicotinamide chloride have been tentatively identified as tryptophan-118, tryptophan-104, and tryptophan-60.