Selective Sulfenylation of Tryptophan Residues in α-Lactalbumin of Bovine Milk

Tryptophan residues of bovine α-lactalbumin, one of the two protein components of lactose synthetase, were selectively sulfenylated with 2-nitrophenylsulfenyl chloride. When the reaction was performed in 0.1 m acetic acid for 1 hour, a modified protein containing 1.0 nitrophenylsulfenyl tryptophan residue per α-lactalbumin molecule was formed and isolated in 42% yield. Upon reduction, carboxymethylation, and trypsin digestion, this sulfenylated α-lactalbumin (NPS1-αLA) yielded two yellow peptides. Amino acid composition of these peptides indicated that in NPS1-αLA, tryptophan residues 60 and 118 were equally sulfenylated to the extent of 0.5 nitrophenylsulfenyl tryptophan per α-lactalbumin. NPS1-αLA cross-reacts with anti-α-lactalbumin antibodies, but, unlike the native protein, it has no biological activity in the lactose synthesis reaction, and its electrophoretic mobility on polyacrylamide gels is different from that of native α-lactalbumin.