Selective Sulfenylation of Tryptophan Residues in a-Lactalbumin of Bovine Milk*

Tryptophan residues of bovine cr-la&albumin, one of the two protein components of lactose synthetase, were selectively sulfenylated with Z-nitrophenylsulfenyl chloride. When the reaction was performed in 0.1 M acetic acid for 1 hour, a modified protein containing 1 .O nitrophenylsulfenyl tryptophan residue per a-la&albumin molecule was formed and isolated in 42% yield. Upon reduction, carboxymethylation, and trypsin digestion, this sulfenylated cu-la&albumin (NPS1-aLA) yielded two yellow peptides. Amino acid composition of these peptides indicated that in NPS1-CYLA, tryptophan residues 60 and 118 were equally sulfenylated to the extent of 0.5 nitrophenylsulfenyl tryptophan per cr-lactalbumin. NPS1-aLA cross-reacts with anti-a-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 a-lactalbumin.