Photoaffinity Labeling of Human Lysosomal β-Hexosaminidase B

The carbene precursor 3-azi-1-[([6-3H]-2-acetamido-2-deoxy-1-β-D-galactopyranosyl)thio]-butane (also designated [3H]-1-ATB-GalNAc) has been used as a photoaffinity label for human lysosomal β-hexosaminidase B (Hex B, EC 3.2.1.52) purified to apparent homogeneity from postmortal liver. [3H]-1-ATB-GalNAc behaved as an active site-directed inhibitor, which bound covalently to Hex B upon photolysis at 350 nm and resulted in 15% inactivation of enzyme activity. Up to 75% of the inactivation of Hex B was prevented by including the competitive inhibitor 2-acetamido-2-deoxy-D-glucono-1,5-lactone in the photoaffinity experiment. Incubation of [3H]-1-ATB-GalNAc with the enzyme followed by irradiation and subsequent separation of the three polypeptides composing the β-subunit led mainly to labeling of the βa-polypeptide. Subsequent proteolysis of βa with trypsin and separation of the resulting peptides by high pressure liquid chromatography yielded one prominently labeled peptide fraction. Edman degradation resulted in the sequence E339ISEVFPDQFIHLGGDEVEFK359. However, no modified amino acid was detected, indicating that the photoaffinity label was presumably bound to the peptide by a labile ester linkage. This was proven when the radiolabel was almost completely released from the peptide by treatment with aqueous ammonium hydroxide. Simultaneously, Glu-355 was converted into Gln-355, which is located within a region of Hex B that shows considerable homology with the α-subunit of human hexosaminidase A and other hexosaminidases from various species.