Investigation of the active site of Escherichia coli beta-D-galactosidase by photoaffinity labelling.

3,7-Anhydro-2-azi-1,2-dideoxy-D-glycero-L-manno-(8-3H)octitol++ + (1a) and 3-azibutyl 1-thio-beta-D-(6-3H)galactopyranoside (2a) were synthesised from the unlabelled compounds by reaction with galactose oxidase, then reduction with sodium borotritide. Whereas 1a was an efficient photoaffinity reagent for the beta-D-galactosidase from E. coli, 2a was ineffective. Three 3H-labelled peptides were isolated after digestion of the labelled enzyme with trypsin, one of which was an octapeptide (Trp 158 to Ser 165), which is remote from the segments detected as part of the active site of the enzyme.