Several rapid coupling diazonium salts (fast garnet GBC, fast red B and fast blue R) have been shown to cause the retention by fixation in tissue sections of the buffer-extractable portion of a particle-bound aminopeptidase catalyzing the hydrolysis of l-leucyl β-naphthylamide. The enzyme retained by these diazonium salts is identical to a cobalt-activated aminopeptidase which remains bound to tissues after buffer washing, but is distinct from a soluble sulfhydryl-dependent aminoptipedase extracted from tissues during incubation. No one of the three diazonium salts tested at equivalent coupling concentrations produced more retention of enzyme in tissue section than another, while the surfactants and salts present in commercial garnet GBC had no discernible effect in preventing enzyme extraction.
