[Role of tryptophan in the enzymatic activity of histidine decarboxylase from Micrococcus sp. n].

The effect of N-bromosuccinimide (BSI) on micrococcal histidine decarboxylase in 0.07 M phosphate buffer, pH 5.6 was studied. Data from spectral and amino acid analyses suggest that at 20-fold molar excess of BSI three of 12 tryptophane residues undergo selective modification, resulting in 80-85% loss of the enzyme activity. Using fluorescent method and polyacrylamide gel electrophoresis, it was shown that modification of these reactive tryptophane residues does not cause structural changes of the enzyme. Presumably tryptophane residue responsible for enzymatic activity are either located in the enzyme active site of close to it. At 40-50-fold molar excess of BSI 6 to 9 tryptophane and 2 to 3 cysteine residues are subjected to modification; the other 3 tryptophane residues are unaffected by BSI. These are probably located deep inside the histidine decarboxylase molecule. The maximum of the protein fluorescent spectrum during modification of the 40-50-fold molar excess of BSI is shifted towards higher wavelength values, thus suggesting conformational changes of the enzyme. It can be therefore assumed that the enzyme molecule contains at least 2 groups of structurally and catalytically essential tryptophane residues which significantly differ in reactivity. Some diazonium salts were shown to inhibit micrococcal histidine decarboxylase. The kinetics of the inhibiting effect of these compounds were investigated.