[Specific modification of free lysine amino groups of histidine decarboxylase from Micrococcus sp.  n. by trinitrobenzene sulfonic acid]. 
It has been found that 14 lysine residues are accessible for trinitrobenzene sulfonic acid (TNBS) in the molecule of histidine decarboxylase (HDC).  The other 62 lysine residues in the molecule of native HDC are masked and inaccessible for TNBS.  It is demonstrated that the SH- and alpha-amino groups of methionine are not modified by TNBS.  A correlation between the decarboxylase activity of the enzyme and the degree of its trinitrophenylation has been studied.  HDC, whose molecule contains 3--9 TNP groups, retains up to 90--97% of its initial activity.  Trinitrophenylation of 14 lysine residues induces inactivation of HDC by 33--34%, which probably depends on conformational changes or steric hindrances, occurring in the catalytic site of the modified active centre of HDC.  Using circular dichroism and fluorescence methods as well as disc-electrophoresis in polyacrylamide gel, it has been shown that trinitrophenylation does not cause any significant changes in the enzyme structure.  The TNP groups have been found to be localized in the large and small subunits of the HDC molecule.