The invention concerns the introduction of a coupling group to 6-O-Alkylguanine DNA alkyltransferase from a 6-O-Alkylguanine-coupling group deriv. that enables the immobilization of the enzyme for subsequent anal. The coupling group is bound covalently to the enzyme; the immobilization to the surface is either by covalent or non-covalent binding. The coupling group can be biotin; the 6-O-Alkylguanine DNA alkyltransferase-biotin is immobilized to streptavidin coated surface. Independently from the coupling group, the enzyme can be fluorescent or enzyme-labeled for anal. purposes. [on SciFinder (R)]