ABSTRACT 2,3-Butanedione (biacetyl), 2,3-pentanedione, methylglyoxal, phenylglyoxal, 1,2-cyclohexanedione and related compounds have been used as specific arginyl probes in protein and peptide chemistry. Modification procedures based on the reactions of the above compounds have led to the clarification of the role of arginyl residues in numerous protein functions, including binding of substrate, inhibitor or coenzyme, catalytic activity, energy conservation, transport, binding of proteins to membranes, and other functions. Depending on irradiation conditions, these and other related carbonyl compounds may, however, cause extensive changes in the structure of an enzyme regardless of the essentiality or unessentiality of arginyl residues of enzyme activity. Particularly 2,3-butanedione, 2,3-pentanedione and 1-phenyl-1,2-propanedione are powerful photoinactivators of enzymes in the visible region. This process results in the practically total, time-dependent loss of tryptophan fluorescence of the enzyme, accompanied by other irreversible structural changes. Some carbonyl compounds photoinactive enzymes also in the UV light; methylglyoxal, phenylglyoxal and 1,2-cyclohexanedione effectively inactivate enzymes at 254 nm and in the far UV region. These photoinactivations can be prevented by carrying out the reactions in anoxic media or adding singlet oxygen quenchers to the reaction mixture. These photochemical interferences have been known for at least twenty years. However, several scientists are still unaware of the complex photochemical consequences and the possible, serious misinterpretations of results when using these compounds for probing of arginyl residues under photo-oxidative conditions. It is obvious that a large number of studies which have not considered these photosensitised reactions, should be repeated considering these light-induced effects. On the other hand, such photochemical inactivations of enzymes can also be used to induce wanted changes in protein structure. Photosensitised reactions can take place also when using modifiers other than diketones and ketone aldehydes. It is advisable to carry out chemical modifications in the dark unless there is unequivocal evidence that a photosensitised reaction is not involved.
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