Effects of urea and trimethylamine-N-oxide on enzyme activity and stability

The interactions of urea, trimethylamine-N-oxide (TMAO), and related solutes on a number of enzymes were examined. Urea inhibited enzymatic activity and accelerated the thermal inactivation of catalase, whereas TMAO activated some enzymes but inhibited others. The effects of urea and of TMAO, whether parallel or in opposition, were exerted independently. Thus, in those cases where TMAO increases enzymatic activity, it did so to the same relative degree, whether or not urea was present. TMAO markedly decreased the rate of thermal inactivation of catalase, indicating that it does favor compact protein structures. The assumption that TMAO factors compaction of protein structure, whereas urea has the contrary effect, does not lead to the expectation that TMAO must always oppose the effect of urea on enzymatic activity, since the most compact form of an enzyme may not always be the most active form.