Bile acids deactivate certain
enzymes, such as prolyl
endopeptidases (PEPs), which are investigated as candidates for
protease-based
therapy for
celiac sprue. Deactivation by
bile acids presents a problem for therapeutic
enzymes targetted to function in the upper intestine. However,
enzyme deactivation by
bile acids is not a general phenomenon.
Trypsin and
chymotrypsin are not deactivated by
bile acids. In fact, these pancreatic
enzymes are more efficient at cleaving large dietary substrates in the presence of
bile acids. We targeted the origin of the apparently different effect of
bile acids on prolyl
endopeptidases and pancreatic
enzymes by examining the effect of
bile acids on the kinetics of cleavage of small substrates, and by determining the effect of
bile acids on the thermodynamic stabilities of these
enzymes. Physiological amounts (5 mM) of
cholic acid decrease the thermodynamic stability of Flavobacterium meningosepticum PEP from 18.5 ± 2 kcal/mol to 10.5 ± 1 kcal/mol, while thermostability of
trypsin and
chymotrypsin is unchanged.
Trypsin and
chymotrypsin activation by bile and PEP deactivation can both be explained in terms of a common mechanism:
bile acid-mediated
protein destabilization.
Bile acids, usually considered non-denaturing
surfactants, in this case act as a destabilizing agent on PEP thus deactivating the
enzyme. However, this level of global thermodynamic destabilization does not account for a more than 50% decrease in
enzyme activity, suggesting that
bile acids most likely modulate
enzyme activity through specific local interactions.