426 2. ANALOGS OF ENZYME REACTION COMPONENTS 



against van der Waals' forces being a major factor in the hydroxyl inter- 

 actions, and suggest hydrogen bonding. The terminal carboxylate group 

 also participates, since glucaro-3,6-lactone is much more inhibitory than 

 glucurone. (See formulas on page 425). 



Some inhibitor constants for various /^-glucuronidases are shown in Table 

 2-23. Not enough /^-glucuronidases have been tested with glucaro-l,4-lac- 

 tone to determine the variations in susceptibility, but from the limited 

 data it appears that the animal enzymes are quite sensitive whereas the 

 plant enzyme baicalinase is much less readily inhibited. It is clear that 

 glucaro-l,4-lactone is one of the most potent inhibitors known and that 

 none of the other analogs so far examined on /^-glucuronidase is comparable, 

 although galactaro-l,4-lactone is undoubtedly a very effective inhibitor. It 

 is interesting that the /?-galacturonidases of limpet and preputial gland are 

 approximately as susceptible to these lactones as are the /^-glucuronidases 

 (Marsh and Levvy, 1958). 



Limpet or-glucuronidase shows quite a different pattern of inhibition (see 

 accompanying tabulation for inhibitions at 5 mM with phenyl-a-glucur- 



onide at 1 raM with relative binding energies calculated on the basis of 

 competitive inhibition). a-Glucuronides are more inhibitory than /5-glucur- 

 onides, as expected, but no particularly potent inhibitors were found {Kj^ 

 for glucuronate is 0.54 mM and for galacturonate 4.7 mM). The following 

 do not inhibit: mannurone, glucaro-l,4-lactone, mannono-l,4-lactone, and 

 galactarate. The difference between the a and /? enzymes with respect to 

 glucaro-l,4-lactone is especially striking. 



