INHIBITION OF ENZYMES 



811 



in Fig. 7-28 to indicate more clearly the relative rates of inhibition (see 

 Eq. 1-12-14 and Figs. 1-12-3 and 1-12-9), the slopes being proportional to 

 the bimolecular rate constants. One notes that most of the curves deviate 

 from linearity, frequently at high inhibitions; this is probably an expression 

 of the different relative reactivities of the SH groups on. a single enzyme, 

 some reacting initially at a rapid rate and others reacting more slowly. 

 The rate constants have been calculated for some enzymes, e.g., 18.8 liters/ 

 mole/sec for glutamate decarboxylase (Shukuya and Schwert, 1960), 51 li- 

 ters/mole/sec for muscle phosphorylase (Madsen and Cori, 1956), and 61.4 

 liters/mole/sec for heart lactate dehydrogenase (Takenaka and Schwert, 

 1956), in all cases p-MB being the inhibitor. The effects of temperature and 

 mercurial concentration on the rates of inhibition are well illustrated for 

 alcohol dehydrogenase (Fig. 7-26) and /?-fructofuranosidase (Fig. 7-27), re- 

 spectively. Glutamate decarboxylase presents an interesting phenomenon, 

 in that exposure of the enzyme to low temperatures appears to liberate 

 additional SH groups (Fig. 7-27). 



)9 - FRUCTOFURANOSIDASE 



GLUTAMATE DECARBOXYLASE 



80 " 60 120 180 240 300 360 



MIN TIME »- MIN 



Fig. 7-27. Rate titrations of various enzymes with p-MB. The reaction 

 of SH groups was determined by absorption changes at 250 m/<. Phos- 

 phorylase a from rabbit muscle: p-MB = 0.04 mM, pH 6.7, and 21° 

 (Madsen and Cori, 1956.) Lactate dehydrogenase from heart: p-MB 

 = 0.00448 Mm, pH 6.8, and 25° (Takenaka and Schwert, 1956.) 

 ^-Fructofuranosidase from Neurospora: p-MB concentrations given in 

 the graph, pH 6.8, and 0° (Metzenberg, 1963). Glutamate decarboxylase 

 from E. coli: preincubation with p-MB for 4 hr at either 0° or 25°, 

 and reaction run at 25° and pH 6.5 (Shukuya and Schwert, 1960). 



