332 2. ANALOGS OF p:;nzyme reaction components 



is similarly inhibited (see' accompanying tabulation), and here glutarate 

 also appears to present a relatively good fit to the active site (Mills and 

 Cochran, 1963). Glutamate was 15 mM in all cases. The reverse reaction 

 catalyzed by the glutamate dehydrogenases (both NAD- and NADP- 



linked) from Fusarium oxysporum is also inhibited by glutarate, the K,^ 

 for a-ketoglutarate being 2.1 mM, and the K^ for glutarate 1.52 mM 

 (Sanwal, 1961). 



Glutaminase 



The deamidation of glutamine is inhibited by the product glutamate 

 and this is not a reversal of the equilibrium but a competition for the 

 active center, as first pointed out by Krebs (1935). L-Glutamate and d- 

 glutamate inhibit guinea pig kidney glutaminase equally (98% at 25 mM 

 when glutamine is 8.7 mM). Inhibition by glutamate has been confirmed 

 for the enzyme from guinea pig kidney (van Baerle et al., 1957), pig kidney 

 (Klingman and Handler, 1958), dog kidney (Sayre and Roberts, 1958), 

 and rat brain (Blumson, 1957). The inhibition has generally been found 

 to be noncompetitive with respect to glutamine, but oddly is competitive 

 with phosphate on the phosphate-activated glutaminase from dog kidney. 

 The ammonium ion is, however, strictly competitive with glutamine on 

 both the pig and dog kidney enzymes. Another type of glutaminase (called 

 glutaminase II), which is transaminating in the presence of pyruvate and 

 is obtained from guinea pig kidney, is not inhibited by even 100 mM 

 glutamate (Goldstein et al., 1957). Sayre and Roberts (1958) pictured the 

 active center as containing two cationic groups, one binding the phosphate 

 and one the glutamine carboxylate group; the negatively charged phosphate 

 also interacts with the positive or-amino group of glutamine. Since the active 

 enzyme is the phosphate complex, it is easy to see why phosphate would 

 antagonize inhibitions produced by certain substances (e.g., dyes such as 

 bromosulfalein or bromcresol green which complex with both enzyme 

 cationic sites), but it is difficult to understand why glutamate inhibits 



