MISCELLANEOUS ANALOG INHIBITIONS 615 



The question of the site, or sites, of tungstate inhibition is not yet settled. 

 Bulen believes that the primary effect is a depression of molybdate uptake 

 and has provided evidence that that there is no antagonism of the enzymi- 

 caUy functioning molybdate. The growth of the crown-gall organism Agro- 

 bacterium tumefaciens is also inhibited in nitrate medium (about 50% at 

 0.05 milf), but when ammonia is added there is only slight inhibition at 

 1 ToM (Kurup and Vaidyanathan, 1963). Molybdate antagonizes the growth 

 depression. A decrease in nitrate reductase activity during the inhibition 

 was observed. Before the problem of the site of inhibition can be finally 

 settled, more work must be done on isolated molybdenum-dependent en- 

 zymes. The NADH-dependent nitrate reductase from wheat is not inhibited 

 by 1 mM tungstate (Spencer, 1959). In any event, this represents a unique 

 type of competitive inhibition which is basically due to the similar structures 

 and properties of tungstate and molybdate. 



Inhibition of Penicillinases 



Some of the data are summarized in Table 2-38, and it is evident that the 

 penicillinases from different bacteria exhibit various patterns of sensitivity. 

 In particular, the gram-negative and gram-positive organisms possess dif- 

 ferent types of enzyme, the former usually being more sensitive to penicillin 

 analogs. The inhibitions are generally competitive. The exact mechanism of 

 the inhibition, however, is not clear, since there is some evidence that the 

 analogs alter the configuration of the enzyme (Garber and Citri, 1962; Citri 

 and Garber, 1963). In the first place, the analogs accelerate the tempera- 

 ture inactivation of penicillinase and, in the second place, the inhibition by 

 6-(2,6-dimethoxybenzamido)penicillanate is accompanied by the appear- 

 ance of groups sensitive to iodine. These penicillinase inhibitors are of use 

 in determining the mechanism of penicillin resistance in bacteria; if the 

 resistance is due to the increased synthesis of penicillinase, the inhibitor 

 wiU abolish the resistance. (Hamilton-Miller et al., 1964). 



Inhibition of Cycle Enzymes by y-Hydroxy-a-ketoglutarate 



Glyoxylate and oxalacetate condense under physiological conditions to 

 form a product which inhibits certain steps in the tricarboxylate cycle. 

 The reaction is quite rapid at 20° and pH 7.4, and since both glyoxylate 

 and oxalacetate are produced normally in cells, it was of some interest to 

 study the properties of this condensation product, which was considered to 

 be «-hydroxy-/?-oxalosuccinate by Ruffo et al. (1962 a). It was found to be 

 competitive with respect to citrate and cis-aconitate, and to inhibit 50% 

 at concentrations around 0.12 mM. When glyoxylate is added to mito- 

 chondria there is respiratory inhibition and some accumulation of citrate 

 if oxalacetate is present, which is due to the inhibition of aconitase and 



