14 

 also functions as a SDH, competition for two products 



(dehydroquinate and dehydroshikimate) would occur and this 

 could account for the inhibition seen with SHK added to the 

 reaction mix. This might also explain the earlier report of 

 the NAD* specific SDH activity by Ogawa and Tateoka (64) , if 

 the enzyme was actually a nonspecific QDH. The qa gene 

 cluster of Neurospora crassa encodes a functional domain for 

 a QDH (NADH specific) that will also utilize SHK/NADH (34) . 

 Properties. The range of molecular mass for DQT/SDH 



(or SDH, where DQT was not studied) is from 44,000 to 

 73,000. It is not clear as to whether the lower M^ of 

 44,000 might represent a monofunctional SDH. SDH activity 

 is usually assayed in the reverse of the physiological 

 direction with the substrates, SHK and NADP*, rather than 

 the forward direction with DHS and NADPH: 



NADPH 

 Dehydroshikimate < ^ . > Shikiraate 



NADP" 



Values of Km have been determined and vary from 0.060 to 1.3 

 mM for SHK, from 0.010 to 0.10 for NADP% and from 0.130 to 

 0.180 mM DHQ. Table 1-1 gives estimates of M,. and Km values 

 for DQT and SDH from several plant species, as well as M^ 

 from E. coli and from S. cerevisiae. 



Inhibition studies with HgClj, p-chloromercuri benzoate 

 and oxidized glutathione have suggested that SDH 



