190 Essays in Biochemistry 



hydroxyl in this position was available. Recently neo-inositol (III) 

 which carries a polar hydroxyl group in position c was synthesized by 

 Angyal ; 1X his kind gift of a small amount of this isomer permitted 

 its use as a substrate for A. suboxydans. It was not attacked; appar- 

 ently the presence of an equatorial hydroxyl group in position c is re- 

 quired for oxidation by the microorganism. 



Not oxidized 



III 



The results of these studies can be summarized in three rules defin- 

 ing the steric requirements for the oxidation of inositols, deoxyinosi- 

 tols, and ketoinositols by A. suboxydans (Fig. 4). 



Fig. 4. The steric requirements for oxidation by A. suboxydans. 



1. Only polar hydroxyl groups are oxidized. 



2. The carbon in meta position to the one carrying the polar hy- 

 droxyl group (in counterclockwise direction if north polar, clockwise 

 if south polar) must carry an equatorial hydroxyl group. 



3. The carbon in para position to the one carrying the polar hydroxyl 

 group must carry an equatorial hydroxyl group. 



These structural requirements demonstrate three points of contact 

 between substrate and enzyme and can account for the oxidation of epi- 

 inositol (VI) to D-2-keto-epi-inositol (IX), an asymmetric synthesis 

 which theoretically demands a three-point attachment of substrate to 

 enzyme. The carbon atom carrying the polar hydroxyl group seems 

 to be particularly susceptible to dehydrogenation, as shown by the 

 recent report that platinum catalyzes the specific conversion of myo- 

 inositol (II) to 2-keto-?/i?/o-inositol (X) with oxygen as hydrogen ac- 

 ceptor. 12 The initial attack of the A. suboxydans enzyme is presum- 

 ably directed against the equatorial hydrogen atom located on the 

 carbon carrying the polar hydroxyl group; the enzyme-substrate com- 



