16 METABOLIC PATHWAYS IN MICROORGANISMS 



Glycerol, Glycerol enters the pentose cycle via dihy- 

 droxyacetone (Figs. 1.3 and 1.4). This conversion is reached 

 by two alternate pathways (25), both of which are in the 

 soluble fraction (soluble at 30,000g). One, active at pH 

 6.0, is independent of ATP and DPN, and yields dihydrox- 

 yacetone directly. The other, with a pH optimum around 

 8.5, requires the participation of ATP and Mg^+, in a 

 kinase reaction, to yield glycerol-a-phosphate. The latter 

 is then oxidized by a DPN-dependent dehydrogenase to 

 form DHA-PO4. 



Sorbitol, Sorbitol (Fig. 1.4) is oxidized on either end of 

 the molecule by soluble extracts of A. suhoxydans, depend- 

 ing upon which pyridine nucleotide is present (26, 27). In 

 the presence of TPN, L-sorbose is formed; with DPN, d- 

 fructose is produced (see Table 1.6). The fructose can then 

 be phosphorylated or further oxidized via the pentose cycle. 

 These two pathways occur in addition to the previously 

 demonstrated sorbitol dehydrogenase in the particulate frac- 

 tion of the cells, which form L-sorbose. 



The DPN-linked enzyme has been purified about sixteen- 

 fold so that it is free from the TPN (sorbose-producing) 

 enzyme, and is also free of mannitol dehydrogenase activity 

 (27). An effective purification step consists of heating the 

 enzyme in the presence of a polyhydroxy compound and 

 pyridine nucleotide; the sorbitol dehydrogenase is pro- 

 tected by these compounds, while many other proteins are 

 denatured and may be removed by centrifuging. 



Although earlier work (30, 31) has suggested the presence 

 of some relatively non-specific polyol dehydrogenases in 

 mammalian tissues, the present experience with A. suhoxy- 

 dans indicates a higher degree of specificity at least as far as 

 sorbitol is concerned. The purified DPN enzyme, active 

 for sorbitol, was completely inactive toward mannitol. 



