Glycerolphosphate Dehydrogenases

Two different types of enzymes catalyze the reduction of dihydroxy-acetone phosphate to L-a-glycerol phosphate. The first enzyme requires DPN as the cofactor and has been crystallized from extracts of muscle tissue. The TN is 26,000 per 10 g. of enzyme protein per minute at 20°C. and at pH 7.0. The equilibrium constant greatly favors the reduction of dihydroxyacetone phosphate  

The crystalline protein shows a significant absorption band at 260-mju, suggesting that this dehydrogenase may contain bound DPN similarly to muscle triosephosphate dehydrogenase. 

The second type of a-glycerolphosphate dehydrogenase was first described by Green as a particulate system which was coupled to cytochrome c. This particulate system has now been solubilized by treatment with sodium desoxycholate. On further purification the enzyme was separated from triosephosphate dehydrogenase, isomerase, and catalase activities and no longer was capable of reducing cytochrome c. It also did not react with DPN, TPN, or FAD, although it readily reduced suitable dyes. The reaction product was dihydroxyacetone phosphate. 

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Figure 10-5. Intrahepatic metabolism of free fatty acids (FFA). CPT I, CPT II, carnitine palmitoyltransferase I, II, respectively LCFA, long-chain fatty acid VLDL, very low-density lipoprotein. 1, Long-chain acyl-CoA synthase 2, acetoacetyl-CoA thiolase 3, hydrox-ymethylglutaryl-CoA synthase 4, hydroxymethylglutaryl-CoA lyase 5, 3-hydroxybutyrate dehydrogenase 6, acetyl-CoA carboxylase 7, fatty acid synthase 8, glycerolphosphate acyltransferase Reprinted with permission from Girard et al. (1992).

Evidence for the presence of the enzymes of the histidine pathway in plants appears to be limited to the work of Winter et al. (1971a) who demonstrated the presence of ATP-phosphoribosyltransferase, the first enzyme of the pathway, imidazole glycerolphosphate dehydratase and histidinol phosphatase in extracts from the shoots of barley, oats, and peas, and to the unpublished observations of Davies (see Davies, 1971) on the presence of histidinol dehydrogenase in rose tissue culture cells. The specific activity of ATP-phosphoribosyltransferase was greatest in peas and oats and least in barley. The enzymes from oats and barley were thermolabile losing activity after 30 min at 37°C. The specific activities of imidazole glycerolphosphate dehydratase were very low but it was possible to purify the enzyme to some extent. The values for imidazole glycerolphosphate for the barley enzyme was 0.6 mM which compares with values for jthe yeast and bacterial enzymes of 0.3 and 0.4 mM, respectively. Histidinolphosphatase was purified 20-fold but the authors considered that two phosphatases were still present. 

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