Primary hyperoxaluria type 2: enzymology.

Deficiency of the enzyme D-glycerate dehydrogenase (D-GDH) which also has glyoxylate reductase (GR) activity, is believed to be the underlying cause of primary hyperoxaluria type 2 (PH2). We have established the reaction kinetics of this enzyme in human liver and using these parameters have developed a microassay for the measurement of D-GDH and GR on needle liver biopsies obtained from patients with suspected primary hyperoxaluria. Tissue distribution studies of the two enzyme activities suggest that more than one enzyme with D-GDH activity is present in human tissues and the one with associated GR activity is mainly confined to the liver. The clinical significance of these findings for diagnosis and treatment is discussed.

Kinetic analysis and tissue distribution of human D-glycerate dehydrogenase/glyoxylate reductase and its relevance to the diagnosis of primary hyperoxaluria type 2.

The enzyme D-glycerate dehydrogenase (D-GDH; EC 1.1.1.29), which is also believed to have glyoxylate reductase (GR; EC 1.1.1.26/79) activity, plays a role in serine catabolism and glyoxylate metabolism and deficiency of this enzyme is believed to be the cause of primary hyperoxaluria type 2 (PH2). The pH optima and kinetic parameters of D-GDH and GR in human liver have been determined and assays developed for their measurement. Maximal activities were observed at pH 6.0, 8.0 and 7.6 for the D-GDH forward, D-GDH reverse and GR reactions, respectively. The apparent Km values for the substrates in these reactions were as follows: D-GDH forward reaction, 0.5 mmol/L hydroxypyruvate and 0.08 mmol/L NADPH; D-GDH reverse reaction, 20 mmol/L D-glycerate and 0.03 mmol/L NADP and for the GR reaction 1.25 mmol/L glyoxylate and 0.33 mmol/L NADPH. The forward D-GDH and GR assays were adopted for routine use, the low activity of the reverse D-GDH reaction being of little use for routine analyses. D-GDH and GR activity in 13 normal livers ranged from 350-940 nmol per min per mg protein (median 547) and 129-209 nmol per min per mg protein (median 145), respectively. D-GDH activity in kidney, lymphocytes and fibroblasts fell within the range of values seen in the liver but GR activity was approximately 30% in the kidney and barely detectable in lymphocytes and fibroblasts. Analysis of liver and lymphocyte samples from patients with PH2 showed that GR activity was either very low or undetectable while D-GDH activity was reduced in liver but within the normal range in lymphocytes. These results suggest that there is more than one enzyme with D-GDH activity in human tissues but only one of these has significant GR activity. We conclude that a definitive diagnosis of PH2 requires measurement of GR and D-GDH in a liver biopsy.