p hosphate - d ependent e nzymes
4 Glycine, Serine and the One - Carbon Pool
4.3 Glycine o xidase and g lyoxylate m etabolism
retardation. When there is still a signifi cant residual activity of the glycine cleavage system, the condition is episodic, and affected children show mild mental retardation with episodes of delirium, chorea and vertical gaze palsy during febrile illnesses.
Standard treatment for the condition is the administration of benzoate to lower serum glycine by forming hippuric acid, and dextromethorphan as an antagonist of glycine at the NMDA receptors in the central nervous system.
In ketotic hyperglycinaemia, the failure of glycine metabolism is secondary to propionic acidaemia (due to propionyl CoA carboxylase defi ciency) or methylmalonic aciduria (due to methylmalonyl CoA mutase defi ciency).
When the patient ’ s condition is well controlled by a low protein diet, the serum concentration of glycine is normal and liver biopsies show normal activity of the glycine cleavage system. However, post mortem measurement of the glycine cleavage system in livers from two patients who died in meta-bolic acidosis showed very low activity, especially of the H - protein, which is the lipoamide carrier protein in the multi - enzyme complex. This suggests that there is repression of the synthesis of the protein by one or other of the abnormal metabolites that accumulate when the condition is poorly control-led (Hayasaka et al ., 1982 ).
carbon dioxide and forming 2 - oxo - 3 - hydroxyadipate. This can undergo further decarboxylation to 2 - oxo - hydroxyglutarate and reduction to yield 2 - oxoglutarate, thus providing a cyclic pathway for complete catabolism of glyoxylate. The importance of this pathway is shown by observations that, in rats, both magnesium defi ciency and thiamin defi ciency lead to a decrease in glyoxylate oxidation and accumulation of glyoxylate and oxalate, although the enzymes involved in the conversion of 2 - oxo - 3 - hydroxyadipate to oxoglu-tarate have not been characterized (Bais et al ., 1991 ; Rattan et al ., 1993 ; Sidhu et al ., 1987 ).
In plants and bacteria, pyruvate decarboxylase catalyzes the decarboxyla-tion of glyoxylate to formaldehyde. Glyoxylate inhibits the decarboxyladecarboxyla-tion of pyruvate to acetaldehyde, but pyruvate and acetaldehyde accelerate the Figure 4.4 Metabolic fates of glyoxylate.
Lactate dehydrogenase EC 1.1.1.27, glyoxylate reductase EC 1.1.1.26, glyoxylate carboxylyase EC 4.1.1.47, tartronic semialdehyde reductase EC 1.1.1.60, 2 - hydroxy - 3 - oxo - adipic acid synthase EC 2.2.1.5.
HC COO -O glyoxylate
COO-COO -oxalate
HC COO
-O CO2 HC O
HC COO
-OH
tartronic semialdehyde NADH
NAD+ HC O HC
COO -OH
D-glyceric acid tartronic semialdehyde
reductase glyoxylate carboxylyase
C O COO -CH2
2-oxoglutarate CH2
COO
-CO2
C HC
CH2
CH2
COO
-COO -OH O
2-hydroxy-3-oxoadipic acid HC
C CH2 CH3
COO -O OH
2-oxo-3-hydroxyglutaric acid CO2
NADH
NAD+ H2O
NAD+ NADH
lactate dehydrogenase
2-hydroxy-3-oxoadipic acid synthase
non-enzymic NAD(P)+
NADPH glyoxylate reductase
H2C COO -OH glycolate
O2
H2O2
glycolate oxidase
decarboxylation of glyoxylate, because acetaldehyde forms a condensation product with formaldehyde bound to thiamin at the active site and enhances its removal.
4.3.1 Primary h yperoxaluria
Any glyoxylate that accumulates in the cell and is not metabolized by one of the pathways discussed in section 4.3 is a substrate for lactate dehydrogenase, forming oxalate. Oxalate has a low solubility, and excessive excretion of oxalate leads to crystallization of calcium oxalate in the kidney and urinary tract. As renal function is progressively impaired, so systemic oxalosis develops, with crystallization of oxalate throughout the body.
Type I primary hyperoxaluria is due to defects in alanine - glyoxylate transaminase. Some 150 different mutations in the alanine - glyoxylate transaminase gene have been reported. In some cases, the effect of the muta-tion is that the enzyme has little or no activity (either a very low V max or an abnormally high K m ). In other cases, there is no detectable enzyme protein.
Sometimes the defect is in the coenzyme binding site, and doses of vitamin B6 of the order of 250 – 500 mg/day (as compared with reference intakes of the order of 15 – 20 mg/day) permit maintenance of more or less normal enzyme activity (see section 3.6 for a discussion of other vitamin B 6 dependency syndromes).
However, the commonest cause of primary hyperoxaluria is mis - targeting of the enzyme, so that it is found in mitochondria rather than peroxisomes.
Human alanine - glyoxylate transaminase is a peroxisomal enzyme; however, in carnivores the enzyme is mainly mitochondrial, and in rodents it is equally distributed between mitochondria and peroxisomes. There is a common poly-morphism of human alanine - glyoxylate transaminase, with proline rather then leucine at position 11. About 95 per cent of the Pro11Leu protein folds and dimerizes normally, and is imported into the peroxisomes, but 5 per cent is imported into mitochondria before it can fold and dimerize. It then folds, and is active, inside the mitochondria.
A second mutation, Gly170Arg, delays protein folding and dimerization and, when this is superimposed on the Pro11Leu polymorphism, it results in the unfolded protein being transported into mitochondria, where it folds and dimerizes, with only about 10 per cent of the enzyme being in the peroxisomes where glyoxylate is formed and needs to be metabolized. Mitochondria can only import unfolded or loosely folded proteins, whereas peroxisomes can import the folded and dimerized enzyme with its pyridoxal phosphate cofac-tor bound (Danpure, 1993, 2004, 2005 ).
Type II primary hyperoxaluria is very much rarer than type I, and it is caused by defects in glyoxylate reductase, which also catalyzes the intercon-version of d - glycerate and hydroxypyruvate (see Figure 4.8 ). In this
condi-tion, not only oxalate but also glycerate is excreted in the urine. d - Glycerate reductase is less severely affected than glyoxylate reductase, which is virtually undetectable, suggesting that there are other enzymes that can catalyze the reduction of d - glycerate, but not of glyoxylate (Cramer et al ., 1999 ; Giafi &
Rumsby, 1998 ; Kemper et al ., 1997 ).