Glycine semialdehyde

The urea so formed is distributed throughout the body water and excreted. The renal clearance of urea is less than the glomerular filtration rate because of passive tubular back-diffusion. Diffusion of urea in the intestine leads to formation of ammonia, which enters the portal blood and is converted to urea in liver. Reentry of ornithine into mitochondria initiates the next revolution of the urea cycle. Ornithine can be converted to glutamate-y-semialdehyde (which is in equilibrium with its cyclic form A -pyrroline-5-carboxylate) by ornithine aminotransferase and de-carboxylated to putrescine by ornithine decarboxylase. Ornithine is also produced in the arginine-glycine trans-amidinase reaction. 

Succinate-glycine cycle, glycine-succinate cycle, Shemin cycle a bypass of the TCA-cycle of particular importance in the metabolism of red blood cells. It converts succinyl-CoA and glycine into 5-aminolevu-linate, which is the biosynthetic precursor of the Porphyrins (see). Alternatively, 5-aminolevulinate is de-aminated to 2-oxoglutarate semialdehyde, and the cycle is completed by formation of succinyl-CoA via 2-oxoglutarate. One turn of the cycle converts glycine into 2 molecules CO2 and 1 molecule NHj. 2-Oxoglu-tarate semialdehyde can also be converted into succinate and a Cl-unit. 

Examination of plasma and urine revealed elevated quantities of jff-ala-nine, 3-hydroxypropionic acid, (R)- and (S)-3-aminoisobutyric acid, (R)-and (S)-3-hydroxyisobutyric acid and (S)-2-hydroxymethylbutyric acid. Direct enzymatic assay of methylmalonate semialdehyde dehydrogenase is unavailable revealed homozygosity for DNA analysis 1336 G>A transversion which substituted an arginine for a highly conserved glycine at amino acid residue 446. 

Ornithine + glyoxylic acid glutamic semialdehyde glycine sss... 

Shemin has pointed out that the oxidative deamination of aminolevulinate and the subsequent oxidation of ketoglutarate semialdehyde to ve ketoglutarate constitute a second cycle which would effect complete oxidation of glycine. The significance of this is still obscure. 

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