Glycine encephalopathy
| Glycine encephalopathy (non-ketotic hyperglycinemia) | |
|---|---|
 |
|
| Glycine | |
| Classification and external resources | |
| OMIM | 605899 |
| MeSH | D020158 |
Glycine encephalopathy (also known as non-ketotic hyperglycinemia or NKH) is a rare autosomal recessive disorder of glycine metabolism. After phenylketonuria, glycine encephalopathy is the second most common disorder of amino acid metabolism. The disease is caused by defects in the glycine cleavage system, an enzyme responsible for glycine catabolism. There are several forms of the disease, with varying severity of symptoms and time of onset. The symptoms are exclusively neurological in nature, and clinically this disorder is characterized by abnormally high levels of the amino acid glycine in bodily fluids and tissues, especially the cerebral spinal fluid.
Glycine encephalopathy is sometimes referred to as "nonketotic hyperglycinemia" (NKH), as a reference to the biochemical findings seen in patients with the disorder, and to distinguish it from the disorders that cause "ketotic hyperglycinemia" (seen in propionic acidemia and several other inherited metabolic disorders). To avoid confusion, the term "glycine encephalopathy" is often used, as this term more accurately describes the clinical symptoms of the disorder.
Glycine is the simplest amino acid, having no stereoisomers. It can act as a neurotransmitter in the brain, act as an inhibitor in the spinal cord and brain stem, while having excitatory effects in the cortex of the brain. Glycine is metabolized to final end products of ammonia and carbon dioxide through the glycine cleavage system (GCS), an enzyme complex made up of four protein subunits. Defects in these subunits can cause glycine encephalopathy, although some causes of the disease are still unknown. Normally, GCS shows its highest enzymatic activity in liver, brain and placental tissue. One of its main functions is to maintain normal glycine levels in the brain. Defects in GCS cause an increase of glycine concentration in blood plasma and cerebrospinal fluid. The exact pathophysiology of the disorder is not known, but it is considered likely that buildup of glycine in the brain is responsible for the symptoms.
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