Gamma-cystathionase or cystathionase, abbreviated as CTH OR CSE, is also known as cystathionine gamma-lyase. It is an enzyme that catalyzes, or breaks down cystathionine into cysteine, αlpha –ketobutyrate and ammonia. This enzyme also plays a role in the catalysis of L-homoserine, L-cystine, and L-cysteine. Pyridoxal phosphate, the active form of vitamin B6, is a co-enzyme of cystathionase and is essential for the function of cystathionase. With the aid of pyridoxal phosphate, cystathionase breaks apart the sulfur-gamma carbon bond of cystathionine, resulting in the release of cysteine.

Cysteine, an essential amino acid, is the main product of this catalysis and essential in the synthesis of glutathione which is a very important anti-oxidant. Ammonia is mainly a waste product, and α -Ketobutyrate plays a further role in the citric acid cycle or Kreb’s cycle, for the efficient conversion of macromolecules (carbohydrates, fats and proteins) into energy.

A genetic metabolic disorder called cystathioninuria, or cystathionase deficiency, leads to inability of the body to break down cystathionine which in turn is associated with a wide range of diseases. It is diagnosed by an increased amount of cystathionine in urine.

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