The AASS gene provides instructions for making an enzyme called aminoadipic semialdehyde synthase. This enzyme is found in most tissues, with the highest amounts found in the liver. Aminoadipic semialdehyde synthase is involved in the breakdown of the amino acid lysine, a building block of most proteins. It is called a bifunctional enzyme because is performs two functions. One function, called lysine-ketoglutarate reductase, breaks down lysine to a molecule called saccharopine. The other function, called saccharopine dehydrogenase, breaks down saccharopine to a molecule called alpha-aminoadipate semialdehyde.
At least five mutations in the AASS gene have been found to cause hyperlysinemia. Most of these mutations change single amino acids in aminoadipic semialdehyde synthase. These mutations are thought to decrease or eliminate enzyme activity, resulting in an inability to break down lysine. Lysine that is not broken down accumulates in the blood, but it typically causes no health problems.
When mutations in the AASS gene impair the breakdown of saccharopine, this molecule builds up in blood and urine. This buildup is sometimes referred to as saccharopinuria, which is considered to be a variant of hyperlysinemia. It is unclear if saccharopinuria causes any symptoms.
- alpha-aminoadipate semialdehyde synthase
- aminoadipic semialdehyde synthase
- lysine-2-oxoglutarate reductase
- lysine-ketoglutarate reductase /saccharopine dehydrogenase