The cofactor hydrobiopterine is oxidized during tyrosine formation. The reaction of phenylalanine hydroxylase itself is irreversible. Tyrosine degradation is a liver resident process. This enzyme is specific for both tyrosine and phenylalanine. The ring structure of homogentisate is subsequently broken and the linear C8 unit degraded in two reaction steps to fumarate and acetoacetate, one citric acid cycle intermediate and one ketone body.
Glucocorticoids stimulate amino acid degradation in liver which yields fumarate and acetoacetate.
Fumarate is converted to oxaloacetate which stimulates gluconeogenesis. Thus liver can synthesize and export glucose into the blood plasma. Acetoacetate is secreted and used by neurons and muscle tissue for oxidative degradation. Tryptophan degradation in human also yields precursors for glucose synthesis. Tryptophan stimulates its own degradation by allosterically activating tryptophan oxygenase .