Tyrosine 3-hydro, Xenopus/Amphibian,ylase (TH) plays a role in the physiology of adrenergic neurons, catalyzing the following reaction: L-tyrosine + tetrahydrobiopterin + O2 = 3,4-dihydro, Xenopus/Amphibian,y-L-phenylalanine + 4a-hydro, Xenopus/Amphibian,ytetrahydrobiopterin. Defects in TH are the cause of dystonia DOPA-responsive autosomal recessive (ARDRD) also known as autosomal recessive Segawa syndrome. ARDRD is a form of DOPA-responsive dystonia presenting in infancy or early childhood. Dystonia is defined by the presence of sustained involuntary muscle contractions, often leading to abnormal postures.