SPR and hyperinsulinemic hypoglycemia, familial, 4: As well as being a cofactor for tyrosine hydroxylase, tetrahydrobiopterin is also a required cofactor for phenylalanine hydroxylase, but patients with sepiapterin reductase deficiency do not exhibit overt hyperphenylalaninaemia as is seen in other autosomal recessive disorders of tetrahydrobiopterin synthesis as alternative enzymes are able to replace sepiapterin reductase in peripheral tissues and can thus compensate to some degree.