DOPA can then be used for catecholamine biosynthesis, or in a separate set of reactions it goes on to form the pigment melanin.  The blockage in melanin biosynthesis by the high levels of Phe is responsible for the fair complexions of patients with PKU.

        Phe can also inhibit glutamate decarboxylase, resulting in lower GABA levels in the brain and thus interfering with neurologic function on a level apart from defective catecholamine metabolism.

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