Impaired Arachidonic (20:4n-6) and Docosahexaenoic (22:6n-3) Acid Synthesis by Phenylalanine Metabolites as Etiological Factors in the Neuropathology of Phenylketonuria

The recent literature on polyunsaturated fatty acid metabolism in phenylketonuria (PKU) is critically analyzed. The data suggest that developmental impairment of the accretion of brain arachidonic (20:4n-6) and docosahexaenoic (22:6n-3, DHA) acids is a major etiological factor in the microcephaly and mental retardation of uncontrolled PKU and maternal PKU. These fatty acids appear to be synthesized by the recently elucidated carnitine-dependent, channeled, mitochondrial fatty acid desaturases for which α-tocopherolquinone (α-TQ) is an essential enzyme cofactor. α-TQ can be synthesized either de novo or from α-tocopherol. The fetus and newborn would primarily rely on de novo α-TQ synthesis for these mitochondrial desaturases because of low maternal transfer of α-tocopherol. Homogentisate, a pivotal intermediate in the de novo pathway of α-TQ synthesis, is synthesized by 4-hydroxyphenylpyruvate dioxygenase. The major catabolic products of excess phenylalanine, viz. phenylpyruvate and phenyllactate, are proposed to inhibit α-TQ synthesis at the level of the dioxygenase reaction by competing with its 4-hydroxyphenylpyruvate substrate, thus leading to a developmental impairment of 20:4n-6 and 22:6n-3 synthesis in uncontrolled PKU and fetuses of PKU mothers. The data suggest that dietary supplementation with carnitine, 20:4n-6, and 22:6n-3 may have therapeutic value for PKU mothers and for PKU patients who have been shown to have a low plasma status of these essential metabolites.