HMGCR and Hyperphenylalaninemia: Thirdly, other factor associated with the decreased plasma CoQ level of PKU patients may be the elevated blood phenylalanine concentrations30, since experimentally induced hyperphenylalaninemia in mice has been reported to inhibit the activities of the brain and liver enzymes, 3-hydroxy-3-methylglutaryl-CoA reductase, and mevalonate-5-pyrophosphate decarboxylase31, which are both essential for CoQ and Chol biosynthesis.