PCBD2 and allergic asthma: Also, other genes encoding for enzymes indirectly involved in metabolism of Phe, such as pterin-4-alpha-carbinolamine dehydratase 2 (PCBD2), catechol-O-methyltransferase (COMT), and kynurenine aminotransferase 1 (KYAT1), were significantly upregulated, altogether suggesting potentially faster turnover of intracellular Phe in Th2 cells of patients with allergic asthma.