Phytanic acid catabolism in mammals. Phytanic acid in ruminant fats is derived from phytol produced during the bacterial degradation of chlorophyll in their rumen (first stomach). After conversion to its CoA thioester, phytanic acid undergoes α-oxidation, yielding pristanic acid. This fatty acid then undergoes three subsequent rounds of β-oxidation in the peroxisome. The resulting medium chain fatty acid exits the peroxisome and translocates to the mitochondrion where the remaining carbon chain is degraded by β-oxidation. Abbreviations for the enzymes listed include: HACL1 (aka HPCL2) = 2-hydroxyphytanoyl-CoA lyase; PHYH = phytanoyl-CoA α-hydroxylase; PDH = pristanal dehydrogenase, whose gene is not yet known. We note that phytanic acid can also be degraded by β-oxidation; however, this activity of this pathway is relatively minor [40, 44].