Phytanoyl-CoA hydroxylase

Refsum s disease. This disorder, first described nearly 60 years ago, was recently been shown due to a defect in the enzyme phytanoyl-CoA hydroxylase. Phytanic acid is a 3-methyl fatty acid that because of this methyl group cannot be oxidized directly. It is degraded by a peroxisomal a-oxidation to pristanic acid, a 2-methyl fatty acid which can be degraded by P-oxidation. The principal clinical features of Refsum s disease are progressive polyneuropathy, retinal degeneration, hearing loss, cardiomyopathy and ichthyosis, beginning in late childhood or later. 

Patients with Refsum disease may have extremely high phytanic acid levels, up to 1500 pmol/1, whereas pristanic acid is low (< 1 pmol/1) as a consequence of the phytanoyl-CoA hydroxylase deficiency. Less pronounced phytanic acid elevations will be observed in RCDP type 1 patients, which applies to both the classical form as well as the variant forms. Values may range from 200 to 900 pmol/1, somewhat depending on age. There is some discussion on the time of onset of phytanic acid accumulation in the classical neonatal RCDP-patients. Normal plasma phytanic acid levels (0.7-5.8 pmol/1) were recorded in the authors laboratory in patients aged less than 1 week. Two- to three-week-old RCDP patients had increased phytanic acid levels of 9.1 -13.2 pmol/1. Classical patients invariably had undetectable plasmalogen levels of the erythrocytes at any age. 

A number of inherited deficiencies of peroxisomal enzymes have been described. Zellweger s syndrome, which results from defective peroxisomal biogenesis, leads to complex developmental and metabolic phenotypes affecting principally the liver and the brain. One of the metabolic characteristics of these diseases is an elevation of C26 0, and C26 1 fatty acid levels in plasma. Refsum s disease is caused by a deficiency in a single peroxisomal enzyme, the phytanoyl CoA hydroxylase that carries out a-oxidation of phytanic acid. Symptoms include retinitis pigmentosa, cerebellar ataxia, and chronic polyneuropathy. Because phytanic acid is obtained solely from the diet, placing patients on a low-phytanic acid diet has resulted in marked improvement. 

Jansen, G.A., Mihalik, SJ., Watkins, PA, Moser, H.W., Jakobs, C., Denis, S. Wanders, R.JA. (19%) Biochem. Biophys. Res. Commun. 229, 205-210. Phytanoyl-CoA hydroxylase is present in human fiver, located in peroxisomes, and deficient in Zellweger syndrome direcl, unequivocal evidence for the new, revised pathway of phytanic acid a-oxidation in humans. 

Thirdly, Mihalik et al discovered a new enzyme which converts phytanoyl-CoA into 2-hydroxyphytanoyl-CoA in a reaction involving 2-oxoglutarate, Fe and ascorbate. The existence of this enzyme was confirmed in subsequent studies both in rat and man. Interestingly, phytanoyl-CoA hydroxylase is localiz in peroxisomes. This has been... 

Indeed, all phytanoyl-CoA hydroxylases studied by us thus far, which includes human PhyH dl show a perfeet PTS2-signature in Ae NH2-terminal portion of the protein. 

Rhizomelic Chondrodysplasia Punctata Type 1 (RCDP Type I). Patients suffering from RCDP type I which is due to a non-fiinetional PTS2-receptor, also show deficient oxidations of phytanic acid. Recent data show that this results from a deficiency of phytanoyl-CoA hydroxylase. The deficiency of PhyH probably results from its mis-locahzation to the cytosol as a consequence of the non-functional PTS2-receptor and the apparent instability of the PhyH-protein in the cydosol. Pulse/chase experiments are underway to resolve this. 

We recently found the enzyme defect in RD to be at the level of phytanoyl-CoA hydroxylase (PhyH), a peroxisomal enzyme catalysing the first step of the a-oxidation of phytanic acid, which involves conversion of phytanoyl-CoA to 2-hydroxyphytanoyl-CoA. Furthermore we identified the human PHYH cDNA after purification of PhyH... 

Table 2. Mutations in the phytanoyl-CoA hydroxylase cDNA in 8 patients with Refsum disease. Patient Mutation effect on coding sequence...

PHYH Phytanoyl-CoA hydroxylase (Refsum disease) Lipid metabolism/fatty acid alpha-oxidation... 

Refsum disease (adult form) (phyta-noyl-CoA hydroxylase deficiency) Phytanoyl-CoA hydroxylase Generalized lOpter-pll.2 266500... 

Mutation analysis C26 0 levels, C26 0 -oxidation, ALDP analysis Acyl-CoA oxidase activity D-bifunctional enzyme activity Phytanoyl-CoA hydroxylase activity... 

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