Charles E. Mize

Alpha-decarboxylation, an important pathway for degradation of phytanic acid in animals

Abstract Patients suffering from Refsums disease, an inherited disorder of the nervous system ( Refsum, 1946 ), accumulate large stores of phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) in their blood and tissues ( Klenk and Kahlke, 1963 ). There appears to be little or no endogenous synthesis of phytanic acid either in normal experimental animals ( Avigan, Steinberg and Cammermeyer, 1966 ; Mize et al, 1966 ) or in patients with Refsums disease ( Steinberg et al, 1965 , 1966a ) and the metabolic error is presumed to lie in a relative inability to degrade phytanic acid. It has been suggested on the basis of indirect evidence that an error in omega-oxidation might be involved ( Eldjarn, 1965 ), but there is as yet no direct information on the normal pathway for degradation of phytanic acid. The results reported below show that the normal rat rapidly converts phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) to its α-decarboxylation product, pristanic acid (2,6,10,14-tetramethylpentadecanoic acid).

Studies on the Metabolic Error in Refsum's Disease*

Studies utilizing mevalonic acid-2-(14)C and D(2)O as precursors failed to provide evidence for an appreciable rate of endogenous biosynthesis of phytanic acid in a patient with Refsums disease. Orally administered tracer doses of phytol-U-(14)C were well absorbed both by seven normal control subjects (61 to 94%) and by two patients with Refsums disease (74 and 80%). The fraction of the absorbed dose converted to (14)CO(2) in 12 hours was 3.5 and 5.8% in Refsums disease patients and averaged 20.9% in seven control subjects. Labeled phytanic acid was demonstrated in the plasma of both control subjects and patients given phytol-U-(14)C, establishing phytol in the diet as a potential precursor of phytanic acid. This labeled phytanic acid had disappeared almost completely from the plasma of the seven control subjects by 24 to 48 hours, whereas it persisted at high concentrations in the plasma of the two patients for many days. We conclude that the phytanic acid accumulating in Refsums disease is primarily of exogenous origin and that patients with Refsums disease have a relative block in the degradation of phytanic acid and possibly other similar branched-chain compounds. This may relate to a deficiency in mechanisms for release of phytanic acid from stored ester forms or, more probably, to reactions essential to oxidative degradation of the carbon skeleton.

A major pathway for the mammalian oxidative degradation of phytanic acid.

Abstract A series of branched-chain saturated fatty acids accumulated in tissues of weanling mice fed regular chow supplemented with 2% phytanic acid (3,7,11,15 tetramethylhexadecanoic acid). They were identified by combined gas-liquid chromatography-mass spectrometry as phytanic acid, α-hydroxyphytanic acid, and the 19-, 16- and 14-carbon homologs of phytanic acid. Trace quantities of the 9-carbon homolog have also been detected by gas-liquid chromatography. When the regular chow is supplemented with 2% phytol (3,7,11,15-tetramethylhexadec-2-en-1-ol), Δ 2 -phytenic acid accumulated in liver in addition to the compounds listed above. No evidence was found for the presence of α-ketophytanic acid, or of 15- and 17-carbon intermediates. The latter two would be expected if carbon dioxide fixation to the branch-methyl group were required in phytol or phytanate metabolism, as in the bacterial metabolism of farnesoic acid. Dietary supplementation with 3,7,11-trime thyldodecanoic acid produces accumulation in liver of the fed compound only. Intravenous injection of uniformly 14 C-labeled phytanic acid in mice led to significant incorporation of radioactivity associated on gas-liquid chromatography with 19-16-, 14- and 11-carbon isoprenoid acids. Rat liver in vivo also rapidly converted in high yield uniformly 14 C-labeled phytanic acid or 2,3-dideuterophytanic acid to uniformly 14 C-labeled pristanic acid and 2-deuteropristanic acid, respectively. The oxidation of uniformly 14 C-labeled phytanic acid to 14 CO 2 was not impaired in biotindeficient rats. The proposed main metabolic pathway of phytanic acid in mammals consists of an α-oxidative process leading through α-hydroxyphytanic acid to pristanic acid, followed by a series of β-oxidative steps. The chemical syntheses of a number of isoprenoid acids are described.

Absorption and metabolism of uniformly 14C-labeled phytol and phytanic acid by the intestine of the rat studied with thoracic duct cannulation

Abstract 1. 1. Tracer doses of uniformly 14C-labeled phytol were absorbed from the intestine of the rat to the extent of approx. 50% in 24 h. When the doses were increased to 300 mg, a smaller fraction was absorbed but the total quantity absorbed reached values as high as 65 mg. Over 70% of the absorbed phytol entered through the intestinal lymphatics. Phytol and cetyl alcohol were absorbed more slowly and less completely than were phytanic acid and palmitic acid, and less exclusively by way of the lymphatic route. 2. 2. Over half of the [14C] phytol radioactivity absorbed into the thoracic-duct lymph was still in phytol, some of which was free but a larger part of which was in the form of phytyl esters of fatty acids. The remainder of the label was in acidic derivatives of phytol, principally phytanic acid and phytenic acid (probably at least 3 isomers), which were present almost entirely in triglycerides and other complex lipids. The same derivatives were found in germ-free rats. No dihydrophytol was found in the lymph after phytol had been fed, and no phytenic acid was found after phytanic acid had been fed. Administered phytanic acid was absorbed into the lymph largely without change, except for its incorporation into triglycerides and other complex lipids. In contrast to the results with phytol, cetyl alcohol was largely converted to fatty acid during absorption. 3. 3. Significant quantities of radioactivity from [14C]phytol (administered to rats with intact thoracic ducts) were excreted in the bile, principally in the form of derivatives much more polar than phytanic acid.

Localization of the oxidative defect in phytanic acid degradation in patients with refsum's disease

The rate of oxidation of phytanic acid-U-(14)C to (14)CO(2) in three patients with Refsums disease was less than 5% of that found in normal volunteers. In contrast, the rate of oxidation of alpha-hydroxyphytanic acid-U-(14)C and of pristanic acid-U-(14)C to (14)CO(2), studied in two patients, while somewhat less than that in normal controls, was not grossly impaired. These studies support the conclusion that the defect in phytanic acid oxidation in Refsums disease is located in the first step of phytanic acid degradation, that is, in the alpha oxidation step leading to formation of alpha-hydroxyphytanic acid. The initial rate of disappearance of plasma free fatty acid radioactivity after intravenous injection of phytanic acid-U-(14)C (t(1/2) = 5.9 min) was slower than that seen with pristanic acid-U-(14)C (t(1/2) = 2.7 min) or palmitic acid-1-(14)C (t(1/2) = 2.5 min). There were no differences between patients and normal controls in these initial rates of free fatty acid disappearance for any of the three substrates tested. There was no detectable lipid radioactivity found in the plasma 7 days after the injection of palmitic acid-1-(14)C or pristanic acid-U-(14)C in either patients or controls. After injection of phytanic acid-U-(14)C, however, the two patients showed only a very slow decline in plasma lipid radioactivity (estimated t(1/2) = 35 days), in contrast to the normals who had no detectable radioactivity after 2 days. Incorporation of radioactivity from phytanic acid-U-(14)C into the major lipid ester classes of plasma was studied in one of the patients; triglycerides accounted for by far the largest fraction of the total present between 1 and 4 hr.

Refsum's Disease—A Recently Characterized Lipidosis Involving the Nervous System: Combined Clinical Staff Conference at the National Institutes of Health

Excerpt Dr. Daniel Steinberg: About the time of World War II, a young Norwegian neurologist, Sigvald Refsum, became intrigued with four patients referred to the Neurology Department of the Rikshosp...