Minor pathways of oxidation of fatty acids- Lecture-1 (Alpha oxidation)

The β oxidation accounts for the bulk of energy production from fatty acids in humans. These reactions must be supplemented by other mechanisms so that all types of ingested fatty acids can be oxidized.

Overview of minor pathways of the biological importance of fatty acid oxidation

1) α- Oxidation- Oxidation occurs at C-2 instead of C-3, as in β oxidation

2) ω- Oxidation – Oxidation occurs at the methyl end of the fatty acid molecule.

3) Peroxisomal fatty acid oxidation- This occurs for the chain shortening of very-long-chain fatty acids.

Details

1) α- Oxidation

α- Oxidation- Takes place in the microsomes of the brain and liver, involves the decarboxylation process for the removal of a single carbon atom at one time with the resultant production of an odd chain fatty acid that can be subsequently oxidized by beta-oxidation for energy production. It is a strictly anaerobic process. No prior activation of the fatty acid is required. The process involves hydroxylation of the alpha carbon with a specific α-hydroxylase enzyme that requires Fe⁺⁺ and vitamin C/FH4 as cofactors.

There are systems in many tissues for the hydroxylation of α –carbon of shorter chain fatty acids in order to start their oxidation.

The biological significance of alpha oxidation

1) Oxidation of phytanic acid

Although the use of the α- Oxidation scheme is relatively less in terms of total energy production, it is significant in the metabolism of dietary fatty acids that are methylated. A principal example of these is Phytanic acid which is produced from dietary phytol, a constituent of chlorophyll of plants. Phytanic acid is a branched-chain (3, 7, 11, 15-tetramethylhexadecanoic acid) fatty acid (figure-1) and is also a significant constituent of milk lipids and animal fats. Normally it is metabolized by an initial α- hydroxylation followed by dehydrogenation and decarboxylation. Beta oxidation cannot occur initially because of the presence of 3- methyl groups, but it can proceed after decarboxylation. The whole reaction produces three molecules of propionyl co A, three molecules of Acetyl co A, and one molecule of iso butyryl co A (figure-2).

Figure-1- showing the structure of Phytanic acid (3, 7, 11, 15-tetramethylhexadecanoic acid).

Figure-2- Phytanic acid is oxidized by Phytanic acid α oxidase (α- hydroxylase enzyme) to yield CO2 and odd chain fatty acid Pristanic acid that can be subsequently oxidized by beta-oxidation. This process involves hydroxylation of the alpha carbon, removal of the terminal carboxyl group and concomitant conversion of the alpha hydroxyl group to a terminal carboxyl group, and linkage of CoA to the terminal carboxyl group. This branched substrate will function in the beta-oxidation process, ultimately yielding propionyl-CoA, acetyl Co As and, in the case of phytanic acid, 2-methyl propionyl CoA (Iso butyryl Co A)

2)Formation of Hydroxy fatty acid

The hydroxy fatty acids produced as intermediates of this pathway like Cerebronic acid can be used for the synthesis of cerebrosides and sulfatides

3) Formation of odd chain fatty acids

 Odd chain fatty acid produced upon decarboxylation in this pathway can be used for the synthesis of sphingolipids and can also undergo beta-oxidation to form propionyl co A and Acetyl co A.The number of acetyl co A depend upon the chain length. Propionyl co A is converted to Succinyl co A to gain entry into the TCA cycle for further oxidation.

Clinical significance of alpha oxidation of fatty acids

Refsum disease (RD)

Refsum disease (RD) is a neurocutaneous syndrome that is characterized biochemically by the accumulation of phytanic acid in plasma and tissues. Refsum first described this disease. Patients with Refsum disease are unable to degrade phytanic acid because of the deficient activity of Phytanic acid oxidase enzyme catalyzing the first step of phytanic acid alpha-oxidation.

Peripheral polyneuropathy, cerebellar ataxia, retinitis pigmentosa, and  Ichthyosis (rough, dry and scaly skin) are the major clinical components. The symptoms evolve slowly and insidiously from childhood through adolescence and early adulthood.

Biochemical defect and pathogenesis

Refsum disease is an Autosomal recessive disorder characterized by defective alpha-oxidation of phytanic acid. Consequently, this unusual, exogenous C20branched-chain (3, 7, 11, 15-tetramethylhexadecanoic acid) fatty acid accumulates in the brain, blood, and other tissues. It is almost exclusively of exogenous origin and is delivered mainly from dietary plant chlorophyll and, to a lesser extent, from animal sources. Blood levels of phytanic acid are increased in patients with Refsum disease. These levels are 10-50 mg/dL, whereas normal values are less than or equal to 0.2 mg/dL, and account for 5-30% of serum lipids. Phytanic acid replaces other fatty acids, including such essential ones as Linoleic and Arachidonic acids, in lipid moieties of various tissues. This situation leads to an essential fatty acid deficiency, which is associated with the development of ichthyosis (figure 3). Refsum disease is rare, with just 60 cases observed so far.

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Figure-3- Dryness of skin caused due to deficiency of essential fatty acids in Refsum disease.

Clinical manifestations

Classic Refsum disease manifests in children aged 2-7 years; however, diagnosis usually is delayed until early adulthood. Infantile Refsum disease makes its appearance in early infancy. Symptoms develop progressively and slowly with neurologic and ophthalmic manifestations. The disease is characterized by

• Night blindness due to degeneration of the retina (retinitis pigmentosum)
• Loss of the sense of smell (anosmia)
• Deafness
• Concentric constriction of the visual fields
• Cataract
• Signs resulting from cerebellar ataxia
  • Progressive weakness
  • Foot drop (Figure-4)
  • Loss of balance
• Some individuals will have shortened bones in their fingers or toes (figure-5).

Figure-4 showing foot drop.

Figure-5- showing shortened bones

• The children usually have moderately dysmorphic features that may include epicanthal folds, a flat bridge of the nose, and low-set ears.
• Skin appears scaly and dry (figure 3).

Laboratory Diagnosis

• Levels of plasma cholesterol and high- and low-density lipoprotein are often moderately reduced.
•  Blood phytanic acid levels are elevated.
• Cerebrospinal fluid (CSF) shows a protein level of 100-600 mg/dL.
• Routine laboratory investigations of blood and urine do not reveal any consistent diagnostic abnormalities.
• Phytanic oxidase activity estimation in skin fibroblast cultures is important

Imaging

Skeletal radiography is required to estimate bone changes.

Treatment

• Eliminate all sources of chlorophyll from the diet.
  • The major dietary exclusions are green vegetables (source of phytanic acid) and animal fat (phytol).
  • The aim of such dietary treatment is to reduce the daily intake of phytanic acid from the usual level of 50 mg/d to less than 5 mg/d.
• Plasmapheresis – Patients may also require plasma exchange (plasmapheresis) in which blood is drawn, filtered, and reinfused back into the body, to control the buildup of phytanic acid.

• • The main indication for Plasmapheresis in patients with Refsum disease is a severe or rapidly worsening clinical condition.
  • A minor indication is the failure of dietary management to reduce a high plasma phytanic acid level.

Prognosis –in untreated patients generally is poor. Dysfunction of myelinated nerve fibers and the cardiac conduction system leads to central and peripheral neuropathic symptoms, impaired vision, and cardiac arrhythmias. The latter frequently are the cause of death.

In early diagnosed and treated cases, phytanic acid decreases slowly, followed by improvement of the skin scaling and, to a variable degree, the reversal of recent neurological signs. Retention of vision and hearing are reported.

Pharmacological upregulation of the omega-oxidation of phytanic acid may form the basis of the new treatment strategy for adult Refsum disease in the near future.