Phytanic Acid Metabolism
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The relative proportions of large and small intestine differs among humans and great apes; a difference attributable to the variation in dietary intake among these species. These differences may have lead to systemic changes of phytanic acid at the physiological and cellular levels between humans and great apes. Additionally, humans evolved differences in the rates of phytanic acid catabolism, as over-accumulation of phytanic acid is associated with a number of physiological disorders
Humans and great apes show significant differences in their gut proportions, which correlate well with differences in dietary intake. Humans consume more cooked food and animal products and, therefore, the major of their gut volume is occupied by small intestine. However, the great apes major gut volume is large intestine, which allows them to derive higher metabolic energy from plant products than humans. One of the major consequences of the different gut morphologies is phytanic acid metabolism. Phytanic acid is produced in the hindgut by microbes, by fermentation of plant products to phytol. Over-accumulation of phytanic acid in humans is associated with many diseases such as peripheral polyneuropathy, cerebellar ataxia, retinitis pigmentosa, anosmia, and hearing loss. This can also result in cardiac arrhythmias, shortened metacarpals or metatarsals, and ichthyosis. The differences in the levels of phytanic acid (storage and metabolism) between humans and great apes are suggestive of an evolutionary strategy to diminish phytanic acid over-accumulation.
Humans display differential catabolism of phytanic acid derived from phytol (plant product) and animal products (herbivores). This was determined from the following observations:
- Phytanic acid levels in RBCs differ between humans and all other great apes, which have roughly the same intake of phytol.
- In humans phytanic acid is oxidized in skin cultured fibroblast cells at a different rate than in great apes.
- The genes responsible for catabolism of phytanic acid are up regulated in humans as compared to great apes.
- The genes responsible for lipid metabolism show differential expression among human and great ape populations.
The difference seems to be derived, as it is not observed in any other apes. To date there are no studies investigating the differences among different hominids.
It is hard to generalize among all human populations as only a limited number of humans have been tested, only one mechanism of phytanic acid metabolism has been explored, and studies are limited for human populations with differing diets.
The following are all hypothesized to contribute to prevention of phytanic acid over-accumulation in humans:
- The decreased gut volume to body mass ratio may lead to lower accumulation of phytols.
- The rate at which phytanic acid dissociates and oxidizes is faster in humans (shown for fibroblast cells). Phytanic acid activates the PPAR-alpha transcription factor which influences lipid metabolism.
- Although speculative, human microbial flora may be less efficient at the fermentation process leading to phytol.
Phytanic acid accumulation leads to a number of diseases and, therefore, negative selective pressure resulted in processes resulting in increased phytanic acid degradation in humans.
Based on the medical conditions caused by phytanic acid over-accumulation, differences in phytanic acid catabolism could have influenced differences between human and great ape nervous, cardiovascular, and skeletal systems.
Two examples of possibly convergent evolution are below:
- Freshwater sponges contain terpenoid acids such as 4,8,12-trimethyltridecanoic, phytanic and pristanic acids, which indicate that these acids may have chemo taxonomical significance for both marine and freshwater sponges.
- Insects, such as the sumac flea beetle, are reported to use phytol and its metabolites (e.g. phytanic acid) as chemical deterrents against predation. These compounds originate from host plants.
Dietary influences on tissue concentrations of phytanic acid and AMACR expression in the benign human prostate., , Prostate, 02/2015, Volume 75, Issue 2, p.200-10, (2015)
Identification of differences in human and great ape phytanic acid metabolism that could influence gene expression profiles and physiological functions., , BMC Physiol, 2010, Volume 10, p.19, (2010)