Do-It-Yourself Vitamin C

People can’t live without vitamin C, but everyone’s heard a story about that one college student who tried to live off of nothing but chips and ramen [1]. And were it not for a mutation a few millions of years ago, the story wouldn’t end in the clinic with a diagnosis of scurvy.  We could synthesize all the vitamin C we need with a few repairs to this week’s gene of interest: L-gulono-γ-lactone oxidase or GLO.

Vitamin C has a several important jobs. It’s an antioxidant – neutralizing nasty free-radicals [2].  It stimulates the immune system, possibly by acting as an oxidizer to destroy bacteria and viruses during infection [3].  And it’s required to make collagen, a protein found in nearly every tissue in the body [4].

Vitamin C deficiency leads to scurvy, a disease forever linked to pirates and sailors. On lengthy sea voyages, fresh fruits and vegetables didn’t last long. Without any vitamin C, sailors became sluggish, their gums would bleed, old wounds would ache, then jaundice, fever, convulsions, and finally death [5]. A horrible way to go and easily remedied by an orange or two. Multiple times in history citrus was identified and dismissed as a cure for scurvy.  Many were confused why sucking on a lime might cure scurvy, but the juice was less effective and preserved fruit even less so. Vitamin C degrades in light and heat, so processing and preservation would make the food useless. It wasn’t until the 1930’s that vitamin C was isolated and identified as the cure to scurvy.

Raw meat can even cure scurvy, because most animals (like plants) make their own vitamin C.  Humans are among a minority of animals – along with guinea pigs, bats, and other primates – that can’t synthesize their own vitamin C.  Most animals convert glucose into vitamin C using several specialized enzymes [6].  We lost the ability due to mutations in the gene for GLO, the enzyme responsible for the last step in vitamin C synthesis [7].

We still have the GLO gene, it just doesn’t work anymore.  Ever since that first mutation knocked out GLO gene function 61 million years ago, more mutations have eaten away at the gene.  Every gene is made up of several sections of coding DNA (called exons) separated by sections of non-coding DNA. Only the exons are used to build a protein. Of the 12 exons in the GLO gene, humans have lost 7 [7].

Human GLO pseudogene compared to the functional GLO gene from a rat [7].

GLO is an example of a pseudogene – a gene that’s lost its function.  It differs from other mutated genes we’ve discussed because it’s harmless, as long as we eat enough vitamin C to compensate. Early primates may have lost the GLO gene because their diet was rich in vitamin C. Gorging on fruits and veggies, there was no real disadvantage to loosing natural vitamin C production [6].

Our genome is likely full of psuedogenes. Each one sits dormant in our genome, a relic of our evolution. For example, scientists have identified 390 different olfactory genes, responsible for our sense of smell, but there are another 468 olfactory pseudogenes [8]. Compared to what we’ve lost, our nose is basically worthless. Think of all the smells we’re missing.

So that’s why you should eat your fruits and veggies – because evolution has failed you. But considering the American diet, it’s probably a good thing we can’t make our own vitamin C. If not for a horrifying death by scurvy, too many people would opt to avoid anything green for tasty, tasty junk food.

---

References
[1] “Scurvy Is a Serious Public Health Problem”. Slate. 20 November 2015.
[2] Padayatty SJ, Katz A, Wang Y, Eck P, Kwon O, Lee JH, Chen S, Corpe C, Dutta A, Dutta SK, Levine M. (2003). “Vitamin C as an antioxidant: evaluation of its role in disease prevention”. J Am Coll Nutr 22 (1): 18–35. PMID 12569111
[3] Wintergerst ES, Maggini S, Hornig DH.(2006) “Immune-enhancing role of vitamin C and zinc and effect on clinical conditions”. Ann Nutr Metab 50(2):85-94. PMID 16373990
[4] Peterkofsky B (1991). “Ascorbate requirement for hydroxylation and secretion of procollagen: relationship to inhibition of collagen synthesis in scurvy”. Am. J. Clin. Nutr. 54 (6): 1135S–1140S. PMID 1720597
[5] “Scott and Scurvy”. Idlewords.com. 6 March 2010.
[6] “Plagiarized Errors and Molecular Genetics”. Talkorigins.org. 5 May 2003.
[7] Drouin G, Godin JR, Pagé B. (2011). “The Genetics of Vitamin C Loss in Vertebrates”. Curr Genomics. 12 (5): 371–378. PMCID PMC3145266
[8] “The Smell of Evolution”.  National Geographic. 11 December 2013.