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Roundup, as we all know, is a herbicide that kills weeds. It does this by preventing the production of amino acids in plants. One of these amino acids is tyrosine. Tyrosine is a nonessential amino acid so it is produced in the human body. So is Roundup toxic because it could also affect the human tyrosine synthesis pathways? Or are the pathways for tyrosine synthesis different for humans and plants?

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    $\begingroup$ "Glyphosate specifically inhibits an enzyme that is essential to plant growth; this enzyme is not found in humans or other animals, contributing to the low risk to human health from the use of glyphosate according to label directions." monsanto.com/products/pages/… $\endgroup$
    – AMR
    Oct 25, 2015 at 4:03

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Glyphosate works by inhibiting 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase. Plants and microbes have this gene, and humans do not.

So, Roundup is toxic to plants because because it inhibits the production of critical amino acids as you surmised (EPSPS makes 5-enolpyruvyl-shikimate-3-phosphate (ESP) which is used as precursor to not only amino acids but also other plant hormones and aromatic chemicals). It does not do this in humans.

It is a very controversial compound though, and weight of evidence for human toxicity has not clearly overwhelmingly swung one way or the other yet. There are some very impassioned believers on both sides of the fence, and I suggest reading pubmed epidemiology studies to make up your own mind.

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The pathways are different. Humans synthesize tyrosine by hydroxylating phenylalanine. Plants use the shikimate pathway to produce all of the aromatic amino acids. Roundup inhibits an enzyme in the shikimate pathway but does not affect the human enzyme involved in producing phenylalanine. None of this is to say, however, that it is or isn't toxic.

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    $\begingroup$ The MSDS shows it to be pretty much non-toxic. LD50 is greater than 5g/kg in rats... California wants to list it as a carcinogen but that may be more political than fact based. $\endgroup$
    – AMR
    Oct 25, 2015 at 4:22
  • $\begingroup$ @AMR Thank you for the info. It seems most things are "known to the state of California" to cause cancer. $\endgroup$
    – canadianer
    Oct 25, 2015 at 4:59

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