When there are errors in genetic expression of enzymes, biochemically the body does go wonky. Many things may fail to work properly. That is why we have syndromes and disease. When certain cofactors (vitamins) are missing or are needed in dependency states (pyroluria), you cannot blame the substrate, which in this case is the essential amino acid tryptophan.

However, in reality tryptophan is NOT toxic. It is an essential amino acid, which means we have to have it ...or suffer.

Genetic errors in metabolism occur for many other substances, and I guess the interpretation of that is what is being questioned here.

A very special case, does not generalize to everyone, or to the general population. When things break down, due to deficiency or genetic error, the conclusions are not generalizable to others who do NOT have those errors.

This is a very good resource, and does not include tryptophan "toxicity".
http://lpi.oregonstate.edu/infocente...cin/index.html

The only significant tryptophan toxicity occurred during manufacture years ago in Japan, which led to EMS due to a chemical contaminant during that manufacture (which was called Peak X). This has been corrected and tryptophan is now available as a single amino acid again. Much of what I have read states that high doses are self limiting, and that more conservative use
reaps better results.