I work out.  I take a whey protein supplement.  The whey protein is made of amino acids bonded together.  On the back of the can, it has a chart of the typical amino acid masses that are in the average serving.  For example, it says that there is 450mg of Tryptophan per serving. 

When Tryptophan is taken in its free-form, 450mgs would have very noticable mental effects because  my body would convert it into serotonin.  However, when I take the tryptophan, as a BCAA in the whey, there are no mental effects.

I realize that my body uses amino acids for other chemical synthesis besides protein.  It seems though, that my body would prefer to use a free-form amino acid for these non-protein purposes.

Is this true? Does the body primarily use free-form amino acids for non-protein purposes. It certainly seems that way from my experience. There doesn't seem to be any good info on the net about this.  I'm sure a biologist must know.

Thanks

It is most unlikely that if you ate 450mgs of "free" Tryptophan it would have any central or "mental" effects. Some but not all would indeed be converted to 5HT (serotonin) but that would be in the periphery. The amount that would then cross the blood brain barrier to act on the brain would be very small indeed. 5HT as a neurotransmitter is locally synthesised in the brain - very little of that comes from peripheral synsthesis.

There are case reports of toxic effects of tryptophan ingestion (in body builders) that caused liver and skin problems - but that was ingestion of 15-10 grams or even more (ie 30-40 times higher than you describe), and even then central effects were not terribly prominent!