There could be lots of answers to this question, depending on the aminoacid, etc. The vast majority of specific free-form aminoacid actions would be related to intestinal absorption and its mechanism, since in the portal circulation we can find only free amino acids (unless you are a baby < 48h and other special cases). For example, local consumption of glutamine etc. Other interesting actions might occur by taking a great dose of a specific amino acid, a dose that would be difficult to obtain with food (for example, 10gr of carnitine, if we include it in amino acids). And everything would have to do with the four basic amino acid transport systems of the intestines etc... lengthy subject...