A fast and sensitive method for measuring picomole levels of total free amino acids in very small amounts of biological tissues

In the present study we describe a simple and fast method to measure the co ncentration of total free amino acids in very small amounts of biological t issues. The procedure described here is based on the reaction of free amino acids with o-phthaldialdehyde (OPA) in the presence of a reducing agent, b eta -mercaptoethanol (MET), to give a complex which can be measured by fluo rescence. It is a very rapid process and has the same reliability as the co nventional ninhydrin method of Moore and Stein but is about 500 times more sensitive. The sensitivity of the new protocol is such to permit the determ ination with high reliability of very small amounts of free amino acids at picomole levels, either in a standard amino acid mixture or in biological t issues, without chromatographic separation of the amino acids. It is partic ularly useful when the amount of the sample is very low, e.g. on a single p ituitary or pineal gland of small animals or on single cells, such as oocyt es or eggs, as well as single ganglions or axons of marine invertebrates.