An automated fluorometric procedure for histidine in blood

We have adapted the sample procurement procedure of blood spotted on filter paper for use with an automated fluorometric method for histidine which is based on the reaction of the amino acid with O-phthaldialdehyde in alkaline solution. Histidine was eluted from blood spotted on filter paper by an alcoholic diluent, and samples and standards were analyzed at a rate of 40/h with a 1:2 sample-to-wash ratio. The reaction product was stabilized by addition of a phosphoric acid solution and measured in a standard fluorometer. The fluorescence is linear for increasing concentrations of histidine, carry-over is negligible, and reproducibility is excellent. The mean percentage recovery was 92% over a concentration range of 2–40 mg/100 ml histidine added to whole blood. Blood obtained from 130 newborn infants 1–3 days after birth revealed a mean and standard deviation of 3.7 ± 0.9 mg/100 ml. The method is rapid and sensitive, and should be applicable to detection of histidinemia and to studies on other clinical and biological problems in human populations.