On-line flow-injection monitoring of the enzyme inductor l-phenylalanine in the continuous cultivation of rhodococcus sp. M4

The o-phthaldialdehyde (OPA) assay for amino acids was adapted for flow injection analysis. With a dual-channel manifold and 2-μl injections, injection rates of 90–100 h^?1 were possible. Depending on the selected fluorimeter sensitivity, linear response ranges of 0.1–1.2 mM or 1–30 mM were obtained for l-phenylalanine with a relative standard deviation (RSD) of 0.9% for the slope of the calibration line. For ten injections of a 0.1 mM standard, the RSD was 1.6%. The detection limit was 0.01 mM. When the flow-injection method was combined with a fermentation sampling device and controlled by a timer, it was possible to monitor the decrease of l-phenylalanine on-line over a period of 87 h in the initial phase of a continuous cultivation of Rhodococcus sp. M4 producing the intracellular enzyme l-phenylalanine dehydrogenase. Correlation of the results with those obtained with an amino acid analyzer was good. Because OPA reacts with all primary amino groups, the applicability of the proposed method is restricted to cases in which only one amino acid and only small amounts of other amino group donators are present.