Enzymatic synthesis of carbon-11 labeled methionine and its derivatives with immobilized γ-cyano-α-aminobutyric acid synthase

No carrier added (nca) 2-amino-4-([11C]methylthio) butyric acid ([11C]methionine, [11C]MET) was prepared by the reaction of O-acetyl-image-homoserine and [11C]methanethiol using immobilized γ-cyano-α-aminobutylic acid synthase. If one passed [11C]methyl iodide through a DMF solution containing NaSH at 120°C, approximately 90% of the radioactivity was converted to [11C]methanethiol with the radiochemical purity of >99%, which was trapped in acetone cooled at −30 to −50°C. After the buffer containing a substrate for the enzyme reaction was added, the aqueous acetone solution was passed through a small column packed with the immobilized enzyme (6.0 mm i.d.×17 mm length) at room temperature for approximately 1 min. Approximately 80% of [11C]methanethiol applied to the column was recovered as only [11C]MET. After dryness was achieved in vacuo, the residue was dissolved in physiological saline and the solution was passed through a membrane filter. The radiochemically and enatiomerically pure [11C]MET was ready for injection within 15 min after the end of irradiation. [11C]MET analogues such as [11C]ethionine and [11C]propionine were also prepared from corresponding [11C]alkanethiols.