NEW CHEMOENZYMATIC SYNTHESIS OF VERY HIGH SPECIFIC RADIOACTIVITY [S-35](S)METHIONINE

[S-35] H2SO4 was quantitatively reduced to [S-35] H2S with a pretreate d mixture of HI/HCl/H3PO2. [S-35] H2S was used in a (S) homoserine sul fhydrylase [EC.4.2.99.10] catalysed thiol exchange reaction with O-ace tyl-(S) homoserine to afford [S-35] (S) homocysteine. The latter was m ethylated (CH3I/NaOH) in situ giving rise to [S-35] (S) methionine whi ch was purified by HPLC. Overall yield 10-15%; specific radioactivity : > 46 Tbq/mmol e.g. > 1243 Ci/mmol; chemical and radiochemical puriti es: > 98%.