Biotransformation of benzaldehyde t o L-phenylacetylcarbinol (L-PAC) as a key intermediate for L-ephedrine has been evaluated using immobilized pyruvate decarboxylase (PDC) from Candida uti/is. PDC immobilized in spherical polyacrylamide beads was found t o have a longer half-life compared with free enzyme. In a batch process, the immobilized PDC generally produced lower L-PAC than free enzyme at the same concentrations of substrates due to increased by-products acetaldehyde and acetoin and reduced benzaldehyde uptake. With immobilized PDC, L-PAC formation occurred at higher benzaldehyde concentrations (up to 300 m M ) with the highest L-PACconcentration being 181 mM(27.1 g/L). For a continuous process, when 50 m M benzaldehyde and 100 m M sodium pyruvate were fed into a packed-bed reactor at 4°C and pH 6.5, a productivity of 3.7 mM/h (0.56 g/L . h) L-PAC was obtained at an average concentration of 30 mM (4.5 g/L). The half-life of immobilized PDC reactor was 32 days. 0 1996 John Wiley & Sons, Inc. Key words: biotransformation * L-phenylacetylcarbinol immobilization pyruvate decarboxylase Candida utilis version to by-products acetaldehyde and acetoin. The main factors limiting the further development of the L-PAC process are the high benzaldehyde toxicity and the product inhibition which resulted from accumulation of L-PAC (and possibly also acetaldehyde).
Previously it has been reported that cell and enzyme immobilization in suitable gels and matrices could minimize substrate inhibition effects via diffusional limitation and substrate gradients which exist within the immobilizing material.s*7,8 This concept has been investigated in the present study using immobilized PDC partially purified from C. utilis and evaluating L-PAC production under conditions similar to those for the partially purified free enzyme.14 The potential use of immobilized PDC in a continuous biotransformation process has been assessed, also, as a possible means of achieving long-term L-PAC production.