A racemic mixture of a-hydroxy acids can be transformed to the desired optically active L-amino acid by means of a reaction route that goes through the intermediate formation of the corresponding a-keto acid. The continuous realization of this process is possible in a multienzyme membrane reactor, which has been described earlier as to its technical characteristics.'The feasibility of the process is demonstrated by the transformation of (LD)-hCtate via pyruvate to L-alanine, as first suggested by Mosbach ( See FIG. I). ' In the process, a form of NAD covalently bound to water-soluble polyethylene glycol' (PEG-20000-NAD) together with the three enzymes L-LDH, D-LDH and ALADH is retained by an ultrafiltration membrane. A necessary condition for the steady-state continuous realization of the process is that the concentration of the intermediate (pyruvate) in the reactor has to be held at some level above zero by continuous feeding. If this is not done, the soluble intermediate will be washed out from the reactor and the reaction path will be shifted entirely to the formation of the reduced coenzyme form, NADH, that is, the reaction will be stopped. On the other hand, it is conceivable that large intermediate concentrations will favor the reverse reaction to lactate and theoxidized coenzyme form, NAD'; thus, an optimal feed concentration of the intermediate will be expected.The dependence of the rates of the four individual reaction steps on substrate and product concentrations has been investigated by measuring initial rates and subsequently verified by measuring in the entire range of conversion. The data-fitting has been accomplished by a nonlinear optimization p r~c e d u r e .~The theoretical analysis below is based on the initial rate kinetic data.On the basis of the assumption that the sorption capacities of the enzymatic complexes are practically too low to have any measurable dynamic effect, the following model differential equations can be established for the dynamics of the reaction system under consideration: -d(lac) lac, -lac --dt 7 R , + Rz d(a1a) ala, -ala + R , -R4 --dt T 91(3)