Kinetic aspects of the bioconversion of L-tyrosine into L-DOPA by cells of Mucuna pruriensL. Entrapped in different matrices

Plant cells of Mucuna pruriens L. entrapped In calcium alginate, calcium pectinate, agarose, or gelatine were able to convert L-tyrosine to L-DOPA, which was released Into the medium. Michaelis-Menten kinetics could be applied on the entrapped cells, based on the measurement of initial rates of L-DOPA production. The calculated apparent affinity constants were comparable with the affinity constants obtained with enzyme preparations. Comparison of the apparent maximum rate of bioconversion of the entrapped cells and the maximum rate of bioconversion of a derived cell homogenate indicated that the systems were not operating optimally. Measurement of the effective diffusion coefficients of L-tyrosine pointed out that this substrate could diffuse freely into the matrices. From the initial rates of bioconversion and the effective diffusion coefficients, the observable modulus was calculated for each system. The obtained values confirmed that the diffusional supply rate of L-tyrosine was not the limiting factor. For oxygen, which was utilized for byconversion as well as for cell respiration, the calculated observable moduli was directed toward strong oxygen transfer limitations. The values found for the oxygen consumption indicated that the entrapped cells remained partly or totally viable in the four matrices tested. Based on the highest viability and the highest rates of bioconversion, it was concluded that alginate-entrapped cells of M. pruriens formed the most suitable biocatalytic system for the production of L-DOPA from L-tyrosinre.