Enzymic Synthesis of Lignin Precursors

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Abstract
Isoenzyme 2 of cinnamyl‐alcohol dehydrogenase from soybean suspension cultures was purified about 3800‐fold to apparent homogeneity by an improved purification procedure involving biospecific elution of the enzyme from a NADP+‐agarose column. On sodium dodecylsulfate gels the dehydrogenase showed only one protein band with Mr 40000 ± 500. The enzyme is strongly inhibited by thiol reagents. Various metal chelators as well as the non‐chelating 7,8‐benzoquinoline also inhibited enzyme activity. Inhibition by 10 mM 1,10‐phenanthroline could be partially reversed by addition of Zn2+. 1,10‐Phenanthroline and 7,8‐benzoquinoline are non‐competitive inhibitors with respect to NADP+. The presence of zinc in the dehydrogenase was proved by atomic absorption spectroscopy and by specific incorporation of 65Zn into the enzyme. In steady‐state kinetics inhibition patterns were obtained which are consistent with an ordered bi‐bi mechanism in which NADP(H) is the first substrate to bind and the last product released. The cinnamyl‐alcohol dehydrogenase belongs to the A‐specific dehydrogenases and removes the pro‐R hydrogen from coniferyl alcohol. The enzyme shows many similarities with alcohol dehydrogenases from horse and rat liver and from yeast.