Enzymic Synthesis of Lignin Precursors

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 40 000 +/- 500. The enzyme is strongly inhibited by thiol reagents. Various metal chelators as well as the nonchelating 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.