Two isoenzymes of an NADP' -dependent cinnamyl alcohol dehydrogenase and an NAD + -dependent aliphatic alcohol dehydrogenase were extracted from cell suspension cultures of soybean (Glycine max L., var. Mandarin) which form lignin during growth. These enzymes could be separated from each other by chromatography on DEAE-cellulose and hydroxyapatite. The cinnamyl alcohol dehydrogenase isoenzymes were partially purified by (NH&SO, fractionation, and column chromatography on DEAE-cellulose, SephadexG-100, and hydroxyapatite. The molecular weight of the enzymes were estimated by the elution volumes from a Sephadex G-100 column and were found to be about 43000 (isoenzyme 1) and 69000 (isoenzyme 2). Maximum rates of reaction were observed in the case of coniferyl alcohol oxidation at pH 9.2 (isoenzyme 1) and pH 8.8 (isoenzyme 2); in the reverse reaction pH 6.5 was optimal for isoenzyme 2.Whereas isoenzyme 1 is specific for coniferyl alcohol, isoenzyme 2 can also oxidize cinnamyl alcohol and a number of substituted cinnamyl alcohols. K , values for substituted cinnamaldehydes are 3 -11 times lower than for the corresponding alcohols. Neither isoenzyme reacted with benzyl alcohol, anisic alcohol or ethanol. Substrate inhibition for the forward and reverse reaction was found with isoenzyme 2 but not with isoenzyme 1. The equilibrium constant was determined to be about lo9 in favour of coniferaldehyde reduction. The possible role of the cinnamyl alcohol dehydrogenase in lignin biosynthesis is discussed.We have previously shown that an enzyme preparation from cell suspension cultures of soybean (Glycine max) catalyses the NADPH-dependent reduction We now describe the partial purification of two specific cinnamyl alcohol dehydrogenase isoenzymes from soybean cell suspension cultures. MATERIALS AND METHODS MatrriulsSubstituted cinnamic acids and coniferyl alcohol were purchased from C. Roth (Karlsruhe). Lithium aluminium-tris (tert-butyloxyhydride) was obtained from Fluka AG (Buchs, SG, Switzerland). All biochemicals were purchased from Boehringer Mannheim GmbH (Mannheim) or Serva (Heidelberg).Svnthesis of Substituted Cinnamyl Alcohols and Cinnamaldehydes p-Coumaryl alcohol [7] and sinapyl alcohol [8] were synthesized according to published methods. In analogy to these methods, 3,4-dimethoxycinnamyI alcohol (A, , , = 267 nm) was obtained from methyl 3,4-dimethoxycinnamate by reduction with LiAIH,. p-Coumaryl, coniferyl and sinapyl alcohol were purified by paper chromatography on Whatman 3 MM (prewashed with 10mM EDTA, 10% acetic acid, methanol, water) with the solvent system of butanol (water-saturated) containing 2 7,; NH3 [9]. For the purification of 3,4-dimethoxycinnamyl alcohol, 2 %, acetic acid was used as solvent system.

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.

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