A new S-adenosylmethionine decarboxylase from soybean axes

A new active S-adenosylmethionine decarboxylase (EC 4.1.1.50) (SAMDC II) was extracted from soybean (Glycine max) axes. The enzyme was purified to homogeneity by ammonium sulfate fractionation, DEAE-Sepharose and methylglyoxalbis(guanylhydrazone) (MGBG)-Sepharose 6B chromatographies. The molecular weight of the native enzyme was 110,000, while the subunit molecular weights were 66,000 and 58,000, indicating a heterodimeric structure. The Km value of the enzyme for S-adenosylmethionine was 16 microM, which is two times higher than that of previously reported S-adenosylmethionine decarboxylase (SAMDC I) (8.1 microM). The specific activity of SAMDC II during the seed growth increased rapidly and reached its maximum on the second day after germination whereas that of SAMDC I reached its peak on the fourth day. MGBG was shown to inhibit SAMDC II competitively like SAMDC I. Carbonyl and sulfhydryl group specific reagents modified SAMDC II, resulting in the loss of enzymatic activity. Agmatine, the product of arginine decarboxylation catalyzed by arginine decarboxylase, inhibited the SAMDC II competitively (Ki = 40 microM) while it inhibited the SAMDC II non-competitively (Ki = 600 mM). The possible role of the chronological appearance of SAMDC II and SAMDC I, and properties of the enzyme are briefly discussed in connection with polyamine biosynthesis in soybean axes.