| dc.description |
Homospermidine synthase, catalyzing the formation of homospermidine [H<sub>2</sub>N(CH<sub>2</sub>)<sub>4</sub>NH-(CH<sub>2</sub>)4NH<sub>2</sub>] from putrescine and NAD<sup>+</sup> with concomitant liberation of NH<sub>3</sub>, was purified 600-fold over the crude extract with a yield of about 14% to homogeneity from Acinetobacter tartarogenes ATCC 31105. The enzyme had a native molecular mass of 102 kDa, with a pI of 5.0, and was apparently composed of two identical subunits (52 kDa), suggesting that a single protein catalyzes two serial reactions, oxidation of putrescine to 4-aminobutyral-dehyde and subsequent reduction of the putative Schiff base formed between this aldehyde and a second molecule of putrescine to homospermidine. The K<sub>m</sub> values for putrescine and NAD<sup>+</sup> were 280 and 18 μM, respectively. 1, 3-Diaminopropane and cadaverine were inactive as substrates, and NAD<sup>+</sup> could not be replaced by NADP<sup>+</sup>. 1, 3-Diaminopropane and NADH were potent competitive inhibitors. The enzyme had a pH optimum of 8.7, was most active at 30°C, and required K<sup>+</sup> and dithiothreitol for full activity. Putrescine and NAD<sup>+</sup> protected the enzyme from the inhibition by thiol reagents. The NH<sub>2</sub>-terminal amino acid sequence was AQWPVYGKISGPVVI. Some of these properties were compared with those of the homospermidine synthases from a photosynthetic bacterium, Rhodopseudomonas viridis and a plant, Lathyrus sativus. |
