Purification and characterization of homospermidine synthase in Acinetobacter tartarogenes ATCC 31105

Homospermidine synthase, catalyzing the formation of homospermidine [H2N(CH2)4NH-(CH2)4NH2] from putrescine and NAD+ with concomitant liberation of NH3, 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-aminobutyraldehyde and subsequent reduction of the putative Schiff base formed between this aldehyde and a second molecule of putrescine to homospermidine. The Km values for putrescine and NAD+ were 280 and 18 microM, respectively. 1,3-Diaminopropane and cadaverine were inactive as substrates, and NAD+ could not be replaced by NADP+. 1,3-Diaminopropane and NADH were potent competitive inhibitors. The enzyme had a pH optimum of 8.7, was most active at 30 degrees C, and required K+ and dithiothreitol for full activity. Putrescine and NAD+ protected the enzyme from the inhibition by thiol reagents. The NH2-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.