Purification and Properties of a Unique Nucleotide Pyrophosphatase/Phosphodiesterase I That Accumulates in Soybean Leaves in Response to Fruit Removal

Several unique proteins accumulate in soybean (Glycine max) leaves when the developing fruits are removed. In the present study, elevated levels of nucleotide pyrophosphatase and phosphodiesterase I activities were present in leaves of defruited soybean plants. The soluble enzyme catalyzing these reactions was purified nearly 1000-fold, producing a preparation that contained a single 72-kD polypeptide. The molecular mass of the holoenzyme was approximately 560 kD, indicating that the native enzyme was likely octameric. The purified enzyme hydrolyzed nucleotide-sugars, nucleotide di- and triphosphates, thymidine monophosphate p-nitrophenol, and inorganic pyrophosphate but not nucleotide monophosphates, sugar mono- and bisphosphates, or NADH. The subunit and holoenzyme molecular masses and the preference for substrates distinguish the soybean leaf nucleotide pyrophosphatase/phosphodiesterase I from other plant nucleotide pyrophosphatase/phosphodiesterase I enzymes. Also, the N-terminal sequence of the soybean leaf enzyme exhibited no similarity to the mammalian nucleotide pyrophosphatase/phosphodiesterase I, soybean vegetative storage proteins, or other entries in the data bank. Thus, the soybean leaf nucleotide pyrophosphatase/phosphodiesterase I appears to be a heretofore undescribed protein that is physically and enzymatically distinct from nucleotide pyrophosphatase/phosphodiesterase I from other sources, as well as from other phosphohydrolytic enzymes that accumulate in soybean leaves in response to fruit removal.