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

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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.

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