An Essential Arginyl Residue in the Tonoplast Pyrophosphatase from Etiolated Mung Bean Seedlings

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Abstract
Tonoplast membrane of etiolated mung bean (Vinga radiata. L.) seedlings contained H+-translocating pyrophosphatase (PPase). Modification of tonoplast vesicles and partially purified PPase from etiolated mung bean seedlings with arginine-specific reagents, phenylglyoxal (PGO) and 2,3-butanedione (BD), resulted in a marked decline in H+-translocating PPase activity. The half-maximal inhibition was brought about by 20 millimolar PGO and 50 millimolar BD for membrane bound and 1.5 millimolar PGO and 5.0 millimolar BD for soluble PPase, respectively. The substrate, Mg2+-pyrophosphate, provided partial protection against inactivation by these reagents. Loss of activity of partially purified PPase followed pseudo-first order kinetics. The double logarithm plots of pseudo-first order rate constant versus reagent concentrations gave slopes of 0.88 (PGO) and 0.90 (BD), respectively, suggesting that the inactivation may possibly result from reaction of at least one arginyl residue at the active site of H+-translocating PPase.