Comparative kinetic studies on the two interconvertible forms of Streptococcus faecalis inorganic pyrophosphatase

Role of H(+)-pyrophosphatase activity in the regulation of intracellular pH in a scuticociliate parasite of turbot: Physiological effects

The scuticociliatosis is a very serious disease that affects the cultured turbot, and whose causal agent is the anphizoic and marine euryhaline ciliate Philasterides dicentrarchi. Several protozoans possess acidic organelles that contain high concentrations of pyrophosphate (PPi), Ca(2+) and other elements with essential roles in vesicular trafficking, pH homeostasis and osmoregulation. P. dicentrarchi possesses a pyrophosphatase (H(+)-PPase) that pumps H(+) through the membranes of vacuolar and alveolar sacs. These compartments share common features with the acidocalcisomes described in other parasitic protozoa (e.g. acid content and Ca(2+) storage). We evaluated the effects of Ca(2+) and ATP on H (+)-PPase activity in this ciliate and analyzed their role in maintaining intracellular pH homeostasis and osmoregulation, by the addition of PPi and inorganic molecules that affect osmolarity. Addition of PPi led to acidification of the intracellular compartments, while the addition of ATP, CaCl2 and bisphosphonates analogous of PPi and Ca(2+) metabolism regulators led to alkalinization and a decrease in H(+)-PPase expression in trophozoites. Addition of NaCl led to proton release, intracellular Ca(2+) accumulation and downregulation of H(+)-PPase expression. We conclude that the regulation of the acidification of intracellular compartments may be essential for maintaining the intracellular pH homeostasis necessary for survival of ciliates and their adaptation to salt stress, which they will presumably face during the endoparasitic phase, in which the salinity levels are lower than in their natural environment.

Regulatory role of polyamine in the acid phosphatase from potato tubers

Effects of polyamine and metal ions on the new type of acid phosphatase purified from potato (Solanum tuberosum L. Irish Cobbler) tubers were analyzed. The enzyme belongs to nonspecific acid phosphatase family (EC 3.1.3.2), which hydrolyzes various phosphorylated substrates. The enzyme hydrolyzed inorganic pyrophosphate as a preferred substrate, and exhibited the hyperbolic kinetics with respect to the substrate, inorganic pyrophosphate in the absence of metal cations. Polyamine activated the enzyme effectively by lowering the K(m) value without appreciable changes in the maximal velocity. The most effective polyamines as activators were spermine and spermidine. Mg(2+) ion increased the K(m) value without affecting the maximal velocity of the enzyme, but Ca(2+) ion decreased both the K(m) and V(max) values. Increasing concentrations of spermine also decreased the K(m) value irrespective of Mg(2+) ion included, but gave a constant K(m) and V(max) values in the absence and presence of Ca(2+) ion. Action of spermine and metal ions can be explained by the complex formation with the substrate pyrophosphate. The acid phosphatase from potato can utilize the pyrophosphate-spermine or pyrophosphate-Ca(2+) complex as the preferred substrates. However, the enzyme can use the pyrophosphate-Mg complex with a weak affinity for the active site. Polyamine activates acid phosphatase in the absence and presence of metal cations, and activation by polyamine of the enzyme may contribute to the stimulation of starch biosynthesis and the control of glycolysis/gluconeogenesis by regulating PPi levels in growing potato tubers.

Substrate kinetics of the tonoplast h-translocating inorganic pyrophosphatase and its activation by free mg

To clarify the kinetic characteristics and ionic requirements of the tonoplast H(+)-translocating inorganic pyrophosphatase (H(+)-PPiase), PPi hydrolysis and PPi-dependent H(+) transport were studied in tonoplast vesicles isolated from leaf mesophyll tissue of Kalanchoë daigremontiana Hamet et Perrier de la Bâthie. The tonoplast H(+)-PPiase showed an absolute requirement for a monovalent cation and exhibited hyperbolic kinetics with respect to cation concentration. H(+)-PPiase activity was maximal in the presence of K(+) (K(50) approximately 3 millimolar), with PPi-dependent H(+) transport being more selective for K(+) than PPi hydrolysis. When assayed in the presence of 50 millimolar KCl at fixed PPi concentrations, H(+)-PPiase activity showed sigmoidal kinetics with respect to total Mg concentration, reflecting a requirement for a Mg/PPi complex as substrate and free Mg(2+) for activation. At saturating concentrations of free Mg(2+), H(+)-PPiase activity exhibited Michaelis-Menten kinetics towards MgPPi(2-) but not Mg(2)PPi, demonstrating that MgPPi(2-) was the true substrate of the enzyme. The apparent K(m) (MgPPi(2-)) for PPi hydrolysis (17 micromolar) was significantly higher than that for PPi-dependent H(+) transport (7 micromolar). Free Mg(2+) was shown to be an allosteric activator of the H(+)-PPiase, with Hill coefficients of 2.5 for PPi hydrolysis and 2.7 for PPi-dependent H(+) transport. Half-maximal H(+)-PPiase activity occurred at a free Mg(2+) concentration of 1.1 millimolar, which lies within the range of accepted values for cytosolic Mg(2+). In contrast, cytosolic concentrations of K(+) and MgPPi(2-) appear to be saturating for H(+)-PPiase activity. We propose that one function of the H(+)-PPiase may be to act as an ancillary enzyme that maintains the proton-motive force across the vacuolar membrane when the activity of the tonoplast H(+)-ATPase is restricted by substrate availability. As ATP levels decline in the cytosol, free Mg(2+) would be released from the MgATP(2-) complex, thereby activating the tonoplast H(+)-PPiase.

Effects of spermine and spermidine on the inorganic pyrophosphatase of Streptococcus faecalis. Interactions between polyamines and inorganic pyrophosphate

We have shown a dual role for Mg2+ in the hydrolysis of PPi catalysed by inorganic pyrophosphatase (PPase; EC 3.6.1.1) of Streptococcus faecalis; Mg2+ is necessary for the formation of the substrates, Mg1PPi2- and Mg2PPi0, and it also acts as an allosteric activator [Lahti + Jokinen (1985) Biochemistry 24, 3526-3530]. No activity can be observed with S. faecalis PPase in the absence of bivalent cations, which indicates that free PPi cannot serve as a substrate for this enzyme. However, significant activities were observed in the presence of spermine and spermidine, even though no bivalent cations were present. It was shown by particle-induced gamma-ray emission and particle-induced X-ray-emission analysis that the polyamines used were not contaminated with Mg2+ or any other bivalent cations that could support PPase activity. Hence it is obvious that polyamines are able to form a complex with PPi that serves as a substrate for PPase. The apparent stability constants for the 1:1 adducts of spermine and spermidine were estimated by a resin competition method. The values obtained at pH 7.5 were 2.7 X 10(3) M-1 and 6.4 X 10(2) M-1 respectively. Kinetic results further suggested that polyamines can also substitute for Mg2+ as an activator in vitro. The physiological significance of these polyamine effects were discussed.