Corn (Zea mays L. cv Trojan T929) coleoptile membranes were fractionated on isopycnic sucrose density gradients. Two peaks of ATPdriven H"-transport activity, corresponding to the previously characterized tonoplast (1.07 grams per cubic centimeter) and Golgi (1.13 grams per cubic centimeter) fractions (Chanson and Taiz, Plant Physiol 1985 78: 232-240) were localized. Coincident with these were two peaks of inorganic pyrophosphate (PPi)-driven H-transport. At saturating (3 millimolar) concentrations of Mg2":ATP, the rate of proton transport was further enhanced by the addition of 3 millimolar PPi, and the stimulation was additive, i.e. equal to the sum of the two added separately. The specific PPi analog, imidodiphosphate, antagonized PPi-driven H'-transport, but had no effect on ATP-driven transport. Moreover, PPidependent proton transport in both tonoplast-enriched and Golgi-enriched fractions was strongly promoted by 50 millimolar KNO3, unlike the ATPdependent Hf-pumps of the same membranes. Taken together, the results indicate that PPi-driven proton transport is mediated by specific membrane-bound HW-translocating pyrophosphatases. Both potassium and a permanent anion (NO3-> Cl-), were required for maximum activity. The PPi-driven proton pumps were totally inhibited by N,N'-dicyclohexylcarbodiimide, but were insensitive to 100 millimolar vanadate. The PPi concentration in coleoptile extracts was determined using an NADH oxidation assay system coupled to purified pyrophosphate:fructose 6-phosphate 1-phosphotransferase (EC 2.7.1.90). The total pyrophosphate content of corn coleoptiles was 20 nanomoles/gram fresh weight. Assuming a cytoplasmic location, the calculated PPi concentration is sufficient to drive proton transport at 20% of the maximum rate measured in vitro for the tonoplast-enriched fraction, and 10% of the maximum rate for the Golgi-enriched fraction.The role of transmembrane proton pumps as primary transport mechanisms in plant cells is now well established. The most thoroughly studied proton pumps are the H+-translocating Mg2+:ATPases, which utilize the energy of hydrolysis of Mg2+:ATP to transport protons across the lipid bilayer (25). Proton-pumping Mg2+:ATPases have been identified on the plasma membrane (5, 9, 18, 22, 27), tonoplast (2,5,6,13,(15)(16)(17), and Golgi membranes (3, 4) in a variety of plant tissues. A Mg2`:pyrophosphatase activity has also been shown to be associated with plant tonoplasts (12,28,30).

Human phosphofructokinase (PFK) exists in tetrameric isozymic forms, at least in vitro. Muscle and liver contain homotetramers M4 and L4, respectively, whereas red cells contain five isozymes composed of M (muscle) and L (liver) type subunits, i.e., M4, M3L, M2L2, and ML3, and L4. Homozygous deficiency of muscle PFK results in the classic glycogen storage disease type VII characterized by exertional myopathy and hemolytic syndrome beginning in early childhood. The genetic lesion results in a total and partial loss of muscle and red cell PFK, respectively. Characteristically, the residual red cell PFK from the patients consists of isolated L4 isozyme; the M-containing hybrid isozymes are completely absent. In this study, we investigated an 80-yr-old man who presented with a 10-yr history of progressive weakness of the lower limbs as the only symptom. The residual red cell PFK showed the presence of a few M-containing isozymes in addition to the predominant L4 species, indicating that the genetic lesion is a "leaky" mutation of the gene coding for the M subunit. The presence of a small amount of enzyme activity in the muscle may account for the atypical myopathy in this patient.