A novel role for subunit C in mediating binding of the H+-V-ATPase to the actin cytoskeleton

Primary proton transport by V-ATPases is regulated via the reversible dissociation of the V(1)V(0) holoenzyme into its V(1) and V(0) subcomplexes. Laser scanning microscopy of different tissues from the tobacco hornworm revealed co-localization of the holoenzyme and F-actin close to the apical membranes of the epithelial cells. In midgut goblet cells, no co-localization was observed under conditions where the V(1) complex detaches from the apical membrane. Binding studies, however, demonstrated that both the V(1) complex and the holoenzyme interact with F-actin, the latter with an apparently higher affinity. To identify F-actin binding subunits, we performed overlay blots that revealed two V(1) subunits as binding partners, namely subunit B, resembling the situation in the osteoclast V-ATPase (Holliday, L. S., Lu, M., Lee, B. S., Nelson, R. D., Solivan, S., Zhang, L., and Gluck, S. L. (2000) J. Biol. Chem. 275, 32331-32337), but, in addition, subunit C, which gets released during reversible dissociation of the holoenzyme. Overlay blots and co-pelleting assays showed that the recombinant subunit C also binds to F-actin. When the V(1) complex was reconstituted with recombinant subunit C, enhanced binding to F-actin was observed. Thus, subunit C may function as an anchor protein regulating the linkage between V-ATPase and the actin-based cytoskeleton.     

Aspartyl proteinase from cucumber (Cucumis sativus) seeds. Preparation and characteristics

Aspartyl proteinase (EC 3.4.23) from cucumber seeds was purified by ammonium sulphate fractionation, chromatography on immobilized pepstatin and gel filtration on Sephacryl S-200. The preparation obtained, homogeneous on polyacrylamide-gel electrophoresis in acidic and alkaline media, has a molecular mass of 42,000, pI of 5.2, and shows the highest activity with denatured haemoglobin at pH 3.2. The proteinase is stable in slightly alkaline medium, whereas it is inactivated in acidic medium, especially in the presence of NaCl. The enzyme activity is affected neither by the inhibitors of serine proteinases, sulfhydryl-proteinases and metalloproteinases, nor by divalent metal ions, whereas the enzyme is inactivated by the inhibitors of aspartyl proteinases: 1,2,3-epoxy(p-nitrophenoxy)propane, diazoacetyl-DL-norleucine and pepstatin.     

Brain acetylcholine, adenine nucleotides and their degradation products after intraperitoneal and intracerebral adenosine administration.

The paper describes adenosine effects on the acetylcholine synthesis and the profiles of adenine nucleotides, adenosine, inosine, and hypoxanthine in the rat brain in vivo after intracerebral (intraventricular) and intraperitoneal administration of adenosine. Intracerebral as well as extracerebral adenosine injection caused a dose- and time-dependent increase of the cerebral acetylcholine level, which was not accompanied by an equal development of the contents of adenine compounds and their degradation products. However, a considerable turnover of adenosine was observed in the brain after both routes of administration concerning the nucleotide as well as the degradation pathway. The kinetics of the purified enzymes of choline acetyltransferase and acetylcholinesterase were not influenced by adenosine. By this, the adenosine-caused increase of the cerebral acetylcholine cannot be explained by a direct molecular attack of adenosine on the enzymes of the synthesis or degradation of acetylcholine. An indirect mechanism which includes cAMP was discussed as a possible interpretation at present.     

Animal plasma membrane energization by proton-motive V-ATPases

Proton-translocating, vacuolar-type ATPases, well known energizers of eukaryotic, vacuolar membranes, now emerge as energizers of many plasma membranes. Just as Na⁺ gradients, imposed by Na⁺/K⁺ ATPases, energize basolateral plasma membranes of epithelia, so voltage gradients, imposed by H⁺ V-ATPases, energize apical plasma membranes. The energized membranes acidify or alkalinize compartments, absorb or secrete ions and fluids, and underwrite cellular homeostasis. V-ATPases acidify extracellular spaces of single cells such as phagocytes and osteoclasts and of polarized epithelia, such as vertebrate kidney and epididymis. They alkalinize extracellular spaces of lepidopteran midgut. V-ATPases energize fluid secretion by insect Malpighian tubules and fluid absorption by insect oocytes. They hyperpolarize external plasma membranes for Na⁺ uptake by amphibian skin and fish gills. Indeed, it is likely that ion uptake by osmotically active membranes of all fresh water organisms is energized by V-ATPases. Awareness of plasma membrane energization by V-ATPases provides new perspectives for basic science and presents new opportunities for medicine and agriculture. BioEssays 21:637–648, 1999. © 1999 John Wiley & Sons, Inc.     

The Emerging Role of Copeptin

Direct measurement of the nonapeptide vasopressin has been limited by analyte instability ex vivo and in vivo rapid degradation, low serum concentrations requiring a sensitive assay and inherent secretory pulsatility. Copeptin is a 39 amino acid glycopeptide cleavage product of vasopressin synthesis with high stability, providing a marker of vasopressin secretion. Copeptin measurement has applications in diagnosis of diabetes insipidus and other diseases with altered vasopressin secretion. This review summarises our current understanding of serum copeptin measurement in diabetes insipidus and possible future applications of copeptin assays. As vasopressin is a stress hormone, there is emerging evidence on the use of copeptin for diagnosis and prognostication of disorders such as syndrome of inappropriate anti-diuretic hormone secretion, diabetes mellitus, critical illness, stroke, cardiovascular disease, respiratory disease, renal disease and thermal stress. Copeptin concentration measurement is likely to improve the diagnostic reliability of diabetes insipidus and, as a marker of stress, may have diagnostic or prognostic utility in specific clinical circumstances. Further studies are needed to determine if goal-directed therapy using plasma copeptin concentrations may improve patient outcomes.