9-Aminoacridine, a fluorescent probe of the thiamine carrier in yeast cells

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Carrier-mediated cellular uptake of pharmaceutical drugs: an exception or the rule?

It is generally thought that many drug molecules are transported across biological membranes via passive diffusion at a rate related to their lipophilicity. However, the types of biophysical forces involved in the interaction of drugs with lipid membranes are no different from those involved in their interaction with proteins, and so arguments based on lipophilicity could also be applied to drug uptake by membrane transporters or carriers. In this article, we discuss the evidence supporting the idea that rather than being an exception, carrier-mediated and active uptake of drugs may be more common than is usually assumed - including a summary of specific cases in which drugs are known to be taken up into cells via defined carriers - and consider the implications for drug discovery and development.

New aspects of inorganic polyphosphate metabolism and function

The review analyzes the results of recent studies on the biochemistry of high-molecular inorganic poly-phosphates (PolyPs). The data obtained lead to the following main conclusions. PolyPs are polyfunctional compounds. The main role of PolyPs is their participation in the regulation of metabolism both at the genetic and metabolic levels. Among the functions of PolyPs known at present, the most important are the following: phosphate and energy storage; regulation of the levels of ATP and other nucleotide and nucleoside-containing coenzymes; participation in the regulation of homeostasis and storage of inorganic cations and other positively charged solutes in an osmotically inert form; participation in membrane transport processes mediated by poly-beta-Ca2+-hydroxybutyrate complexes; participation in the formation and functions of cell surface structures; control of gene activity; and regulation of activities of the enzymes and enzyme assemblies involved in the metabolism of nucleic acids and other acid biopolymers. However, the functions of PolyPs vary among organisms of different evolutionary levels. The metabolism and functions of PolyPs in each cellular compartment of procaryotes (cell wall, plasma membrane, cytosol) and eucaryotes (nuclei, vacuoles, mitochondria, plasma membrane, cell wall, mitochondria, cytosol) are unique. The synthesis and degradation of PolyPs in the organelles of eucaryotic cells are possibly mediated by different sets of enzymes. This is consistent with of the endosymbiotic hypothesis of eucaryotic cell origin. Some aspects of the biochemistry of high-molecular PolyPs are considered to be of great significance to the approach to biotechnological, ecological and medical problems.

Selective photoaffinity labelling of one mitochondrial protein in living cells of Saccharomyces cerevisiae with the fluorescent probe APMC. Identification of the target protein as subunit I of cytochrome c oxidase

The lipophilic, cationic fluorochrome azopentylmethylindocarbocyanine (APMC) specifically stains the mitochondria in living yeast cells (Saccharomyces cerevisiae WT X 2180). It contains a photosensitive diazirine ring and is suitable for photoaffinity labelling. By combining photoaffinity labelling, micro-gel electrophoresis (SDS-PAGE), and detection of the APMC fluorescence with a microfluorimeter, we established a highly sensitive procedure for determining the apparent molecular weight of the APMC-labelled proteins in yeast cells. On vital staining at 0.1 microM APMC for 30 min, only one mitochondrial protein with an apparent molecular weight of 40 kDa is labelled with high intensity. At increased dye concentrations proteins of 47 and 49 kDa are labelled too, however not until all binding sites of the 40 kDa protein are occupied. Obviously, the APMC cations have a pronounced affinity for this protein. It was shown by fractional centrifugation that the labelled 40 kDa protein is a constituent of the inner mitochondrial membrane. One driving force for the accumulation of the APMC cations is the trans-membrane potential (TMP) across the inner mitochondrial membrane. Consequently, uncouplers like dinitrophenol (DNP) and carbonylcyanidechlorophenyl-hydrazone (CCCP), ionophores (valinomycin, gramicidin), and inhibitors of the respiratory chain (myxothiazol, KCN), which decrease the TMP, also diminish the APMC accumulation and labelling. And conversely, drugs, which hyperpolarize the inner membrane (nigericin, atractyloside), favour APMC labelling. Another driving force of APMC accumulation is the dye's lipophilicity, which facilitates dye accumulation by hydrophobic interaction with the very lipophilic proteins of the inner mitochondrial membranes. This was shown by competitive double staining experiments. Thiamine strongly inhibits APMC labelling. Obviously, the transport of the APMC cations is facilitated by the thiamine carrier, and thiamine competes for the same binding sites, which are occupied by the dye cations. Chloramphenicol is an inhibitor of the mitochondrial protein synthesis without affecting the TMP. On preincubation, chloramphenicol completely quenches the signal of the 40 kDa protein. Therefore, this protein must be encoded on the mtDNA. The only 40 kDa protein with adequate properties is the subunit I of cytochrome c oxidase. Obviously, it is the preferred target of the APMC cations on photoaffinity labelling. This assignment agrees with the strong hydrophobicity of the labelled 40 kDa protein, which was tested with various detergents. It also agrees with the solvatochromism of the protein-bound APMC label, and finally with the paralellism of the labelled protein with cytochrome c oxidase on fractional ammonium sulfate precipitation.