Effect of acute hyperketonemia on the cerebral uptake of ketone bodies in nondiabetic subjects and IDDM patients

Using R-beta-[1-(11)C]hydroxybutyrate and positron emission tomography, we studied the effect of acute hyperketonemia (range 0.7-1.7 micromol/ml) on cerebral ketone body utilization in six nondiabetic subjects and six insulin-dependent diabetes mellitus (IDDM) patients with average metabolic control (HbA(1c) = 8.1 +/- 1.7%). An infusion of unlabeled R-beta-hydroxybutyrate was started 1 h before the bolus injection of R-beta-[1-(11)C]hydroxybutyrate. The time course of the radioactivity in the brain was measured during 10 min. For both groups, the utilization rate of ketone bodies was found to increase nearly proportionally with the plasma concentration of ketone bodies (1.0 +/- 0.3 micromol/ml for nondiabetic subjects and 1.3 +/- 0.3 micromol/ml for IDDM patients). No transport of ketone bodies from the brain could be detected. This result, together with a recent study of the tissue concentration of R-beta-hydroxybutyrate in the brain by magnetic resonance spectroscopy, indicate that, also at acute hyperketonemia, the rate-limiting step for ketone body utilization is the transport into the brain. No significant difference in transport and utilization of ketone bodies could be detected between the nondiabetic subjects and the IDDM patients.

Alcohol activates the cerebral reward system in man

OBJECTIVE

We investigated the effect of 0.07% alcohol on regional brain activity at rest and during cognitive performance in order to elucidate the anatomical substrate for the effects of alcohol in man as well as to clarify the interaction between changes in cerebral activity induced by cognitive performance and alcohol inebriation.

METHOD

Regional cerebral blood flow (3D-PET, 15O Butanol) was measured in 13 male, nonalcoholic volunteers. Each subject was scanned 12 times, three scans in each of the following four situations: sober/rest, sober/test and inebriated/rest, inebriated/test. We used statistical parametric mapping and a computerized brain atlas for localization.

RESULTS

Alcohol induced a sense of inebriation and elation as well as a relative increase of the cerebral blood flow in medial parts of the temporal lobes, in the anterobasal parts of the anterior cingulate cortex including the septal region. In addition, there was an increase of blood flow in midline parts of the lower brain stem. Relative decreases of flow were observed in the cerebellum and in the occipital cortex bilaterally. In the sober state, a computerized perceptual maze test and a (silent) serial seven test induced two distinct neocortical activation patterns that were specific to the tasks. Alcohol did not change these patterns and the test performance was also uninfluenced.

CONCLUSIONS

A moderate dose of alcohol selectively activates target structures that pertain to the so-called cerebral reward system and the ascending reticular activating system. Alcohol (approximately 0.07%) appears to have only minor effects in the neocortical systems that are involved in on-line cognitive activity. This apparent independence between the subcortical alcohol target and the neocortical cognitive mechanisms is a new finding that appears to be of importance for an understanding of the effect of moderate doses of alcohol on the brain.

The E. coli SRP: preferences of a targeting factor

Research on the targeting of proteins to the cytoplasmic membrane of E. coli has mainly focused on the so-called 'general secretory pathway' (GSP) which involves the Sec-proteins. Recently, evidence has been obtained for an alternative targeting pathway in E. coli which involves the signal recognition particle (SRP). The constituents of this SRP pathway in E. coli are homologous to those of the well-characterized eukaryotic SRP pathway, which is the main targeting pathway for both proteins translocated across and inserted into the endoplasmic reticulum membrane. However, until recently, no clear function could be assigned to the SRP in E. coli. New studies point to an important role of the E. coli SRP in the assembly of inner membrane proteins.

Expression of an olfactory receptor in Escherichia coli: purification, reconstitution, and ligand binding

An olfactory receptor has been expressed in bacterial cells as a fusion protein with glutathione S-transferase (GST). Overexpression of receptor protein yielding about 10% of the cell protein was achieved with mutants lacking the N-terminus and the first transmembrane region or with mutants carrying three positively charged residues in the first intracellular loop. The overexpressed fusion protein accumulated in inclusion bodies and could be solubilized in detergent. It was purified by metal chelation chromatography based on a C-terminal 6-histidine tag, and the GST portion was removed after proteolytic cleavage. The purified receptor was reconstituted into lipid vesicles and specific binding of odor ligands was shown by photoaffinity labeling and tryptophan fluorescence measurements. Thus, for the first time, an odorant receptor/ligand pair becomes available in large amounts for biophysical and screening studies.

Saccharomyces cerevisiae mitochondria lack a bacterial-type sec machinery

The bacterial Sec genes encode a generalized protein export machinery. Although the mitochondria present in eukaryotic cells are derived from bacterial ancestors, a comprehensive search of the complete genomic sequence for the eukaryotic yeast Saccharomyces cerevisiae did not reveal any close homologs of the bacterial Sec genes, strongly suggesting that yeast mitochondria lack a generalized bacterial-type export system. This finding has implications for the sorting of imported mitochondrial proteins to the intermembrane space compartment, and also for the insertion of mitochondrially encoded proteins into the inner membrane.

Forces involved in the assembly and stabilization of membrane proteins

Hydrophobic organization: Determination of the structure of the bacterial photosynthetic reaction center, bacterial porins, and bacteriorhodopsin allows a comparison of the basic structural features of integral membrane proteins. Structure parameters of membrane- and water-soluble proteins are surprisingly similar, given the different dielectric environments, except for the polarity of residues on the protein surface. Hydrophobic and electrostatic forces: 1) Intramembrane helix-helix interactions that are sensitive to small structure changes can dictate assembly of membrane proteins, as indicated by reconstitution of bacteriorhodopsin from proteolytic fragments and specific dimer formation of the human erythrocyte sialoglycoprotein glycophorin A. 2) Electrostatic interactions have an important role in determining the trans-membrane orientation of integral membrane proteins of the bacterial inner membrane, as expressed by the "positive-inside" rule for the distribution of basic residues on the cis relative to the trans side of the membrane-spanning alpha-helices. The use of this charge asymmetry rule, in conjunction with a hydrophobicity algorithm for prediction of membrane-spanning domains, allows accurate prediction of the folding patterns of such polypeptides across the membrane. A role of electrostatic interactions in assembly and maintenance of the structure of oligomeric integral membrane protein complexes is also implied by the separation and extrusion from the membrane, at high pH, of the major hydrophobic subunits of the cytochrome b6f complex from the chloroplast thylakoid membrane. It is inferred that the hydrophobic helix-helix interactions between the subunits of this complex, whose function is electron transfer and proton translocation, are relatively weak compared to those in bacteriorhodopsin.

Signal peptidases in prokaryotes and eukaryotes--a new protease family

Signal peptidases remove targeting peptides from pre-proteins and play central roles in the secretory pathway, as well as in the delivery of proteins to the mitochondrial intermembrane space and to the lumen of thylakoids. The catalytic mechanism of pre-protein cleavage has long been an enigma, but recent data from site-directed mutagenesis and sequence alignment studies suggest that signal peptidases may constitute a new type of serine protease, mechanistically related to the beta-lactamases.

[Intracellular protein transport--the cell as a mail carrier]

The eukaryotic cell has a highly developed machinery for routing proteins to their correct intra- or extra-cellular locations. Short stretches of amino acids ("signal peptides") serve as address labels that are recognized by receptors on the surface of the appropriate organelle, where-upon translocation across one or more membranes ensues. Premature folding of the nascent protein is prevented by cytoplasmic "chaperones", i.e. proteins that bind to unfolded or partially folded nascent chains. After import into the organelle, other chaperones are required for catalyzing the final folding of the protein, and for promoting its correct assembly into oligomeric protein complexes. Transport along the secretory pathway (endoplasmic reticulum to Golgi to trans-Golgi to plasma membrane) is mediated by vesicles that bud from a donor compartment and fuse with an acceptor compartment.