Amino acid sequence surrounding the lipoic acid cofactor of bovine kidney 2-oxoglutarate dehydrogenase complex

The 2-oxoglutarate dehydrogenase complex was succinylated using 2-oxo[5-14C]glutarate in the presence of N-ethylmaleimide to label the lipoic acid cofactor of the transuccinylase (E2) component. Following peptic digestion, 14C-lipoate-containing peptides were purified and subjected to automated Edman degradation and amino acid analysis. The amino acid sequence surrounding the lipoyllysine residue is reported.     

Phosphorylation of branched-chain 2-oxo acid dehydrogenase complex in isolated hepatocytes

Hepatocytes, isolated from rats fed a low-protein diet, were incubated with [32P]Pi and the phosphoproteins analysed. Immunoprecipitation using antibody against El of branched-chain 2-oxo acid dehydrogenase complex demonstrated phosphorylation of the alpha-subunit of El. Analysis of the tryptic phosphopeptides from the alpha-subunit indicated that two sites were phosphorylated. 4-methyl 2-oxopentanoate and DL-2-chloro 4-methylpentanoate decreased labelling of both sites. No major direct effects of several hormones on phosphorylation of branched-chain 2-oxo acid dehydrogenase was observed.     

Phosphorylation of bovine hormone-sensitive lipase by the AMP-activated protein kinase. A possible antilipolytic mechanism

Hormone-sensitive lipase is phosphorylated at a single site (site 2) in vitro by the AMP-activated protein kinase, without any direct effect on the activity of the enzyme. The amino acid sequence around this site has been determined. Ca2+/calmodulin-dependent protein kinase II also phosphorylates hormone-sensitive lipase predominantly at this site, whilst cyclic-GMP-dependent protein kinase phosphorylates exclusively the regulatory site (site 1) which is also phosphorylated by cyclic-AMP-dependent protein kinase. Phosphorylation of site 2 has been found to inhibit subsequent phosphorylation and activation of hormone-sensitive lipase by the cyclic-AMP-dependent and cyclic-GMP-dependent protein kinases, indicating that site-2 phosphorylation may have an antilipolytic role in vivo.     

Primary structure of the site on bovine hormone-sensitive lipase phosphorylated by cyclic AMP-dependent protein kinase

The primary structure of a region on hormone-sensitive lipase was determined to be: Lys-Thr-Glu-Pro-Met-Arg-Arg-Ser- Val-Ser-Glu-Ala-Ala-Leu-Thr-Gln-Pro-Glu-Gly-Pro-Leu-Gly-Thr-Asp-Ser-Leu-Lys. Ser-8 was the only residue in the intact protein phosphorylated by cyclic AMP-dependent protein kinase. However, Ser-10 also appeared to be present in a phosphorylated form, suggesting that it is a target for a distinct protein kinase in vivo.     

Regulation of cholesterol ester hydrolase by cyclic AMP-dependent protein kinase

Phosphorylation of cholesterol ester hydrolase by cyclic AMP-dependent protein kinase results in activation of both cholesterol ester and triacylglycerol hydrolase activities. Activation against both substrates correlates closely with phosphorylation in time course experiments. Proteolytic digestion of phosphorylated cholesterol ester hydrolase, followed by peptide mapping, indicates the presence of a single phosphorylation site on the enzyme. Phosphoserine is the only phosphoamino acid detected following partial acid hydrolysis of the phosphorylated enzyme.     

Oxidative decarboxylation of 4-methylthio-2-oxobutyrate by branched-chain 2-oxo acid dehydrogenase complex.

Highly purified branched-chain 2-oxo acid dehydrogenase complex (BCOADC) oxidizes 4-methylthio-2-oxobutyrate and 2-oxobutyrate, with Km values of 67 microM and 18 microM respectively. The Vmax. for oxidation of these substrates is 27% and 53% respectively of that for 3-methyl-2-oxobutyrate. Highly purified pyruvate dehydrogenase complex (PDC) oxidizes 2-oxobutyrate (Km 100 microM; Vmax. 49% of that for pyruvate) but not 4-methylthio-2-oxobutyrate, whereas 2-oxoglutarate dehydrogenase complex will not utilize either 2-oxo acid as substrate. BCOADC kinase is inhibited by both 4-methylthio-2-oxobutyrate and 2-oxobutyrate, with half-maximal inhibition by 45 microM and 50 microM respectively. Phosphorylation of BCOADC in isolated adipocytes is inhibited by both 4-methylthio-2-oxobutyrate and 2-oxobutyrate, consistent with their inhibitory action of BCOADC kinase. Phosphorylation of PDC is decreased by 2-oxobutyrate, but not by 4-methylthio-2-oxobutyrate.     

Insertional inactivation of a chromosomal locus that modulates expression of potential virulence determinants in Staphylococcus aureus.

A single insertion of transposon Tn551 into a unique chromosomal locus of Staphylococcus aureus ISP479C has resulted in a pleiotropic effect on the expression of both extracellular and cell wall proteins. In particular, the expression of cell wall protein A and clumping activity with fibrinogen were rendered undetectable in the mutant 1E3 compared with the parent. The secretion of alpha-hemolysin in mutant 1E3 was modestly increased. Southern blot and phenotypic analyses indicated that this locus is distinct from agr, xpr, and sar, three previously described global regulatory loci. Transduction experiments demonstrated that the genotype associated with mutant 1E3 could be transferred back into the parental strain ISP479C. The transductant 1E3-2 displayed a phenotypic profile similar to that of the original mutant. Northern (RNA) blot studies showed that this locus may be involved in modulating target genes at the mRNA level. In the rabbit endocarditis model, there was a significant decrease in both the infectivity rate and intravegetation bacterial density with mutant 1E3 compared with the parent at an inoculum of 10(3) CFU. Since protein A and the fibrinogen-binding protein(s) are major surface proteins that may mediate bacterial adhesion to host tissues, this locus may be an important genetic element involved in the expression of virulence determinants in S. aureus.     

Role of the tubulin-microtubule system in lymphocyte activation

The role of the tubulin-microtubule system was examined in human peripheral blood leukocytes after activation with phytohemagglutinin (PHA). Soluble tubulin and microtubules were measured with a [(3)H]colchicine-binding assay. It was found that the tubulin content of PHA-activated lymphocytes was consistently increased relative to total protein content after 36 h of culture. There was no increase in the proportion of total tubulin synthesis which was present as microtubules at 36 h. Nevertheless, as a result of increased tubulin synthesis, there was a two-to three-fold increase in total microtubular mass. Colchicine, which disrupts microtubles, was used to assess the role of microtubule assembly in the sequence of events which follow lymphocyte activation, namely lymphokine release, protein synthesis, RNA synthesis, and DNA synthesis. Colchicine consistently inhibited DNA synthesis but did not inhibit release of the lymphokine, osteoclast activating factor (OAF). Protein and RNA syntheses were inhibited much less than DNA synthesis. The fact that some effects of PHA on lymphocytes appear to require intact microtubules and at least one does not suggest that the microtubule dependent step in PHA-stimulated lymphocyte activation occurs at a stage after propagation of the signal from the membrane to the cell interior.     

Key role for ceramides in mediating insulin resistance in human muscle cells

Elevated non-esterified fatty acids, triglyceride, diacylglycerol, and ceramide have all been associated with insulin resistance in muscle. We set out to investigate the role of intramyocellular lipid metabolites in the induction of insulin resistance in human primary myoblast cultures. Muscle cells were subjected to adenovirus-mediated expression of perilipin or incubated with fatty acids for 18 h, prior to insulin stimulation and measurement of lipid metabolites and rates of glycogen synthesis. Adenovirus-driven perilipin expression lead to significant accumulation of triacylglycerol in myoblasts, without any detectable effect on insulin sensitivity, as judged by the ability of insulin to stimulate glycogen synthesis. Similarly, incubation of cells with the monounsaturated fatty acid oleate resulted in triacylglycerol accumulation without inhibiting insulin action. By contrast, the saturated fatty acid palmitate induced insulin resistance. Palmitate treatment caused less accumulation of triacylglycerol than did oleate but also induced significant accumulation of both diacylglycerol and ceramide. Insulin resistance was also caused by cell-permeable analogues of ceramide, and palmitate-induced resistance was blocked in the presence of inhibitors of de novo ceramide synthesis. Oleate co-incubation completely prevented the insulin resistance induced by palmitate. Our data are consistent with ceramide being the agent responsible for insulin resistance caused by palmitate exposure. Furthermore, the triacylglycerol derived from oleate was able to exert a protective role in sequestering palmitate, thus preventing its conversion to ceramide.     

Regulation of glycogen synthesis by amino acids in cultured human muscle cells

Insulin and a number of metabolic factors stimulate glycogen synthesis and the enzyme glycogen synthase. Using human muscle cells we find that glycogen synthesis is stimulated by treatment of the cells with lithium ions, which inhibit glycogen synthase kinase 3. Insulin further stimulates glycogen synthesis in the presence of lithium ions, an effect abolished by wortmannin and rapamycin. We report also that amino acids stimulate glycogen synthesis and glycogen synthase, these effects also being blocked by rapamycin and wortmannin. Amino acids stimulate p70(s6k) and transiently inhibit glycogen synthase kinase 3 without effects on the activity of protein kinase B or the mitogen-activated protein kinase pathway. Thus, the work reported here demonstrates that amino acid availability can regulate glycogen synthesis. Furthermore, it demonstrates that glycogen synthase kinase 3 can be inactivated within cells independent of activation of protein kinase B and p90(rsk).