New kinetic parameters for rat liver arginase measured at near-physiological steady-state concentrations of arginine and Mn2+

A cytosolic cell-free system from rat liver containing the last three enzymes of the urea cycle, a number of cofactors and the substrates aspartate and citrulline was shown to synthesize urea at near-physiological rates ranging between 0.40 and 1.25 mumol/min per g of liver. This system was used to determine the kinetic parameters for arginase. With saturating amounts of Mn2+ (30 microM), arginine remained at a steady-state concentration of 5-35 microM depending on the aspartate and citrulline supply. Vmax. at micromolar arginine concentrations was between 1.10 and 1.25 mumol/min per g of liver, the K0.5 (arginine) between 6.0 and 6.5 microM and positive co-operativity was observed (Hill coefficient 2). Omission of Mn2+ caused a significant accumulation of arginine during the incubation, suggesting a regulatory effect of arginase. Under these conditions, Vmax. was 1.10-1.65 mumol/min per g of liver and the Km (arginine) increased up to 14.4-21.1 microM. The apparent Ka for Mn2+ in the presence of physiological concentrations of ATP, Mg2+ and arginine was calculated to be maximally 8 microM. Initial-velocity experiments with millimolar arginine concentrations as the direct substrate gave the following results, which are in good agreement with literature data. In the absence of Mn2+, Vmax. was 71.3 mumol/min per g of liver and the Km (arginine) 1.58 mM. With 30 microM-Mn2+, Vmax. was 69.4 mumol/min per g of liver and the Km (arginine) decreased to 0.94 mM. On the basis of our results, we propose the presence of high-affinity and low-affinity sites for arginine on rat liver arginase and postulate that alterations in arginase activity arising from changes in the concentration of arginine and of the cofactor Mn2+ may contribute to the regulation of ureagenesis in vivo.

Evidence for peptide transport across microsomal membranes

Antigenic peptides bound to class I molecules of the major histocompatibility complex (MHC) are recognized by T-cell receptors during development of an antiviral immune response. T cells respond to peptides derived from cytoplasmic viral proteins as well as viral membrane proteins, indicating that a pathway exists for the transport of proteins or peptides from the cytosol into the compartment(s) where the MHC class I molecules assemble. To investigate this pathway, we have developed an in vitro assay for the transport of peptides into microsomal vesicles. This assay provides evidence for the transport of chemically synthesized peptides (13-21 amino acids) containing N-linked glycosylation acceptor sequences, which serve as glycosylation substrates. Their transport results in depletion of the pool of available dolichol high-mannose oligosaccharides in the lumen of the microsomal vesicles. We have observed transport of peptides derived from antigenic human immunodeficiency virus gag and influenza B nucleoprotein sequences, but transport of a third randomly selected peptide was not detected, suggesting specificity of the transport process. We were not able to demonstrate ATP dependence of this peptide transport process by using apyrase and an ATPase inhibitor. This result was unexpected in light of the recent identification of MHC-linked genes with homology to ATP-binding cassette transporters, which have been proposed to mediate peptide transport.

SEC65 gene product is a subunit of the yeast signal recognition particle required for its integrity

Protein targeting to the endoplasmic reticulum (ER) in mammalian cells is catalysed by the signal recognition particle (SRP), which consists of six protein subunits and an RNA subunit. Saccharomyces cerevisiae SRP is a 16S particle, of which only two subunits have been identified: a protein subunit, SRP54p, which is homologous to the mammalian SRP54 subunit, and an RNA subunit, scR1 (ref. 3). The sec65-1 mutant yeast cells are temperature-sensitive for growth and defective in the translocation of several secreted and membrane-bound proteins. The DNA sequence of the SEC65 gene suggests that its product is related to mammalian SRP19 subunit and may have a similar function. Here we show that SEC65p is a subunit of the S. cerevisiae SRP and that it is required for the stable association of another subunit, SRP54p, with SRP. Overexpression of SRP54p suppresses both growth and protein translocation defects in sec65-1 mutant cells.

In vivo modulation of total and mitochondrial glutathione in rat liver. Depletion by phorone and rescue by N-acetylcysteine

The aim of the present work was to modulate in vivo the level of hepatic mitochondrial glutathione (GSH). Rats were given phorone (diisopropylidene acetone), which in vivo becomes enzymatically conjugated to GSH, and were subsequently treated with N-acetylcysteine (NAC) to rescue GSH. In liver homogenate, a rapid and biphasic (T1/2 less than or equal to 15 min and 1.5 hr) drop of GSH was observed upon phorone administration. NAC treatment led to a restoration (T1/2 about 1 hr) of GSH in the homogenate above control values within 3 hr. The mitochondrial GSH level decreased with T1/2 of about 1.5 hr upon phorone treatment, and was 75% restored by NAC treatment within 3 hr. Hydroperoxide-induced mitochondrial pyridine nucleotide oxidation and Ca2+ release were impeded in GSH-depleted organelles, and NAC treatment restored these processes. The GSH status had no influence on mitochondrial pyridine nucleotide oxidation and Ca2+ released induced by alloxan, which reacts directly and non-enzymatically with pyridine nucleotides. It is concluded that NAC is able to rescue mitochondrial GSH in vivo and restore important mitochondrial functions. The data suggest that NAC may be a useful antidote in oxidative stress-related diseases.

Mutants in three novel complementation groups inhibit membrane protein insertion into and soluble protein translocation across the endoplasmic reticulum membrane of Saccharomyces cerevisiae

We have isolated mutants that inhibit membrane protein insertion into the ER membrane of Saccharomyces cerevisiae. The mutants were contained in three complementation groups, which we have named SEC70, SEC71, and SEC72. The mutants also inhibited the translocation of soluble proteins into the lumen of the ER, indicating that they pleiotropically affect protein transport across and insertion into the ER membrane. Surprisingly, the mutants inhibited the translocation and insertion of different proteins to drastically different degrees. We have also shown that mutations in SEC61 and SEC63, which were previously isolated as mutants inhibiting the translocation of soluble proteins, also affect the insertion of membrane proteins into the ER. Taken together our data indicate that the process of protein translocation across the ER membrane involves a much larger number of gene products than previously appreciated. Moreover, different translocation substrates appear to have different requirements for components of the cellular targeting and translocation apparatus.

C-terminal sequences can inhibit the insertion of membrane proteins into the endoplasmic reticulum of Saccharomyces cerevisiae

We have constructed three gene fusions that encode portions of a membrane protein, arginine permease, fused to a reporter domain, the cytoplasmic enzyme histidinol dehydrogenase (HD), located at the C-terminal end. These fusion proteins contain at least one of the internal signal sequences of arginine permease. When the fusion proteins were expressed in Saccharomyces cerevisiae and inserted into the endoplasmic reticulum (ER), two of the fusion proteins placed HD on the luminal side of the ER membrane, but only when a piece of DNA encoding a spacer protein segment was inserted into the fusion joint. The third fusion protein, with or without the spacer included, placed HD on the cytoplasmic side of the membrane. These results suggest that (i) sequences C-terminal to the internal signal sequence can inhibit membrane insertion and (ii) HD requires a preceding spacer segment to be translocated across the ER membrane.

Analysis of African Burkitt's and high-grade B cell non-Burkitt's lymphoma for Epstein-Barr virus genomes using in situ hybridization

We investigated the association of Epstein-Barr virus (EBV) with African cases of Burkitt's lymphoma (BL) and high grade B cell non-Burkitt's lymphoma (non-BL) occurring in areas where BL is endemic. The presence of EBV genomes was analysed in 24 cases using in situ hybridization with a 35S-labelled EBV probe applied to paraffin sections. EBV DNA was detected in each of 10 cases of BL in which technically satisfactory results were obtained, the virus being homogeneously distributed in all identifiable tumour cells. Two other cases of BL could not be evaluated because of technical problems. In contrast, EBV DNA was not detected in any case of high-grade non-BL (10 centroblastic and two immunoblastic lymphomas). These results confirm previous reports of the strong association of EBV with endemic BL, but suggest that the virus is not important in the pathogenesis of other types of African high-grade B cell lymphoma from regions where BL is endemic.

Tubal schistosomiasis as a cause of ectopic pregnancy in endemic areas?; a report of three cases

Tubal schistosomiasis as a cause of ectopic pregnancy is uncommon. We are reporting three cases of tubal pregnancies in which the histopathological examination showed a bilharzial disease of the tube. Schistosomiasis to Schistosoma haematobium occurs in Gabon with an incidence of 8% to 44% of the adult population, and schistosoma ova have been found in 2.5% of histopathological examinations in removal of any genital tissue. Even if schistosomiasis was not the main causative agent of infertility, in this population which has the lowest fertility index in subsaharan Africa, it might impair an already altered tubal function by salpingitis. We suggest that parasitic examination should be made as a part of the management program of infertility and after an ectopic pregnancy in endemic areas.

The signal recognition particle in S. cerevisiae

We have identified the Saccharomyces cerevisiae homolog of the signal recognition particle (SRP) and characterized its function in vivo. S. cerevisiae SRP is a 16S particle that includes a homolog of the signal sequence-binding protein subunit of SRP (SRP54p) and a small cytoplasmic RNA (scR1). Surprisingly, the genes encoding scR1 and SRP54p are not essential for growth, though SRP-deficient cells grow poorly, suggesting that SRP function can be partially by-passed in vivo. Protein translocation across the ER membrane is impaired in SRP-deficient cells, indicating that yeast SRP, like its mammalian counterpart, functions in this process. Unexpectedly, the degree of the translocation defect varies for different proteins. The ability of some proteins to be efficiently targeted in SRP-deficient cells may explain why previous genetic and biochemical analyses in yeast and bacteria did not reveal components of the SRP-dependent protein targeting pathway.

Hypothyroidism and thyroxin substitution affect the n-3 fatty acid composition of rat liver mitochondria

The effects of hypothyroidism and of daily treatment for up to 21 days with thyroxin (T4, 0.5 micrograms/100 g body weight) on the fatty acid composition of total lipid, phosphatidylethanolamine, and phosphatidylcholine of rat liver mitochondria were studied. The fatty acid compositions of hypothyroid and euthyroid (control) rats of similar age were compared. The n-6 and n-3 polyunsaturated fatty acids (PUFA) were affected differently by the hypothyroid state. The levels of linoleic (18:2n-6), gamma-linolenic (18:3n-6) and dihomo-gamma-linolenic acids (20:3n-6) were higher in hypothyroid rats than in controls, while the level of arachidonic acid (20:4n-6) was lower, which suggests an impairment of the elongase and desaturase activities. The n-3 polyunsaturated fatty acids, eicosapentaenoic (EPA, 20:5n-3) and docosapentaenoic (22:5n-3) acids, were higher in hypothyroid rats, whereas the linolenic acid (18:3n-3) content remained constant. The level of docosahexaenoic acid (DHA, 22:6n-3) was dramatically decreased in hypothyroid rats, while the levels of C22 n-6 fatty acids were unchanged. The differences were probably due to the competition between n-3 and n-6 PUFA for desaturases, elongases and acyltransferases. When hypothyroid rats were treated with thyroxin, the changes induced by hypothyroidism in the proportions of n-6 fatty acids were rapidly reversed, while the changes in the n-3 fatty acids were only partially reversed. After 21 days of thyroxin treatments, the DHA content was only half as high in hypothyroid rats than in euthyroid rats. These results suggest that the conversion of 18:2n-6 to 20:4n-6 is suppressed in the hypothyroid state which favors the transformation of 18:3n-3 to 20:5n-3.(ABSTRACT TRUNCATED AT 250 WORDS)

Evolution of aminoacyl-tRNA synthetase quaternary structure and activity: Saccharomyces cerevisiae mitochondrial phenylalanyl-tRNA synthetase

Phenylalanyl-tRNA synthetases [L-phenylalanine:tRNAPhe ligase (AMP-forming), EC 6.1.1.20] from Escherichia coli, yeast cytoplasm, and mammalian cytoplasm have an unusual conserved alpha 2 beta 2 quaternary structure that is shared by only one other aminoacyl-tRNA synthetase. Both subunits are required for activity. We show here that a single mitochondrial polypeptide from Saccharomyces cerevisiae is an active phenylalanyl-tRNA synthetase. This protein (the MSF1 gene product) is active as a monomer. It has all three characteristic sequence motifs of the class II aminoacyl-tRNA synthetases, and its activity may result from the recruitment of additional sequences into an alpha-subunit-like structure.

An ATP-binding membrane protein is required for protein translocation across the endoplasmic reticulum membrane

The role of nucleotides in providing energy for polypeptide transfer across the endoplasmic reticulum (ER) membrane is still unknown. To address this question, we treated ER-derived mammalian microsomal vesicles with a photoactivatable analogue of ATP, 8-N3ATP. This treatment resulted in a progressive inhibition of translocation activity. Approximately 20 microsomal membrane proteins were labeled by [alpha 32P]8-N3ATP. Two of these were identified as proteins with putative roles in translocation, alpha signal sequence receptor (SSR), the 35-kDa subunit of the signal sequence receptor complex, and ER-p180, a putative ribosome receptor. We found that there was a positive correlation between inactivation of translocation activity and photolabeling of alpha SSR. In contrast, our data demonstrate that the ATP-binding domain of ER-p180 is dispensable for translocation activity and does not contribute to the observed 8-N3ATP sensitivity of the microsomal vesicles.

Identification of potential amino acid residues supporting anticodon recognition in yeast methionyl-tRNA synthetase

Sequence comparisons among methionyl-tRNA synthetases from different organisms reveal only one block of homology beyond the last beta strand of the mononucleotide fold. We have introduced a series of semi-conservative amino acid replacements in the conserved motif of yeast methionyl-tRNA synthetase. The results indicate that replacements of two polar residues (Asn584 and Arg588) affected specifically the aminoacylation reaction. The location of these residues in the tertiary structure of the enzyme is compatible with a direct interaction of the amino acid side-chains with the tRNA anticodon.

Characterization of the rough endoplasmic reticulum ribosome-binding activity

The rough endoplasmic reticulum membranes of mammalian cells contain specific ribosome-binding sites. A purification to apparent homogeneity of a negatively charged protein (ERp180) of relative molecular mass 180,000 (180 K) was reported which was proposed to function as a rough endoplasmic reticulum ribosome receptor. We report here that ribosome-binding site activity quantitatively solubilized from rough endoplasmic reticulum membranes does not cofractionate with ERp180. By contrast, ribosome-binding site activity fractionates as a much smaller, positively charged protein.

Binding sites of the 9- and 14-kilodalton heterodimeric protein subunit of the signal recognition particle (SRP) are contained exclusively in the Alu domain of SRP RNA and contain a sequence motif that is conserved in evolution

The mammalian signal recognition particle (SRP) is a small cytoplasmic ribonucleoprotein required for the cotranslational targeting of secretory proteins to the endoplasmic reticulum membrane. The heterodimeric protein subunit SRP9/14 was previously shown to be essential for SRP to cause pausing in the elongation of secretory protein translation. RNase protection and filter binding experiments have shown that binding of SRP9/14 to SRP RNA depends solely on sequences located in a domain of SRP RNA that is strongly homologous to the Alu family of repetitive DNA sequences. In addition, the use of hydroxyl radicals, as RNA-cleaving reagents, has revealed four distinct regions in this domain that are in close contact with SRP9/14. Surprisingly, the nucleotide sequence in one of these contact sites, predicted to be mostly single stranded, was found to be extremely conserved in SRP RNAs of evolutionarily distant organisms ranging from eubacteria and archaebacteria to yeasts and higher eucaryotic cells. This finding suggests that SRP9/14 homologs may also exist in these organisms, where they possibly contribute to the regulation of protein synthesis similar to that observed for mammalian SRP in vitro.

Effect of hypothyroidism and thyroid hormone replacement on the level of protein kinase C and protein kinase A in rat liver

We investigated the influence of the thyroid hormone status on the levels of protein kinases C (PKC) and A (PKA) in the soluble fraction of rat liver. The immunodetectable PKC level in hypothyroid liver was elevated 7.7-fold, whereas the phorbol-ester binding capacity and the immunodetectable alpha-PKC level were increased 2.4- and 2.6-fold, respectively. Conversely, in hypothyroid livers the abundance of the regulatory type I and the catalytic subunits of PKA were lowered to 42% of the euthyroid level as determined by immunoblotting and by measuring the substrate specific phosphorylation rate of PKA. These changes in the PKC and PKA levels were reversible upon treatment with 0.5 microgram T4/100 g body weight for 2-21 days. The thyroid state dependent alterations in hepatic PKC and PKA levels may be responsible for the known changes in the response of hepatocytes to other hormonal stimuli in hypothyroidism.

Refeeding of rats fasted 36 hours with five different carbohydrates and with malt extract: differential effects on glycogen deposition in liver and muscle, on plasma insulin and on plasma triglyceride levels

Rats fasted for 36 h were refed for 1, 2, 4 or 6 h with a diet containing 12 g/100 g casein, 2 g/100 g NaCl and 86 g/100 g glucose, fructose, maltose, sucrose, starch or malt extract. Blood glucose reached constant levels after 1 to 2 h of refeeding. The increase in plasma insulin paralleled food intake rather than the increase in blood glucose. Plasma triglycerides decreased upon refeeding starch, maltose and malt extract and increased with sucrose and fructose. Recovery of absorbed carbohydrates was highest in rats refed malt extract. Glycogen deposition in muscle was highest in rats fed malt extract and lowest in those fed fructose; sucrose yielded intermediate values. Glucose, maltose and starch resulted in muscle glycogen depositions slightly lower than those obtained with malt extract. In liver, sucrose and fructose were better precursors for glycogen than glucose and starch. With carbohydrates containing only glucose units, much more glycogen was found to be deposited in total muscle than in liver. This asymmetry was less notable or even was reversed with sucrose and fructose. Glycogen deposition in muscle and in liver is influenced by the carbohydrate used for refeeding, and muscle, rather than liver, is the main glycogen storing tissue.

A nascent membrane protein is located adjacent to ER membrane proteins throughout its integration and translation

The immediate environment of nascent membrane proteins undergoing integration into the ER membrane was investigated by photocrosslinking. Nascent polypeptides of different lengths, each containing a single IgM transmembrane sequence that functions either as a stop-transfer or a signal-anchor sequence, were synthesized by in vitro translation of truncated mRNAs in the presence of N epsilon-(5-azido-2-nitrobenzoyl)-Lys-tRNA, signal recognition particle, and microsomal membranes. This yielded nascent chains with photoreactive probes at one end of the transmembrane sequence where two lysine residues are located. When irradiated, these nascent chains reacted covalently with several ER proteins. One prominent crosslinking target was a glycoprotein similar in size to a protein termed mp39, shown previously to be situated adjacent to a secretory protein during its translocation across the ER membrane (Krieg, U. C., A. E. Johnson, and P. Walter. 1989. J. Cell Biol. 109:2033-2043; Wiedmann, M., D. Goerlich, E. Hartmann, T. V. Kurzchalia, and T. A. Rapoport. 1989. FEBS (Fed. Eur. Biochem. Soc.) Lett. 257:263-268) and likely to be identical to a protein previously designated the signal sequence receptor (Wiedmann, M., T. V. Kurzchalia, E. Hartmann, and T. A. Rapoport. 1987. Nature (Lond.). 328:830-833). Changing the orientation of the transmembrane domain in the bilayer, or making the transmembrane domain the first topogenic sequence in the nascent chain instead of the second, did not significantly alter the identities of the ER proteins that were the primary crosslinking targets. Furthermore, the nascent chains crosslinked to the mp39-like glycoprotein and other microsomal proteins even after the cytoplasmic tail of the nascent chain had been lengthened by nearly 100 amino acids beyond the stop-transfer sequence. Yet when the nascent chain was allowed to terminate normally, the major photocrosslinks were no longer observed, including in particular that to the mp39-like glycoprotein. These results show that the transmembrane segment of a nascent membrane protein is located adjacent to the mp39-like glycoprotein and other ER proteins during the integration process, and that at least a portion of the nascent chain remains in close proximity to these ER proteins until translation has been completed.

Supraphysiological dosages of vitamins and their implications in man

Some recent evidence on the benefits and hazards of elevated dosages of vitamins is summarized. Special emphasis is given on the safety of vitamins A, D, K1 and B6. Furthermore, the possibly beneficial effects of vitamins for athletic performance as well as the preventive potential of antioxidative vitamins and of carotenoids against cancer are discussed.

The methionine-rich domain of the 54 kd protein subunit of the signal recognition particle contains an RNA binding site and can be crosslinked to a signal sequence

The 54 kd protein subunit of the signal recognition particle (SRP54) has been shown to bind signal sequences by UV crosslinking. Primary structure analysis and phylogenetic comparisons have suggested that SRP54 is composed of two domains: an amino-terminal domain that contains a putative GTP-binding site (G-domain) and a carboxy-terminal domain that contains a high abundance of methionine residues (M-domain). Partial proteolysis of SRP revealed that the two proposed domains of SRP54 indeed represent structurally discrete entities. Upon proteolysis the intact G-domain was released from SRP, whereas the M-domain remained attached to the core of the particle. Reconstitution experiments demonstrated that the isolated M-domain associates with 7SL RNA in the presence of SRP19. In addition, we observed a specific binding of the M-domain directly to 4.5S RNA of Escherichia coli, which contains a structural motif also present in 7SL RNA. This shows that the M-domain contains an RNA binding site, and suggests that SRP54 may be linked to the rest of SRP through this domain by a direct interaction with 7SL RNA. Using UV crosslinking, we found that in an in vitro translation system the preprolactin signal sequence contacts SRP through the M-domain of SRP54. These results imply that the M-domain contains the signal sequence binding site of SRP54, although we cannot exclude that the G-domain may also be in proximity to bound signal sequences.(ABSTRACT TRUNCATED AT 250 WORDS)