Degradation of 3-hydroxy-3-methylglutaryl-CoA reductase in endoplasmic reticulum membranes is accelerated as a result of increased susceptibility to proteolysis

The endoplasmic reticulum (ER) membrane protein 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is subject to regulated degradation when cells are presented with an excess of sterols or mevalonate. In this report, we demonstrate the degradation of HMG-CoA reductase in ER membranes prepared from cells which have been pretreated with mevalonate or sterols prior to membrane purification. Degradation of HMG-CoA reductase in membranes prepared from pretreated cells is more rapid than in membranes prepared from cells which have received no regulatory molecules. In vitro degradation is blocked by protease inhibitors previously shown to inhibit reductase degradation in vivo and is specific for intact HMG-CoA reductase. The lumenal contents of the ER membranes are dispensible for the regulated proteolysis and the proteases responsible for reductase degradation are stably associated with the ER membrane. Regulated proteolysis of HMG-CoA reductase is inhibited by lactacystin, a newly defined inhibitor of the multicatalytic protease, the proteasome, and in vitro degradation of reductase correlates with the presence of proteasome subunits in purified ER membranes. The ubiquitin system for protein degradation, which has recently been shown to be required for the degradation of several ER membrane proteins, is not required for the degradation of HMG-CoA reductase. Finally, we conclude that the regulated proteolysis of HMG-CoA reductase in response to regulatory molecules such as mevalonate or sterols is mediated by increased susceptibility of the reductase to ER proteases, rather than the induction of a new proteolytic activity.

Transglycosylation of intact sialo complex-type oligosaccharides to the N-acetylglucosamine moieties of glycopeptides by Mucor hiemalis endo-beta-N-acetylglucosaminidase

The endo-beta-N-acetylglucosaminidase (endo-beta-GlcNAc-ase) of Mucor hiemalis, endo-M, was found to transfer the sialo complex-type oligosaccharides from transferrin glycopeptide to the N-acetylglucosamine (GlcNAc) moieties of peptidyl-GlcNAc. Disialo complex-type oligosaccharide of transferrin glycopeptide was transferred to 9-fluorenylmethyloxycarbonyl (Fmoc)-asparaginyl-N-acetylglucosaminide (Fmoc-Asn-GlcNAc) by endo-M in a high yield. The structure of the reaction product was confirmed to be Fmoc-Asn-(GlcNAc)2-Man-(Man-GlcNac-Gal-NeuAc)2 by mass spectrometry. Endo-M also transferred disialo complex-type oligosaccharide to the GlcNAc residue of chemically synthesized H-Ile-Asn(GlcNAc)-Ala-Thr-Leu-OH. Asn-linked asialo complex-type oligosaccharide and Asn-linked high-mannose type oligosaccharide were also effective as oligosaccharide donors. Transfer of disialo complex-type oligosaccharide to the GlcNAc-peptide was the most effective among the three types of oligosaccharides, although the disialo complex-type oligosaccharide attached to the peptide was the poorest substrate for the hydrolytic activity of endo-M.

Purification and characterization of a malic enzyme from the ruminal bacterium Streptococcus bovis ATCC 15352 and cloning and sequencing of its gene

Malic enzyme (EC 1.1.1.39), which catalyzes L-malate oxidative decarboxylation and pyruvate reductive carboxylation, was purified to homogeneity from Streptococcus bovis ATCC 15352, and properties of this enzyme were determined. The 2.9-kb fragment containing the malic enzyme gene was cloned, and the sequence was determined and analyzed. The enzymatic properties of the S. bovis malic enzyme were almost identical to those of other malic enzymes previously reported. However, we found that the S. bovis malic enzyme catalyzed unknown enzymatic reactions, including reduction of 2-oxoisovalerate, reduction of 2-oxoisocaproate, oxidation of D-2-hydroxyisovalerate, and oxidation of D-2-hydroxyisocaproate. The requirement for cations and the optimum pH of these unique activities were different from the requirement for cations and the optimum pH of the L-malate oxidative decarboxylating activity. A sequence analysis of the cloned fragment revealed the presence of two open reading frames that were 1,299 and 1,170 nucleotides long. The 389-amino-acid polypeptide deduced from the 1,170-nucleotide open reading frame was identified as the malic enzyme; this enzyme exhibited high levels of similarity to malic enzymes of Bacillus stearothermophilus and Haemophilus influenzae and was also similar to other malic enzymes and the malolactic enzyme of Lactococcus lactis.

A preliminary description of the crystal structure of gamma-glutamyltranspeptidase from E. coli K-12

The structure of GGT [EC 2.3.2.2] from E. coli K-12 was studied at 3 A resolution by X-ray crystallography. Initial protein phases were calculated using two kinds of Pb2+ derivatives. The phases were refined by non-crystallographic 2-fold symmetry electron density averaging combined with solvent flattening and histogram matching. The GGT molecule has overall dimensions of 60 x 50 x 40 A. There are two antiparallel beta-pleated sheets consisting of 6 and 7 beta-strands. The two beta-sheets form a wall-like structure. Twelve short alpha-helices were detected, of which the maximum length appears to be four helix turns.

Highly efficient wideband continuum generation in a single-mode optical fiber by powerful broadband laser pumping

By pumping a single-mode optical fiber with a powerful broadband nonselective dye laser, we obtain a high-efficiency wideband continuum (530-930 nm) with nonlinear conversion efficiency exceeding 90%. Experimental conditions for a coherent regime of broadband stimulated Raman scattering are created, which in combination with the broadband self-phase modulation and the four-photon parametric processes leads to a spectral broadening and to the continuum formation. The influence of the pump laser spectral linewidth on the nonlinear conversion efficiency is analyzed and investigated by comparative experiments at narrow-band and broadband laser excitations.

[Use of oblique sagittal T2-weighted images in diagnosing rotator cuff tears]

The correlation between oblique sagittal T2-weighted images and oblique coronal T2-weighted images was determined in 86 patients who had symptoms and signs suggesting rotator cuff tears. Abnormal high signal lesions suggesting full and partial thickness tear of the supraspinatus tendon were observed on the oblique coronal images in 20 and 27 of 86 patients, respectively. The oblique sagittal images showed the lesions just the same as the oblique coronal images. In the other 39, abnormal high signal lesions were not observed on oblique coronal images or on oblique sagittal images. The oblique sagittal images were proved not useful to detect tears of the supraspinatus tendon. However, the oblique sagittal T2-weighted images showed posterior extension of high signal lesions and clarified abnormal high signal lesions near the long head of the biceps tendon in two patients. These results suggested that oblique coronal images should always be taken at the first step to identify supraspinatus tendon tear, and oblique sagittal images can be taken as an adjust to the former images.

maoB, a gene that encodes a positive regulator of the monoamine oxidase gene (maoA) in Escherichia coli

The structural gene for copper- and topa quinone-containing monoamine oxidase (maoA) and an unknown amine oxidase gene have been located at 30.9 min on the Escherichia coli chromosome. Deletion analysis showed that the unknown gene was located within a 1.1-kb cloned fragment adjacent to the maoA gene. The nucleotide sequence of this fragment was determined, and a single open reading frame (maoB) consisting of 903 bp was found. The gene encoded a polypeptide with a predicted molecular mass of 34,619 Da which was correlated with the migration on a sodium dodecyl sulfate-polyacrylamide gel. The predicted amino acid sequence of the MaoB protein was identical to the NH2-terminal amino acid sequence derived by Edman degradation of the protein synthesized under the self-promoter. No homology of the nucleotide sequence of maoB to the sequences of any reported genes was found. However, the amino acid sequence of MaoB showed a high level of homology with respect to the helix-turn-helix motif of the AraC family in its C terminus. The homology search and disruption of maoA on the chromosome led to the conclusion that MaoB is a transcriptional activator of maoA but not an amine oxidase. The consensus sequence of the cyclic AMP-cyclic AMP receptor protein complex binding domain was adjacent to the putative promoter for the maoB gene. By use of lac gene fusions with the maoA and maoB genes, we showed that the maoA gene is regulated by tyramine and MaoB and that the expression of the maoB gene is subject to catabolite repression. Thus, it seems likely that tyramine and the MaoB protein activate the transcription of maoA by binding to the regulatory region of the maoA gene.

Destruction of cholera toxin receptor on HeLa cell membrane using microbial endoglycoceramidase

The sensitivity of HeLa cells to cholera toxin decreased by Corynebacterium sp. endoglycoceramidase treatment. This endo-enzyme destroyed the cholera toxin receptor, ganglioside G(M1), on the cell surface membrane by liberating intact oligosaccharide from it, which was confirmed by the decrease of intracellular cAMP accumulation and the results of the analysis of released oligosaccharide with a combination of pyridylamination method and HPLC. Fluorescence microscopy using the immunofluorescence method revealed that the amount of cholera toxin attached to the cells decreased in endoglycoceramidase-treated cells. The enzyme acted on cellular glycosphingolipids without addition of any activator protein which is required by other similar enzymes. Corynebacterium endoglycoceramidase is a useful tool to elucidate the function of glycosphingolipids on the cell surface in situ.

Two distinct quinoprotein amine oxidases are induced by n-butylamine in the mycelia of Aspergillus niger AKU 3302. Purification, characterization, cDNA cloning and sequencing

Two distinct quinoprotein amine oxidases were found in Aspergillus niger mycelia grown on n-butylamine medium and purified using chromatographic techniques. The respective enzymes were termed AO-I, which had already been isolated, and AO-II, a new enzyme found in this study. HPLC indicated that their molecular masses are 150 kDa and 80 kDa, respectively. On SDS/PAGE, the enzymes gave a similar but distinct mobility, which corresponds to 75 kDa for the subunit dimeric AO-I and 80 kDa for monomeric AO-II. The absorption spectra of both enzymes were different from each other; the absorption maxima in the visible region were at 490 nm for AO-I and 420 nm for AO-II. The enzymes showed positive quinone staining, comparable substrate specificity, and sensitivity to inhibitors typical for copper/topa quinone-containing amine oxidases, but they had different copper contents and also differed in their N-terminal sequences. Their peptide maps showed almost identical patterns, with the exception of two additional bands for AO-II. Among the peptides obtained from digestion of AO-II, peptides with sequences corresponding to the N-terminal part of AO-I were detected. Polyclonal antibodies raised against AO-I and AO-II recognized both enzymes, but with different specificities. Using precipitation with AO-I, the antibody prepared against AO-II was purified and was shown to be specific only for AO-II. The cDNA of AO-I was cloned and sequenced. A highly conserved tetrapeptide sequence, Asn-Tyr-Glu-Tyr, was identified in which the first tyrosine residue (Tyr404) that could be converted to topa quinone was present in the 670-residue deduced amino acid sequence. Northern blot analysis indicated that AO-I was highly expressed in A. niger grown on n-butylamine as a single nitrogen source. Genomic Southern blot analysis confirmed that both enzymes are likely to be encoded by the same gene.

Characterization of a lactate oxidase from a strain of gram negative bacterium from soil

A lactate oxidase was purified about 36-fold from a newly screened strain KY6 of gram negative bacterium from soil to yield a homogeneous protein. The native enzyme had a molecular mass of 204 kDa measured by Sephadex G-200 and that of subunit on the SDS-PAGE was found to be 45 kDa. The enzyme was optimally active at pH 7.7 and showed stability at pH range of 5.7 to 9.5 for 24 h at 4 degrees C. The optimum temperature was 70 degrees C and the enzyme activity was stable for 10 min up to 45 degrees C. The half-life of the enzyme activity was about 10 min at 55 degrees C. The best substrate of the enzyme was D-lactate and Km value for D-lactate was 0.14 mM. The Km value for DL-lactate was 0.20 mM. Substrate inhibition of the enzyme was observed at higher concentrations than 20 mM of DL-lactate and 10 mM of D-lactate.

Purification and characterization of beta-D-glucosidase (beta-D-fucosidase) from Bifidobacterium breve clb acclimated to cellobiose

The beta-D-glucosidase (EC. 3.2.1.21) activity of Bifidobacterium breve 203 was increased by acclimation with cellobiose, and the enzyme was purified to homogeneity from cell-free extracts of an acclimatized strain of B. breve clb, by ammonium sulfate fractionation and column chromatographies of anion-exchange, gel filtration, Gigapaite, and hydrophobic interaction. This enzyme had not only beta-D-glucosidase activity but also beta-D-fucosidase activity, which is specific to Bifidobacteria in intestinal flora. The molecular weight of the purified enzyme was estimated to be 47,000-48,000 and the enzyme was assumed to be a monomeric protein. The optimum pH and temperature of the enzyme were around 5.5 and 45 degrees C, respectively. The enzyme was stable up to 40 degrees C and between pH 5 and 8. The isoelectric point of the enzyme was 4.3 and the Km values for p-nitrophenyl-beta-D-glucoside and p-nitrophenyl-beta-D-fucoside were 1.3 mM and 0.7 mM, respectively. This enzyme had also transferase activity for the beta-D-fucosyl group but not for the beta-D-glucosyl group. The N-terminal amino acid sequence of this enzyme was similar to those of beta-D-glucosidase from other bacteria, actinomycetes, and plants.

The sympathetic nervous system in hypertension and renal disease

The sympathetic nervous system plays a critical role in the initiation and development of some types of hypertension and renal diseases. In a subgroup of essential hypertension, sympathetic overactivity is implicated in linking insulin resistance, abnormal intracellular calcium handling and high blood pressure. Endothelium-derived autocrine-paracrine factors such as endothelin and nitric oxide, have been shown to influence the sympathetic nervous system, and thus to modify blood pressure and renal function. Although the action of each endothelial factor had been separately examined, the interaction between endothelin and nitric oxide has recently been found to modulate sympathetic nerve activity in an in-vivo hypertension model. Since elevated sympathetic activity is responsible for cardiovascular complications such as myocardial infarction, it is appropriate to choose antihypertensive drugs that reduce sympathetic nerve activity and attenuate reflex activation of the sympathetic nervous system.

Transcriptional regulation of tyrosine phenol-lyase gene of Erwinia herbicola AJ2985

Induction and repression of tyrosine phenol-lyase (TPL; EC 4.1.99.2) of Erwinia herbicola AJ2985 were examined on the transcriptional level and it was shown that transcription of tpl was increased by the addition of tyrosine and decreased by the addition of glucose in the medium. The 5' flanking region of its gene, tpl, was analyzed and its transcriptional start point was determined. A presumed -35, -10 promoter region, TyrR box, and operator-like region were also found in this region.

Subunit association of gamma-glutamyltranspeptidase of Escherichia coli K-12

gamma-Glutamyltranspeptidase [EC 2.3.2.2] of Escherichia coli K-12 consists of one large subunit and one small subunit, which can be separated from each other by high-performance liquid chromatography. Using ion spray mass spectrometry, the masses of the large and the small subunit were determined to be 39,207 and 20,015, respectively. The large subunit exhibited no gamma-glutamyltranspeptidase activity and the small subunit had little enzymatic activity, but a mixture of the two subunits showed partial recovery of the enzymatic activity. The results of native-polyacrylamide gel electrophoresis suggested that they could partially recombine, and that the recombined dimer exhibited enzymatic activity. The gene of gamma-glutamyltranspeptidase encoded a signal peptide, and the large and small subunits in a single open reading frame in that order. Two kinds of plasmid were constructed encoding the signal peptide and either the large or the small subunit. A gamma-glutamyltranspeptidase-less mutant of E. coli K-12 was transformed with each plasmid or with both of them. The strain harboring the plasmid encoding each subunit produced a small amount of the corresponding subunit protein in the periplasmic space but exhibited no enzymatic activity. The strain transformed with both plasmids together exhibited the enzymatic activity, but its specific activity was approximately 3% of that of a strain harboring a plasmid encoding the intact structural gene. These results indicate that a portion of the separated large and small subunits can be reconstituted in vitro and exhibit the enzymatic activity, and that the expressed large and small subunits independently are able to associate in vivo and be folded into an active structure, though the specific activity of the associated subunits was much lower than that of native enzyme. This suggests that the synthesis of gamma-glutamyltranspeptidase in a single precursor polypeptide and subsequent processing are more effective to construct the intact structure of gamma-glutamyltranspeptidase than the association of the separated large and small subunits.

Baroreceptor function is restored by antihypertensive therapy through lowering of blood pressure in adult SHR

1. We investigated the effects of antihypertensive treatment (8 weeks) with four different agents (trichlormethiazide, atenolol, nicardipine and enalapril) on baroreceptor function in 28 week old spontaneously hypertensive rats (SHR) to measure aortic depressor nerve (ADN) activity. 2. Threshold pressure (Pth) of ADN activity was elevated and the gain sensitivity of the pressure-activity curve, as determined by the maximum gain (Gmax) of a logistic function curve, was depressed in untreated SHR compared to those in untreated Wistar-Kyoto (WKY) rats. 3. Treatment with the four agents similarly reduced blood pressure in SHR. Each of the four agents induced a decrease in Pth and an increase in Gmax to a similar extent in SHR. 4. These findings suggest that antihypertensive therapy in chronic hypertension augments baroreceptor function through the lowering of blood pressure but not through specific pharmacological actions.

Formation of lyso-glycosphingolipids by Streptomyces sp

The actinomycete strain Streptomyces sp. H37 produces a novel glycosphingolipid-degrading enzyme. This strain was capable of converting ganglioside GM1 to lyso-GM1. After cultivation for 5 days in medium containing GM1, peptone, and detergent, GM1 was found to be almost completely converted to lyso-GM1. The product was purified on a DEAE-Sephadex A-25 column and thin layer chromatographies. The purified lyso-GM1 was hydrolyzed by endoglycoceramidase, and the released oligosaccharide moiety was identified as that of GM1 by HPLC using the pyridylaminoderivative method. The counterpart sphingosine moiety was confirmed with TLC. Moreover, the structure of lyso-GM1 was ascertained by 1H-NMR analysis. The maximum formation of lyso-GM1 was found in 50 mM potassium phosphate buffer (pH 7.5) containing 0.1% glycodeoxycholate. Various lyso-glycoshingolipids, including those of ganglio-, neolacto-, and globo-types, were formed from their parent glycosphingolipids using this strain.

Ultrasonographic evaluation of the carpal tunnel syndrome in hemodialysis patients

The carpal tunnel syndrome (CTS) is the most frequent clinical symptom of dialysis-related amyloidosis. We examined CTS by ultrasound imaging and correlation between CTS and bone amyloidosis was sought. One hundred and forty-two wrists of 71 hemodialysis (HD) patients and 28 of 14 normal volunteers (group 1) were studied. Patients on HD were divided into 4 groups based on HD duration: Group 2, less than 5 years; group 3, 5-10 years; group 4, 10-15 years; and group 5, more than 15 years. The frequency of CTS increased gradually with HD duration: 0% in group 2, 35.3% in group 3, 36.4% in group 4 and 74.1% in group 5. The thickness of the palmar radiocarpal ligament (PRL), corresponding to the posterior wall of the carpal tunnel was significantly greater in groups 4 (p < 0.05), and 5 (p < 0.05) than in group 1 (2.25 +/- 1.30 and 3.11 +/- 1.02 in group 4 and 5 vs 1.15 +/- 0.34 mm in group 1); and the width of the carpal tunnel (CT) was significantly larger in group 5 than in group 1 (6.92 +/- 1.78 in group 5 vs 5.53 +/- 1.48 mm in group 1, p < 0.05). Wrists operated on for CTS had significantly increased PRL and CT compared to the control group and patients without CTS. Patients with CTS had wider PRL than the control and HD patients without CTS. Cystic radiolucent lesions (CRL) of carpal bones characteristic of bone amyloidosis were noted in 15 of 71 HD patients. The thickness of PRL and width of CT in HD patients with CRL exceeded those in patients without CRL. These data indicated that ultrasonographic findings of wrists were closely correlated with the degree of CTS and CRL. The usefulness of ultrasonographic evaluation of PRL thickness and CT width in the evaluation of dialysis-related amyloidosis should be evaluated.

Differential modulation of baroreceptor sensitivity by long-term antihypertensive treatment

We investigated the effects of long-term oral treatment with four different classes of antihypertensive drugs (a thiazide diuretic [trichlormethiazide, 10 mg/kg per day]; a beta-blocker [atenolol, 90 mg/kg per day]; a calcium channel antagonist [nicardipine, 150 mg/kg per day]; and an angiotensin-converting enzyme inhibitor [enalapril maleate, 10 mg/kg per day]) on aortic baroreceptor activity in spontaneously hypertensive rats with chronic hypertension (36 weeks of age). Treatment with each of the four drugs, given from 10 to 36 weeks of age, similarly decreased arterial pressure (171 +/- 2 to 144 +/- 1 mm Hg, P < .01) and similarly decreased the threshold pressure for baroreceptors (116 +/- 3 to 103 +/- 1 mm Hg, P < .05). The four antihypertensive drugs also potentiated the maximal gain of the pressure-activity relation in these rats (untreated, 1.08 +/- 0.05% maximum/mm Hg); however, nicardipine and enalapril (1.77 +/- 0.04% and 1.70 +/- 0.06% maximum/mm Hg, respectively) augmented the maximal gain to a greater extent (P < .05 to .01) than did trichlormethiazide or atenolol (1.49 +/- 0.05% and 1.42 +/- 0.02% maximum/mm Hg, respectively). When the initiation of treatment was delayed to 28 weeks of age, no differences were found in the effects on either threshold pressure (104 +/- 1 mm Hg) or maximal gain (1.36 +/- 0.03% maximum/mm Hg) for all four drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular dissection of the role of the membrane domain in the regulated degradation of 3-hydroxy-3-methylglutaryl coenzyme A reductase

We have previously shown that the membrane domain of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase from hamster contains all of the sequences required for both localization to the endoplasmic reticulum and regulated degradation of the enzyme. It has been reported that the enzymatic activity and mRNA levels of HMG-CoA reductase from sea urchin embryos cultured in the presence of regulators were unchanged compared to levels in control embryos (Woodward, H.D., Allen, M.C., and Lennarz, W.J. (1988) J. Biol. Chem. 263, 18411-18418). This observation led us to investigate the possibility that the sea urchin enzyme is not subject to regulated protein turnover. Interestingly, the sea urchin enzyme shares 62% amino acid sequence identity with the hamster enzyme in the membrane domain and shares similar predicted topological features. In the current studies we have compared the degradation phenotypes of the sea urchin HMG-CoA reductase and the hamster HMG-CoA reductase in Chinese hamster ovary cells to further elucidate the role of the membrane domain in enzyme degradation in response to physiological regulators. To accomplish this, we constructed sea urchin HMGal (uHMGal), the structural equivalent of hamster HMGal (httMGal), which has the sea urchin HMG-CoA reductase membrane domain fused to Escherichia coli beta-galactosidase. The uHMGal was stably expressed in CHO cells, and we found that the degradation of uHMGal is not accelerated by sterols, and even in the absence of sterols, it is less stable than hHMGal. We also constructed chimeric hamster/sea urchin HMGal molecules to investigate which amino acid sequences from the hamster enzyme are sufficient to confer sterol-regulated degradation upon the sea urchin enzyme. Our results identify the second membrane-spanning domain of hamster enzyme as important for the regulated degradation of HMG-CoA reductase.

[Idiopathic chronic tubulointerstitial nephritis]

Chronic tubulointerstitial nephritis is a chronic disorder that primarily affects the renal tubules and interstitium while sparing the glomeruli and vasculature. Although there are variable underlying causes, 10% to 25% of the cases of chronic tubulointerstitial nephritis are entirely idiopathic. A thorough search for the underlying causes of chronic tubulointerstitial nephritis is especially important because to avoid or to correct such underlying causes may prevent progression or even allow for some reversal of renal failure. Thus, diagnosis of idiopathic form of chronic tubulointerstitial nephritis is carefully made after the exclusion of potential underlying causes. The major pathologic features of idiopathic chronic tubulointerstitial nephritis are 1) interstitial fibrosis, 2) tubular atrophy and 3) infiltration of the parenchyma with mononuclear inflammatory cells. Of these three features, the extent of interstitial fibrosis is significantly correlated with decreased glomerular filtration rate and progression rate of renal failure. Idiopathic chronic tubulointerstitial nephritis seems to be slowly progressive and develop to end stage renal failure. Since there is no specific treatment for idiopathic chronic tubulointerstitial nephritis, avoidance and correction of exacerbation factors are essential for progression to renal failure. It is acceptable that protein-restricted diet be applied to idiopathic chronic tubulointerstitial nephritis although its effects on the disease still remain equivocal.

Escherichia coli K-12 copper-containing monoamine oxidase: investigation of the copper binding ligands by site-directed mutagenesis, elemental analysis and topa quinone formation

Copper-containing monoamine oxidase (EC 1.4.3.6) from Escherichia coli is a periplasmic enzyme containing topa quinone in addition to divalent copper as a cofactor. The amino acid sequence of E. coli monoamine oxidase was compared to several cloned amine oxidase genes and five well-conserved histidine residues were found. Site-directed mutagenesis studies were performed to determine which histidine residue serves as the binding ligand to the copper. Enzyme activity, absorption spectrum, and atomic absorption spectrophotometry of the mutant enzymes indicated that histidines 470, 554, and 556 are the copper binding ligands. The absorption spectra of phenylhydrazine derivatives suggested that copper is necessary for topa quinone formation.

Effect of site-directed mutations on processing and activity of gamma-glutamyltranspeptidase of Escherichia coli K-12

gamma-Glutamyltranspeptidase [EC 2.3.2.2] of Escherichia coli K-12 is thought to be synthesized from a single precursor polypeptide into a heterodimeric form through post-translational processing. Cells of a gamma-glutamyltranspeptidase-overproducing transformant of E. coli K-12 were fractionated and the localization of the enzyme was examined by Western blot analysis. The periplasmic fraction only contained the mature form of gamma-glutamyltranspeptidase, membrane fraction only contained the precursor of gamma-glutamyltranspeptidase, and no precursor of gamma-glutamyltranspeptidase was detected in the cytoplasmic fraction. Amino acid residues at the cleavage site for processing into the large and small subunits were substituted by site-directed mutagenesis. The processing phenotypes of six mutants were examined by Western blot analysis, and their gamma-glutamyltranspeptidase activities were measured. Mutations at the N-terminal amino acid residues of the small subunit (Thr-391, Thr-392, and His-393) prevented the maturation of the enzyme and the immature mutants exhibited no enzymatic activity. A mutation at the C-terminal residue of the large subunit (Gln-390) had less effect on the processing and enzymatic activity. These results suggest that the sequence of threonyl-threonyl-histidinyl residues at the N-terminal of the small subunit is very important for the processing of E. coli K-12 gamma-glutamyltranspeptidase and this processing is essential to the expression of gamma-glutamyltranspeptidase activity of E. coli K-12.