The effect of amino acid substitution at position 219 of Citrobacter freundii cephalosporinase on extension of its substrate spectrum.

The cephalosporinase of Citrobacter freundii GN346 is a class-C beta-lactamase comprising 361 amino acids. The substitution of the glutamic acid at position 219 in the enzyme by lysine was previously shown to broaden its substrate specificity to unfavorable substrates such as oxyimino cephalosporins [Tsukamoto, K., Ohno, R. & Sawai, T. (1990) J. Bacteriol. 172, 4348-4351]. To investigate the cause of this phenomenon, Glu219 was changed to glutamine, cysteine or tryptophan. All the resultant enzymes showed higher cefuroxime-hydrolytic activities than the wild type, the order of increasing cefuroxime-hydrolytic activity being as follows: Trp greater than Lys greater than Cys greater than Gln greater than Glu. The rate of hydrolysis of cefuroxime by the Trp219 enzyme was approximately 3 x 10(4) times that of the wild-type enzyme. The order of increasing cefuroxime hydrolysis was approximately proportional to the molecular volume of the amino acid substituted and independent of the ionic character of the amino acids. The cysteine residue at position 219 in the Cys219 enzyme allowed its complete reaction with an SH-blocking reagent, 4-chloromercuriphenylsulfonic acid. The modified enzyme with the bulkier residue showed a 45% higher cefuroxime-hydrolytic activity than the untreated enzyme. These results suggested that extension of the substrate spectrum may be attributed to alteration in the configuration of the enzyme around position 219.     

Downhill movement of litter and its implication for ecological studies in three types of forest in Japan

The amount of litter moving down the slope was measured in three types of forest, together with an examination of rain as a factor in bringing this about. The three forest types were a natural mixed stand ofPinus densiflora and hardwood trees (plot A), aCryptomeria japonica plantation (plot S) and aChamaecyparis obtusa plantation (plot H). The amount of moved litter was quite large in plots A and H, but relatively small in plot S. The rain factor had little influence on litter movement in plot A, but was the main cause of movement in plot S and (especially) plot H. It is suggested that measurement of litter input and output not only by traps above ground level, but also by ones on the ground is essential for determining the cycling of elements inC. obtusa forests. It is also suggested that the decomposition of leaf litter should be studied both on the soil surface and in the soil inC. obtusa forests.     

Chemical modification of coenzyme B12-dependent diol dehydrase with pyridoxal 5′-phosphate: Lysyl residue essential for interaction between two components of the enzyme

The apoenzyme of diol dehydrase was inactivated by modification with pyridoxal 5′-phosphate (pyridoxal-P). The inactivation was accompanied by appearance of a new peak at 425 nm which was shifted to 325 nm by reduction with NaBH₄. ?-N-Pyridoxyl lysine was detected by paper chromatography and paper electrophoresis from the hydrolysate of the NaBH₄-reduced enzyme-pyridoxal-P complex. The relationship of inactivation vs pyridoxal-P incorporation as well as kinetic experiments suggests that one lysyl residue per enzyme molecule was essential for catalytic activity, although two to three pyridoxal-P molecules were introduced into the almost completely inactivated enzyme molecule. Both 1,2-propanediol (substrate) and adenosylcobalamin (coenzyme) completely protected the enzyme from inactivation. The result of disc gel electrophoresis showed that the inactivation of diol dehydrase by pyridoxal-P results from irreversible dissociation of the enzyme into subunits upon pyridoxal-P modification. Therefore, it is suggested that this modifiable lysyl residue is essential for subunit interaction to form an active oligomeric enzyme. The inactivated enzyme restored activity by addition of excess component F, but not by S, suggesting that the essential lysyl residue is located in component F of the enzyme. Pyridoxal-P-modified enzyme was no longer able to bind cyanocobalamin (a competitive inhibitor of adenosylcobalamin).     

Reactive sulfhydryl groups of coenzyme B12-dependent diol dehydrase: Differential modification of essential and nonessential ones

The apoenzyme of diol dehydrase was inactivated by four sulfhydryl-modifying reagents, p-chloromercuribenzoate, 5,5′-dithiobis(2-nitrobenzoate) (DTNB), iodoacetamide, and N-ethylmaleimide. In each case pseudo-first-order kinetics was observed. p-Chloromercuribenzoate modified two sulfhydryl groups per enzyme molecule and modification of the first one resulted in complete inactivation of the enzyme. DTNB also modified two sulfhydryl groups, but modification of the second one essentially corresponded to the inactivation. In both cases, the inactivation was reversed by incubation with dithiothreitol. Cyanocobalamin, a potent competitive inhibitor of adenosylcobalamin, protected the essential residue, but not the nonessential one, against the modification by these reagents. By resolving the sulfhydryl-modified cyanocobalamin-enzyme complex, the enzyme activity was recovered, irrespective of treatment with dithiothreitol. From these results, we can conclude that diol dehydrase has two reactive sulfhydryl groups, one of which is essential for catalytic activity and located at or in close proximity to the coenzyme binding site. The other is nonessential for activity. Neitherp-chloromercuribenzoate- nor DTNB-modified apoenzyme was able to bind cyanocobalamin, whereas the iodoacetamide- and N-ethylmaleimide-modified apoenzyme only partially lost the ability to bind cyanocobalamin. The inactivation of diol dehydrase by p-chloromercuribenzoate and DTNB did not bring about dissociation of the enzyme into subunits. Total number of the sulfhydryl groups of this enzyme was 14 when determined in the presence of 6 m guanidine hydrochloride. No disulfide bond was detected.     

Changes in the levels of prostaglandins and thromboxane and their roles in the accumulation of exudate in rat carrageenin-induced pleurisy — a profile analysis using gas chromatography-mass spectrometry —

Injection of γ-carrageenin into t he pleural cavity of rats caused the accumulation of the pleural exudate. When levels of prostaglandins (PGs) and thromboxane (TX) B₂ were quantified by gas chromatography-mass spectrometry as their methyl ester (ME)-dimethyllisopropylsilyl (DMiPS) ether or ME-methoxine-DMiPS ether derivatives, 6-keto-PGF_1α reached the maximum at 1 hr after carrageenin, then PGE₂ and TXB₂ showed peaks at 3 hr and waned off before 9 hr. he PGF_2α level was kept low, but PGD₂, PGE₁ and PGF_1α were not detected. Aspirin (100 mg/kg, i.p.) significantly decreased the PG and TXB₂ levels and suppressed the rate of plasma exudation until 5 hr, but did not at 7 hr, when it was measured by the amount of exuded pontamine sky blue injected intravenously. OKY-025 (300 mg/kg, i.p.), a selective TXA synthetase inhibitor, and tranylcypromine (20 mg/kg, i.p.), a PGI synthetase inhibitor, could not extensively inhibit the accumulation of the exudate. These results suggest that the cyclooxygenase products of arachidonic acid, particularly PGE₂, definitely play an important role in the exudation during the first 5 hr.     

Kinetics of chemical modification of arginine and lysine residues in calf thymus histone H1

The arginine and lysine residues of calf thymus histone H1 were modified with large molar excesses of 2,3-butanedione and O-methylisourea, respectively. Kinetic study of the modification reaction of the arginine residue revealed that the reaction is divided into the two pseudo-first-order processes. About a third (1 Arg) of the total arginine residues of the H1 molecule was rapidly modified without causing any detectable structural change of the molecule, and the slow modification of the remaining arginine residues (2 Arg) led to a loss of the folded structure of H1. In the case of lysine residue modification, 93% (56 Lys) of the total lysine residues of the H1 was modified with the same rate constant, while 7% (4 Lys) of lysine residue remained unmodified. When the reaction was performed in the presence of 6M guanidine-HCl, all of lysine residues were modified. It is concluded that the 2 arginine and 4 lysine residues resistant to modification are buried in interior regions of the H1 molecule and play an important role in the formation of the H1 globular structure, while the other 1 arginine and 56 lysine residues are exposed to solvent.     

1,N6-Etheno-2-aza-adenosine 3', 5'-cyclic phosphate: human erythrocyte membrane binding and activation of membrane protein kinase.

The relative efficiency of 1,N6-etheno-2aza-adenosine 3', 5'-monophosphate (cyclic 2-aza-epsilon AMP), 1,N6-etenoadenosine 3', 5'-monophosphate (cyclic epsilon AMP) and cyclic AMP in activation of membrane protein kinase and binding to membrane was examined using isolated membranes from human erythrocytes. Cyclic 2-aza-epsilon AMP was 81% as active as cyclic AMP in erythrocyte membrane binding and activation of membrane protein kinase. On the other hand, cyclic epsilon AMP was 37% as active toward membrane protein kinase and 29% toward membrane cyclic AMP binding. Since we have previously shown that the fluorescence of cyclic 2-aza-epsilon AMP is highly sensitive to the polarity of solvents, the high efficiency of cyclic 2-aza-epsilon AMP to substitute for cyclic amp suggests that it may be a suitable microenvironmental fluorescent probe for cyclic AMP binding sites.     

Immunological similarity between NADH-cytochrome b5 reductase of erythrocytes and liver microsomes

In a number of animal species soluble NADH-cytochrome b₅ reductase of erythrocytes was compared with membrane-bound NADH-cytochrome b₅ reductase of liver microsomes by using an antibody to purified NADH-cytochrome b₅ reductase from rat liver microsomes. The results obtained indicated clearly that they are immunologically very similar to each other. The data with erythrocyte ghosts suggested that cytochrome b₅ and NADH-cytochrome b₅ reductase are also present in the ghost.     

Kinetic study of alpha-chymotrypsin catalysis with regard to the interaction between the specificity-determining site and the aromatic side chain of substrates.

In order to investigate how changes in the structures of side-chain aromatic groups of specific substrates influence binding and kinetic specificity in alpha chymotrypsin [EC 3.4.21.1]-catalyzed reactions, a number of nucleus-substituted derivatives of the specific ester substrates were prepared and steady-state kinetic studies were carried out at pH 6.5 and 7.8. Ac-Trp(NCps)-OMe was hydrolyzed more readily at low substrate concentration than Ac-Trp-OMe due to its smaller Km(app) value, suggesting that the bulky 2-nitro-4-carboxyphenylsulfenyl moiety interacts with outer residues rather than with those in the hydrophobic pocket and that this interaction increases the binding specificity. Inhibition experiments using the corresponding carboxylate and analogous inhibitors, however, showed that the carboxy group at the para position of the phenyl nucleus of the substituent sterically hinders association with the active site of alpha-chymotrypsin at pH 7.8 but not at pH 6.5. The kcat values of Ac-Trp(CHO)-0Me, Ac-Tyr(3-NO2)-OMe, and Ac-m-Tyr-OMe were much higher than those of the corresponding specific substrates, indicating that derivatives with a substitute as large as a formyl, nitro or hydroxyl group at the xi-position are stereochemically favorable to the catalytic process. Remarkable increases in Km(app) were also observed. The individual parameters for Ac-Dopa-OMe, however, were comparable to those for Ac-Tyr-OMe.     

Attenuation of SARS‐CoV‐2 replication and associated inflammation by concomitant targeting of viral and host cap 2'‐O‐ribose methyltransferases

The SARS‐CoV‐2 infection cycle is a multistage process that relies on functional interactions between the host and the pathogen. Here, we repurposed antiviral drugs against both viral and host enzymes to pharmaceutically block methylation of the viral RNA 2''‐O‐ribose cap needed for viral immune escape. We find that the host cap 2''‐O‐ribose methyltransferase MTr1 can compensate for loss of viral NSP16 methyltransferase in facilitating virus replication. Concomitant inhibition of MTr1 and NSP16 efficiently suppresses SARS‐CoV‐2 replication. Using in silico target‐based drug screening, we identify a bispecific MTr1/NSP16 inhibitor with anti‐SARS‐CoV‐2 activity in vitro and in vivo but with unfavorable side effects. We further show antiviral activity of inhibitors that target independent stages of the host SAM cycle providing the methyltransferase co‐substrate. In particular, the adenosylhomocysteinase (AHCY) inhibitor DZNep is antiviral in in vitro, in ex vivo, and in a mouse infection model and synergizes with existing COVID‐19 treatments. Moreover, DZNep exhibits a strong immunomodulatory effect curbing infection‐induced hyperinflammation and reduces lung fibrosis markers ex vivo. Thus, multispecific and metabolic MTase inhibitors constitute yet unexplored treatment options against COVID‐19.