Nitrate reductase of Neurospora crassa: the functional role of individual amino acids in the heme domain as examined by site-directed mutagenesis

The enzyme nitrate reductase, which catalyzes the reduction of nitrate to nitrite, is a multi-redox center homodimeric protein. Each polypeptide subunit is approximately 100 kDa in size and contains three separate domains, one each for a flavin, a heme-iron, and a molybdopterin cofactor. The heme-iron domain of nitrate reductase has homology with the simple redox protein, cytochrome b5, whose crystal structure was used to predict a three-dimensional structure for the heme domain. Two histidine residues have been identified that appear to coordinate the iron of the heme moiety, while other residues may be important in the folding or the function of the heme pocket. Site-directed mutagenesis was employed to obtain mutants that encode nitrate reductase derivatives with eight different single amino acid substitutions within the heme domain, including the two central histidine residues. Replacement of one of these histidines by alanine resulted in a completely nonfunctional enzyme whereas replacement of the other histidine resulted in a stable and functional enzyme with a lower affinity for heme. Certain amino acid substitutions appeared to cause a rapid turnover of the heme domain, whereas other substitutions were tolerated and yielded a stable and fully active enzyme. Three different single amino acid replacements within the heme domain led to a dramatic change in regulation of nitrate reductase synthesis, with significant expression of the enzyme even in the absence of nitrate induction.     

Th1 type CD4+ T cells may be a potent effector against poorly immunogenic syngeneic tumors

Biotherapy - We examined the possibility that Th1 type CD4+ T cells may be an effector against three kinds of syngeneic tumors such as highly immunogenic B16 melanoma (B16) and two poorly...     

Protection by dexamethasone from a lethal infection with Listeria monocytogenes in mice

Abstract The effects of dexamethasone (DEX) on a lethal infection with Listeria monocytogenes were studied in mice. Mice were completely protected against the lethal infection when treated with 3.3 mg per kg of DEX. The effect was observed only when DEX was injected before infection. The control mice died from day 3 to day 5 of infection, whereas DEX-treated mice could eliminate L. monocytogenes cells from the organs by day 11 of infection. High titres of endogenous tumour necrosis factor (TNF), interleukin-6 (IL-6) and gamma interferon (IFN-γ) were induced in the bloodstreams and organs of the drug-free mice. DEX suppressed IL-6 production, but augmented TNF and IFN-γ production within 24 h of infection, whereas production of all three endogenous cytokines was suppressed in the DEX-treated mice on day 3 of infection when the control mice began to die. These results suggest that DEX shows a protective effect on a lethal infection with L. monocytogenes in mice and that regulation of production of endogenous cytokines might be involved in the effect of DEX.     

Studies on the mechanism of action of guinea pig lymphotoxin. I. Membrane active substances prevent target cell lysis by lymphotoxin.

The effect of various pharamacologic agents on lysis of L cells by guinea pig lymphotoxin (LT) was studied. Among the many drugs tested, only those that affect plasma membrane functions were found to interfere with the cytolytic action of LT. Dimethyl sulfoxide or lidocaine potentiated L cell resistance against lysis. Stronger protection was provided by ouabain. Addition of ouabain to cells previously injured by LT was also effective in reducing cell death. Attempts to detect metabolic disturbances in cells before LT cytolysis were unsuccessful. The biosynthetic rate of DNA, RNA, or protein, and the total cellular content of ATP were not significantly changed in LT-treated cells. The results suggest that LT disturbs some plasma membrane functions of the target cell, perhaps ion transport systems, and consequently induces ionic imbalances between the intra-and extracellular milieu which eventually cause cell death.     

Control of the germination of secondary dormant cocklebur seeds by various germination stimulants

The responsiveness of non-dormant, upper cocklebur (Xanthiumpennsylvanicum Wallr.) seeds to various germination stimulants,such as CO₂ C₂H₄ CS(NH₂)₂, BA and enriched O₂, decreased withincreasing periods of water imbibition and was completely lostin the state of secondary dormancy. Unlike CO₂ BA and CS(NH₂)₂however, C₂H₂ and enriched O₂ effectively prevented the developmentof secondary dormancy, and their combination was the most effectivefor stimulating the germination of seeds which had undergoneimbibition for a long time. CS(NH₂)₂ and BA were effective,not by themselves but either under anaerobiosis or elevatedO₂ tension. Growth of the axial and cotyledonary segments excisedfrom aged seeds remained responsive to these germination stimulantsand could be further stimulated by exogenous C₂H₂. With imbibitionat a lower temperature, the seeds maintained high germinationin response to various stimulants and a high rate of C₂H₂ andCO₂ production during a long period of water imbibition. Theseresults are discussed in terms of the two possible causes forthe loss of responsiveness or induction of the secondary dormancy. (Received June 27, 1978; )     

Fluorescent tracer method for protein SH groups. II

An improved fluorescent tracer technique for protein SH groups is described using a fluorescent thiol reagent, N-(7-dimethylamino-4-methylcoumarinyl)-maleimide. The direct measurement with a scanning fluorometer of the fluorophore-labeled proteins separated by SDS polyacrylamide gel electrophoresis achieved a simple, precise, and sensitive determination of the SH group of the order of picomoles per band. In addition, the direct recording of peaks enabled us to analyze more complex protein systems, compared with our previously reported method using an extraction step from gel slices. As an application, we compared the reactivity of the SH groups of proteins in glycerinated muscle fibers under two conditions, in rigor and in contraction.     

Fine structural localization of thiamine pyrophosphatase and acid phosphatase activities in the mouse pancreatic acinar cell

The fine structural localization of thiamine pyrophosphatase (TPPase) and acid phosphatase (AcPase) was examined in pancreatic acinar cells of fasting and fed mice. The results were not affected by these conditions. TPPase activity was positive in two and sometimes three cisternae of the inner Golgi lamellae as well as in the condensing vacuoles of the trans area, but negative in the rigid lamellae and small vesicles of the trans area. AcPase activity was demonstrated in two and sometimes three cisternae of inner Golgi lamellae, condensing vacuoles, rigid lamellae, lysosomes and smooth or coated vesicles in the trans area. The inner Golgi lamellae and the condensing vacuoles were positive for both enzyme activities. From these facts, the lysosome is considered to be formed not only in the GERL system but also through the rough endoplasmic reticulum-Golgi apparatus route. It is reasonable to consider that Novikoff's GERL is not independent from the Golgi apparatus but represents a part of this organelle.     

Proton translocating ATPase of a thermophilic bacterium. Morphology, subunits, and chemical composition.

1. A stable membrane-bound ATPase [EC 3.6.1.3] (TF0-F1) capable of proton translocation in reconstituted vesicles was purified from the thermophilic bacterium PS3 cultured in medium containing L-[U-14C]amino acids. 2. TF0-F1 was composed of a catalytic moiety (TF1) and a hydrophobic moiety (TF0). TF1 contained 3 polypeptide chains with molecular weights of 56,000, 3 of 53,000, 1 of 32,000, 1 of 15,500, and 1 of 11,000. TF0 contained 1 chain of 19,000, 2 of 13,500, and 5 of 5,400 daltons. TF1 was dissociated into subunits much less readily than F1. 3. TF1 consisted of 95A particles arrayed in hexagonal microcrystals. TF0-F1 consisted of a sphere (TF1) and a stalk plus base (TF0) which was buried in the membrane of the proton translocating vesicles. 4. Vesicles capable of energy transformation were formed when TF1 came in contact with the surface of liposomes containing TF0. On addition of phospholipids, the helix content of TF0 increased 3-fold. The role of F0 in forming channels for protons is discussed. 5. The amino acid compositions of TF0, TF1, and TF0-F1 were compared. TF0 was not hydrophobic, despite its interaction with phospholipids. The phospholipid composition and other properties of the proton translocating vesicles were examined. Vesicles reconstituted from a mixture of phosphatidylethanolamine, phosphatidylgly-cerol, and cardiolipin in the same ratio as in the membranes had the highest activity.