Effects of antibacterial drugs on the total cholesterol balance of cholesterol-fed mice

Mice were fed a diet containing 1.5% cholesterol, and the total cholesterol balance and the distribution of the consumed sterol among excreta, liver, and carcass were determined, using experimental periods of 9–14 days. It was observed that an average of approximately 12% of the cholesterol was either destroyed or chemically modified. When, however, 1% sulfasuxidine and 0.04% streptomycin were added to the diet, all the sterol “put in” was recovered, within the limits of error of the method. These results were tentatively interpreted to indicate that under the conditions of these experiments, microorganisms of the intestine were primarily responsible for the destruction or modification of the consumed sterol, as observed in the absence of antibacterial drugs.     

Affinity labeling of dihydrofolate reductase with an antifolate glyoxal

Dihydrofolate reductases from different species contain several highly conserved arginines, some of which have been shown by x-ray crystallography to have their guanido groups near the p-aminobenzoyl glutamate moiety of enzyme-bound methotrexate. The orientation of one of these (Arg-52) appears to be completely reversed in comparing the crystal structures of Escherichia coli with Lactobacillus casei enzyme (Bolin, J. T., Filman, D. J., Matthews, D. A., Hamlin, R. C., and Kraut, J. (1982). J. Biol. Chem. 257, 13650-13662). We synthesized a novel antifolate containing a glyoxal group designed to react specifically with active-site guanido groups which are able to approach the p-aminobenzoyl carbonyl of methotrexate. The binding of this compound to the enzyme was competitive with dihydrofolate (DHF) in ordinary buffers. In borate buffer at pH 8.0 it inactivated dihydrofolate reductases from both E. coli and L. casei at similar maximum rates, while the chicken liver enzyme was more slowly inactivated. The inactivation was stoichiometric, paralleled the loss of the glyoxal chromophore, and showed saturation kinetics. Inhibitor binding and thus inactivation was enhanced by NADPH, while DHF protected the enzyme. This allowed calculation of the Kd for DHF which was found to be identical with its Km. The stoichiometrically inactivated enzyme displayed the 340-nm chromophore characteristic of 4-aminopteridines bound to dihydrofolate reductase confirming active-site labeling with normal orientation of the ligand. The ligand remained covalently bound to inactivated enzyme upon denaturation at low pH but dissociated at neutral pH. Computer graphic modeling of the crystal structures predicted reaction of Arg-31 but not Arg-52 in L. casei dihydrofolate reductase and of only Arg-52 in the E. coli enzyme. Purification of the CNBr fragments from the inactivated enzymes gave a single labeled peptide for each species. The particular peptide tagged in each case was unaffected by the presence of NADPH and was in excellent agreement with the crystallographic predictions.     

Trace elements in human parotid saliva

Although various proteins and some electrolytes have been measured in human saliva, little systematic data about the major and minor elemental components of this body fluid have been obtained. In order to obtain such data, concentrations of C, Na, P, Cl, K, Ca, Sc, Cr, Fe, Co, Zn, Se, Br, Rb, Sb, I, and Cs in human parotid saliva were measured by instrumental nuclear methods. The data obtained confirmed the relative lack of Zn in saliva of patients with hypogeusia (decreased taste acuity) and suggested that concentrations of Na, Cl, Br, and Ca followed the order: normals > hypogeusia > hyposmia (decreased smell acuity). To compare concentrations of elements in saliva with those in blood and urine, absolute concentrations were normalized to that of Na through the use of a concept called an enrichment factor. On this basis, parotid saliva is relatively depleted in Se, Zn, and Fe and enriched for most other elements relative to blood plasma indicating that the fluid is not simply a transudate of blood plasma. Using this same technique, saliva composition was found more similar to urine than blood plasma, being relatively depleted in Se, Cs, and Co, being enriched in I, Br, and Cr and having about the same relative concentrations of P, Cl, Zn, Fe, Ca, K, and Rb. As the total body concentrations of many of the enriched elements in saliva are extremely small, their enrichment in saliva suggests special roles for these elements in the oral cavity. Because of its accessibility, ease of collection, and interaction with some body constituents, saliva represents a useful, albeit neglected, tool in the diagnosis of some physiological and pathological changes in body function and in understanding important aspects of trace metal metabolism.     

Zinc metabolism in humans: a kinetic model

The development and testing of an integrated kinetic model describing both short and long term zinc metabolism in humans is discussed. The development took place in two stages. The first, based on a five day turnover study, details the early phases of zinc metabolism in several body tissues together with absorption and excretion. This model, however, accounts for only 10% of total body zinc. To describe the kinetics of total body zinc, a long term model which contains a reduced version of the five day model was developed. This model required an additional compartment for the liver system, but as postulated in the short term model, 90% of body zinc turns over slowly in a single compartment. The models are then used to describe the effect on zinc metabolism of different disease states and lietary perturbations. These additional studies enhance the models' validity while providing physiological insights.     

Comparative Localization of Receptors for Plasmin and for Urokinase on MCF7 Cells

The receptor for plasmin and the receptor for urokinase-type plasminogen activator were characterized on MCF 7 cells either independently or simultaneously, using fluorescence microscopy and confocal microscopy. The plasmin receptor was visualized, as previously described by Correc et al. (Int. J. Cancer 50, 767, 1992) using biotinylated plasminogen and fluoresceinated streptavidin. The urokinase receptor was revealed by both polyclonal and monoclonal antibodies reacting specifically with this receptor and by binding of urokinase aminoterminal fragment. On unfixed cells, these methods gave the same heterogeneous patterns of surface staining, consisting of contours and grains, localized mainly at the upper nonadherent face of the tumor cells by confocal microscopy. Only a part of the cells was stained. When both receptors were characterized together, their presence was found on the same cells and they gave almost superimposable patterns in many cases, as shown by confocal microscopy. In contrast, when MCF 7 cells were fixed or permeabilized before staining, quite different patterns were observed: almost all the cells were labeled. The staining was mainly cytoplasmic and localized preferentially in the center and close to the upper face of the cells. Similar results were found with antiserum against urokinase receptor and monoclonal antivinculin antibody. It is likely that the receptor for urokinase and the receptor for plasmin have similar localizations on MCF 7 cells, thus resulting in a functional cooperation.     

The Staphylococcus aureus ileS gene, encoding isoleucyl-tRNA synthetase, is a member of the T-box family.

The Staphylococcus aureus ileS gene, encoding isoleucyl-tRNA synthetase (IleRS), contains a long mRNA leader region. This region exhibits many of the features of the gram-positive synthetase gene family, including the T box and leader region terminator and antiterminator. The terminator was shown to be functional in vivo, and readthrough increased during growth in the presence of mupirocin, an inhibitor of IleRS activity. The S. aureus ileS leader structure includes several critical differences from the other members of the T-box family, suggesting that regulation of this gene in S. aureus may exhibit unique features.     

Synthesis and characterization of 2′-azidoaminopterin as a potential photoaffinity label for folate-utilizing enzymes

A photoreactive analog of aminopterin, 2′-azidoaminopterin (VI), was synthesized and evaluated as a potential inhibitor and photoaffinity label of folate-utilizing enzymes. The compound was tightly bound to dihydrofolate reductase (DHFR) from escherichia coli (MB 1428) with K₁ equal to 3 × 10^?11M and to the enzyme from mouse (S-180) cells with K₁ approximately equal to 2 × 10^?10M. Dissociation constants measured by equilibrium dialysis using radioactive 2′-azidoaminopterin gave a value of K_D = 3.2 × 10^?9M for the bacterial enzyme. The presence of NADPH enhanced the affinity by more than an order of magnitude. Azidoaminopterin is also an inhibitor of thymidylate synthetase from Lactobacillus casei, competitive with methylene-tetrahydrofolate (K_i 7 × 10^?7M). Photolysis of the radioactive inhibitor in complex with DHFR from E. coli led to approximately 3% covalent incorporation of label into protein. The greater part of this attachment was nonspecific as shown by the lack of protection in the presence of methotrexate. Thymidylate synthetase from L. casei was not significantly inactivated upon photolysis in the presence of the inhibitor and deoxyuridylate. Model studies showed that photoreaction of the inhibitor led to covalent linkages with thiol, lysyl amino groups, and the hydroxyl groups of alcohols. Azidoaminopterin may be useful in labeling other enzymes of folate metabolism, although a minor photoproduct reacts nonspecifically with many proteins. The antifolate can be photoconjugated to polylysine as well as to proteins. The polylysine conjugates inhibit DHFR. Difference spectrum analysis of the photoproducts from the irradiation of the DHFR I complex indicates that water reacts efficiently with the enzyme-bound nitrene and must therefore have access to at least part of the bound p-aminobenzoyl group. This analysis suggests that azide analogs of protein ligands may be useful as reporter groups in probing the hydrophobicity of binding sites.     

Domain structure and interactions of recombinant urokinase-type plasminogen activator.

Urokinase-type plasminogen activator (uPA) is a mosaic glycoprotein composed of an epidermal growth factor-like (EGF), a kringle and a serine protease (SP) module. It exists in single and two-chain forms designated HMW pro-uPA and HMW uPA, respectively. A low molecular weight form, LMW uPA, lacks the EGF and kringle modules and is composed of the SP module alone. Recombinant-expressed proteins representing both HMW forms exhibit four reversible unfolding transitions that are resolved by deconvolution of melting curves obtained by differential scanning calorimetry at pH 4.5; no differences in the melting properties of the single and two-chain forms were found. The proteolytic fragment Ser1-Lys135 (EGF-kringle) exhibits two transitions, while the isolated EGF and kringle modules each exhibit a single two-state transition. Thus, both of these modules retain an independently folded compact structure when isolated. The isolated SP module (LMW uPA) exhibits two closely spaced transitions at low pH indicating the melting of two domains of similar stability. Fluorescence-detected melting curves of LMW uPA reveal increasing cooperativity with increasing pH, suggesting an increase in the interaction between the two SP domains. Treatment of both HMW and LMW uPA with the tripeptide inhibitor Glu-Gly-Arg-chloromethylketone dramatically increased the stability of both domains of the SP module which now melt together in a single two-state transition, even at low pH, with no effect on the EGF and kringle modules. From these data one concludes that UK consists of four independently folded domains. Two are formed by the EGF and kringle modules which do not interact with each other or with the SP module. The SP module contains two domains that are independent at low pH but exhibit a tendency to merge into a single cooperative unit at neutral pH or after treatment with the tripeptide inhibitor.