Change of plasma lipoproteins by heparin-released lipoprotein lipase

Lipoprotein lipase (LPL) is known to be attached to the luminal surface of vascular endothelial cells in a complex with membrane-bound heparan sulfate, and released into blood stream by heparin. LPL that catalyzes hydrolysis of triglyceride (TGL) on chylomicron and VLDL into two fatty acids and monoacylglycerol, is also implicated to participate in an enhancement of cholesterol uptake by arterial endothelial cells in vitro. But little is known about the LPL-mediated cholesterol uptake in physiological state. In this study, changes in blood lipid composition and levels of lipoproteins were determined after the injection of heparin in human. The level of LPL in plasma was increased from 0 to 11 mU/ml within 30-40 min post-heparin administration and decreased to the basal level within 2 h. The level of TGL in plasma decreased from 70 mg/dl to 20 mg/dl within 1 h and gradually increased to 80 mg/dl within 4 h. However the level of total cholesterol in plasma remained at 140 mg/dl during an experimental period of 4 h. Analysis of Lipoproteins in plasma by NaBr density gradient ultracentrifugation showed that the level of VLDL decreased from 50 mg/dl to 10 mg/dl within 1-2 h and returned to normal plasm level at 4 h. However there were no significant changes in the level of LDL and HDL. These results suggest that, at least, in normo-lipidemic subjects, increased free plasm LPL acts primarily on VLDL and failed to show any significant uptake of cholesterol-rich lipoproteins in human.

Cytotoxicity of Vibrio vulnificus cytolysin on rat peritoneal mast cells

Histamine has been thought to be a permeability enhancing factor in Vibrio vulnificus infection. The injection of living bacteria or purified V. vulnificus cytolysin (VVC) can cause lethality in mice by inducing hemoconcentration and increased vascular permeability. In the present study, we tried to identify whether histamine release causes the increased vascular permeability that is responsible for the lethal effect of VVC. Treatment of rat peritoneal mast cells with high concentrations of VVC caused the release of whole cellular histamine and lactate dehydrogenase (LDH). At concentrations less than 10 HU/ml, histamine and LDH were not released whereas preloaded 2-deoxy-D-glucose was rapidly effluxed with the concomitant decrease in cellular ATP. VVC-treated mast cells were refractory to the stimulation of histamine secretion by Compound 48/80 but remained fully responsive to Ca2+ plus GTP-gamma-S. These results indicate that histamine can be released from mast cells only when the concentration of VVC is high enough to cause the lysis of cells. At low concentrations, VVC does not induce the release of stored histamine from damaged cells. The intravenous injection of 80 HU purified VVC to rats, which can produce the calculated blood concentration of about 3 HU/ml, caused a marked increase in pulmonary vascular permeability, hemoconcentration and death. However, no increase in blood histamine level was detected. This level of VVC in rat blood was enough to cause severe hemoconcentration and lethality but might not be enough to cause cytolysis of the mast cells and resulting histamine release.

Complementation of the growth defect of an rnpA49 mutant of Escherichia coli by overexpression of arginine tRNA(CCG)

We previously found that overexpression of arginine tRNA(CCG) from Brevibacterium albidum complements the rnpA49 mutation, which is responsible for the thermosensitivity of Escherichia coli RNase P function. In this present work, we show that the E. coli homologue tRNA also complements the same mutation, but other tRNAs do not. These results suggest that the rnpA49 mutation causes a major cellular defect in an RNase P reaction to generate the mature arginine tRNA(CCG).

Human D3 dopamine receptor in the medulloblastoma TE671 cell line: cross-talk between D1 and D3 receptors

In search of a cell line in which the D3 dopamine receptor is expressed endogenously, we found that the neuron-derived human medulloblastoma cell line TE671 expresses the human D3 (hD3) and D1 (hD1) receptor, but neither the D2 or D4 receptors. Exposure of TE671 cells to the D3 agonist 7-OH-DPAT (DPAT), or to the D1 agonist SKF-38393 (SKF) increased the expression of hD3 or hD1 mRNA, respectively. Moreover, whereas DPAT had no effect on hD1 mRNA levels, stimulating the cells with SKF caused an increase in both hD1 and hD3 transcript levels. These results suggest (i) that following ligand stimulation, hD3 and hD1 receptors are upregulated to enhance their own receptor expression, and (ii) that upregulation of hD1 receptor transcripts leads to a stimulation of the hD3 dopamine receptor transcripts.

Transjugular intrahepatic portosystemic shunt: effect of bile leak on smooth muscle cell proliferation

PURPOSE

To evaluate the effect of bile on smooth muscle cell (SMC) proliferation in vitro and in vivo in a porcine transjugular intrahepatic portosystemic shunt (TIPS) model.

MATERIALS AND METHODS

In vitro, SMCs explanted from porcine thoracic aorta were cultured with standard techniques. After initial pilot studies, they were subcultured in one of three groups: 1% porcine serum plus 1% bile, 10% porcine serum plus 1% bile, and 10% porcine serum. Cells were harvested at 3, 10, or 14 days, and DNA, protein, and disintegrations per minute (an indicator of proliferation) were measured. In vivo, TIPS creation was successful in 45 swine. All pigs were euthanized at 10-16 days. The proliferative response within the stent was histologically quantified and correlated for evidence of bile leak.

RESULTS

In pilot studies, 2.5%-10.0% bile solutions caused 100% SMC mortality by 3 days. In the presence of 1% bile (with or without porcine serum), both DNA and protein production decreased significantly compared with that in porcine serum alone (P < .05). In vivo, 13 of 45 specimens (29%) showed bile leak at gross or microscopic examination. SMC proliferation was less overall in animals with versus those without bile leak (difference not significant).

CONCLUSION

These data suggest that the proliferative response in a TIPS is not primarily due to bile leak. Bile leak may promote thrombosis, but it appears to inhibit myointimal proliferation.

Guillain-Barré syndrome: MR imaging findings of the spine in eight patients

PURPOSE

To evaluate magnetic resonance (MR) imaging findings of the spine in patients with Guillain-Barré syndrome.

MATERIALS AND METHODS

MR imaging findings in eight patients (three male, five female; age range, 2-47 years) with Guillain-Barré syndrome were retrospectively reviewed. Guillain-Barré syndrome was diagnosed mainly on the basis of symptoms and also on the basis of supportive ancillary data, such as the results cerebrospinal fluid analysis and electrophysiologic evaluation. In addition, follow-up MR imaging was performed in three patients, who had slight clinical improvement.

RESULTS

All patients had thickening of the intrathecal spinal nerve roots and cauda equina, with varying degrees of enhancement on gadolinium-enhanced axial T1-weighted images. Two enhancement patterns were noted. One was enhancement of both the anterior and posterior spinal nerve roots (n = 2); the other was enhancement of the anterior spinal nerve roots only (n = 6). Follow-up MR imaging in the three patients with slight improvement of symptoms revealed that the thickening and the degree of enhancement of the spinal nerve roots were diminished.

CONCLUSION

Although the enhancement of the intrathecal spinal nerve roots is not specific to Guillain-Barré syndrome and can be seen in neoplasia and other inflammatory processes, the enhancement of only the anterior spinal nerve roots is strongly suggestive of Guillain-Barré syndrome.

Purification and characterization of adenosine diphosphate ribose pyrophosphatase from human erythrocytes

Free ADP-ribose is a turnover product of NAD+, protein-bound polymeric and monomeric ADP-ribose, and cyclic ADP-ribose. But little is known about the specific cellular roles or metabolism of free ADP-ribose. ADP-ribose pyrophosphatase (EC 3.6.1.13), which hydrolyzes ADP-ribose into AMP and ribose-5'-phosphate, was purified from human erythrocytes. Purification was achieved to homogeneity by successive chromatographic steps, resulting in a final purification of 75,790-fold from the hemolysate. The purified enzyme showed a single band with the molecular weight of 34 kDa on SDS-PAGE both in the presence and absence of 2-mercaptoethanol. The molecular weight of the native enzyme calculated by gel filtration was 68 kDa, indicating that the active enzyme is a dimer of identical subunits. The enzyme requiring Mg2+ showed highest activity toward ADP-ribose, and about 40-70% activities with IDP-ribose, ADP-mannose and GDP-mannose. The enzyme showed a Km of 169 +/- 11 microM for ADP-ribose, broad pH optimum around pH 9.5, and pI of 5.1. ADP was a potent noncompetitive inhibitor with a Ki of 16 +/- 1.2 microM. These results suggest that our enzyme is unique, and different from the other ADP-ribose pyrophosphatases reported. ADP-ribose pyrophosphatase may play an important role in the regulation of intracellular steady-state of free ADP-ribose.

Expression of C5 protein, the protein component of Escherichia coli RNase P, from the tac promoter

The accurate function of C5 protein, the protein component of Escherichia coli RNase P, is uncertain in vivo. A controllable expression system for C5 protein was constructed which can be used to investigate effects of C5 protein on various cellular functions including biosynthesis of RNase P in vivo. The semisynthetic rnpA gene encoding C5 protein was fused to the tac promoter of the pKK223-3 expression vector. This tac promoter expression system produced a high level of C5 protein upon induction with isopropyl-beta-D-thiogalacto-pyranoside. When the overexpressed C5 protein was purified and used for reconstitution of RNase P, the reconstituted enzyme was active. The N-terminal amino acid of the overexpressed C5 protein was leucine specified by the second codon of the rnpA gene. The more controllable expression system was constructed by introducing the lacIq gene into the vector sequence itself.

Factors affecting the ability of energy functions to discriminate correct from incorrect folds

Eighteen low and medium resolution empirical energy functions were tested for their ability to distinguish correct from incorrect folds from three test sets of decoy protein conformations. The energy functions included 13 pairwise potentials of mean force, covering a wide range of functional forms and methods of parameterization, four potentials that attempt to detect properly formed hydrophobic cores, and one environment-based potential. the first of the three test sets consists of large ensembles of plausible conformations for eight small proteins, all of which have correct native secondary structure and are reasonably compact. The second is the set of all subconformations in a database of known protein structures applied to the sequences in that database (ungapped threading). The third is a set of ensembles of 1000 conformations each for seven small proteins taken from molecular dynamics simulations at 298 K and 498 K. Our results show that there are functions effective for each challenge set; moreover, success in one test is no guarantee of success in another. We examine the factors that seem to be important for accurate discrimination of correct structures in each of the test sets, and note that extremely simple functions are often as effective as more complex functions.

Mutations in the rpoB gene of Mycobacterium tuberculosis that interfere with PCR-single-strand conformation polymorphism analysis for rifampin susceptibility testing

Rifampin susceptibility of 32 rifampin-resistant and 26 rifampin-susceptible Mycobacterium tuberculosis strains was analyzed by PCR-single-strand conformation polymorphism (SSCP) and DNA sequencing within the 157-bp region of the rpoB gene (Ala500 to Val550). Two false-positive PCR-SSCP results were observed among the susceptible strains due to the silent mutation Gln513 (CAA-->CAG) and the deletion mutation Thr508 and Ser509. Another silent mutation [Leu511 (CTG-->CTA)], combined with the mutation Ser531-->Leu, was observed in a resistant strain. These results suggest that to rule out false-positive PCR-SSCP results, sequencing of the target DNA is required.

Effect of carbonyl cyanide m-chlorophenylhydrazone (CCCP) on the dimerization of lipoprotein lipase

Lipoprotein lipase (LPL), an enzyme playing the central role in triglyceride metabolism, is a glycoprotein and a homodimer of identical subunits. Dimerization and proper processing of oligosaccharide chains are important maturation steps in post-translational regulation of enzyme activity. Indirect evidences suggest that dimerization of LPL occurs in endoplasmic reticulum (ER) or Golgi. In this study, we investigated the dimerization status of LPL in 3T3-L1 adipocytes, using sucrose density gradient ultracentrifugation and carbonyl cyanide m-chlorophenylhydrazone (CCCP), an inhibitor of ER-Golgi protein transport. In the presence of CCCP, no increase of cellular LPL activity was detected during 2 b of recovery period after the depletion of LPL, with heparin and cycloheximide. Only endoglycosidase H (endo H)-sensitive subunits were found in CCCP-treated cells after endo H digestion, suggesting that inactive LPL was retained in ER. In the presence of castanospermine, an inhibitor of ER glucosidase I, LPL subunits of both control and CCCP-treated cells had same molecular weight, indicating that complete oligosaccharides were transferred to LPL subunits in the presence of CCCP. In sucrose density gradient ultracentrifugation, all the LPL protein synthesized in the presence of CCCP was found at the dimeric fractions as in control cells. Most of LPL protein in control cells showed high affinity for heparin, and there was no difference between the control and CCCP-treated cells. These results suggest that dimerization and acquisition of high affinity for heparin of LPL can occur in ER of CCCP-treated cells without acquisition of catalytic activity.

Pulmonary damage by Vibrio vulnificus cytolysin

Vibrio vulnificus is an estuarine bacterium that causes septicemia and serious wound infection. Cytolysin produced by V. vulnificus has been incriminated as one of the important virulence determinants of bacterial infection. Cytolysin (8 hemolytic units) given intravenously to mice via their tail veins caused severe hemoconcentration and lethality. Cytolysin treatment greatly increased pulmonary wet weight and vascular permeability as measured by (125)I-labeled albumin leakage without affecting those factors of other organs significantly. Blood neutrophils were markedly decreased in number after cytolysin injection, with a concomitant increase in the level of pulmonary myeloperoxidase activity, indicating that cytolysin-induced neutropenia might be due to pulmonary sequestration of neutrophils. By microscopic examination, severe perivascular edema and neutrophil infiltration were evident in lung tissues. These results suggest that increased vascular permeability and neutrophil sequestration in the lungs are important factors in lethal activity by cytolysin.

A targeted mutation of the D3 dopamine receptor gene is associated with hyperactivity in mice

While most effects of dopamine in the brain are mediated by the D1 and D2 receptor subtypes, other members of this G protein-coupled receptor family have potentially important functions. D3 receptors belong to the D2-like subclass of dopamine receptors, activation of which inhibits adenylyl cyclase. Using targeted mutagenesis in mouse embryonic stem cells, we have generated mice lacking functional D3 receptors. A premature chain-termination mutation was introduced in the D3 receptor gene after residue Arg-148 in the second intracellular loop of the predicted protein sequence. Binding of the dopamine antagonist [125I]iodosulpride to D3 receptors was absent in mice homozygous for the mutation and greatly reduced in heterozygous mice. Behavioral analysis of mutant mice showed that this mutation is associated with hyperactivity in an exploratory test. Homozygous mice lacking D3 receptors display increased locomotor activity and rearing behavior. Mice heterozygous for the D3 receptor mutation show similar, albeit less pronounced, behavioral alterations. Our findings indicate that D3 receptors play an inhibitory role in the control of certain behaviors.

Glycosylation of lipoprotein lipase in human subcutaneous lipomas

Glycosylation of lipoprotein lipase (LPL) was studied in human subcutaneous lipomas. Heparin-releasable LPL activities were higher in lipomas than those in adjacent normal adipose tissues, and showed good correlation with cellular LPL protein mass. Molecular weight of LPL subunit was 57 kDa in both tissues. After endoglycosidase H-digestion, two types of LPL subunits were found in normal adipose tissues; partially sensitive (55 kDa) and totally sensitive (52 kDa) form. In lipoma tissues, the fraction of partially sensitive form (55 kDa) was increased comparing with control adipose tissues. These results suggest that partially sensitive subunits constitute the major secretable form of LPL in human subcutaneous lipomas.

Manipulation of the immune response by foreign gene expression in the thymus

Retroviral gene transfer vectors have been developed for optimal in vivo gene therapy. Ideally, these vectors should target gene expression specifically to selected tissues or organs. Our studies focus on the development of retroviral vectors for gene delivery to the thymus. The goal of these studies is to utilize thymic expression of exogenous genes to manipulate the immune repertoire. We have characterized the selective thymic tropism of a molecular clone of Gross murine leukemia virus, GD-17, to thymic medullary epithelial cells using immunohistochemical staining and confocal microscopy. Specific expression of viral antigens in the thymus lead to the induction of immunologic tolerance to GMuLV proteins. This tissue specific vector may thus be used to study the requirements of epithelial mediated tolerance induction, and provide a more efficient tool for gene therapy.

The complexity and accuracy of discrete state models of protein structure

The prediction of protein structure depends on the quality of the models used. In this paper, we examine the relationship between the complexity and accuracy of representation of various models of protein alpha-carbon backbone structure. First, we develop an efficient algorithm for the near optimal fitting of arbitrary lattice and off-lattice models of polypeptide chains to their true X-ray structures. Using this, we show that the relationship between the complexity of a model, taken as the number of possible conformational states per residue, and the simplest measure of accuracy, the root-mean-square deviation from the X-ray structure, is approximately (Accuracy) varies; is directly proportional to (Complexity)-1/2. This relationship is insensitive to the particularities of individual models, i.e. lattice and off-lattice models of the same complexity tend to have similar average root-mean-square deviations, and this also implies that improvements in model accuracy with increasing complexity are very small. However, other measures of model accuracy, such as the preservation of X-ray residue-residue contacts and the alpha-helix, do distinguish among models. In addition, we show that low complexity models, which take into account the uneven distribution of residue conformations in real proteins, can represent X-ray structures as accurately as more complex models, which do not: a selected 6-state model can represent protein structures almost as accurately (1.7 A root-mean-square) as a 17-state lattice model (1.6 A root-mean-square). Finally, we use a novel optimization procedure to generate eight 4-state models, which fit native proteins to an average of 2.4 A, and preserve 85% of native residue-residue contacts. We discuss the implications of these findings for protein folding and the prediction of protein conformation.

Protective mechanism of glucose against alloxan-induced pancreatic beta-cell damage

Glucose prevented the alloxan- or H2O2-induced inhibition of insulin secretion in rat pancreatic islets. Hydrogen peroxide was detected during the incubation of islets with alloxan, and this generation of hydrogen peroxide was not affected by glucose. Treatment of beta-cells with alloxan or H2O2 caused elevation of cytosolic free Ca2+ and decrease of cellular NAD+. Glucose blocked the decrease of cellular NAD+ level, but did not abolish the increase of cytosolic Ca2+. These results indicate that glucose protected pancreatic beta-cell damage after the H2O2 generation and Ca2+ influx on a chain of reactions in the diabetogenesis of alloxan.

Structural organization of the murine D3 dopamine receptor gene

We have cloned the gene encoding the murine D3 dopamine receptor and have analyzed its intron-exon structural organization, to gain a better understanding of the detailed architecture of the D2 dopamine receptor genes. Restriction and sequence analysis reveal the presence of six introns, in contrast to the five introns previously reported for the rat D3 receptor. The extra intron is located in the receptor's putative third cytoplasmic loop and generates an intron-exon organization directly analogous to that found in the D2 receptor gene. In addition, we have sequenced the 5' and 3' nontranslated sequences flanking the coding region and have identified a putative poly(A) adenylation signal. These sequences are found to have a far lower homology with the corresponding rat nontranslated sequences than is found for the D2 receptor, suggesting that the control of D3 receptor expression may vary more between species than the control of D2 receptor expression.

Glycosylation, dimerization, and heparin affinity of lipoprotein lipase in 3T3-L1 adipocytes

The relationship between glycosylation, dimerization, and heparin affinity of lipoprotein lipase (LPL) was studied in 3T3-L1 adipocytes. Three forms of LPL subunits were found in normal cells; totally endo H-resistant (57 kDa), partially sensitive (54 kDa), and totally sensitive (51 kDa) forms. LPL in normal cells was active, dimeric, and showed high affinity for heparin. LPL in cells treated with tunicamycin, preventing the transfer of N-linked oligosaccharide chain, was unglycosylated (51 kDa) and inactive. LPL proteins were found as an aggregate, and had low affinity for heparin. After treatment with castanospermine, an inhibitor of ER glucosidase I, 80% of LPL activity was inhibited. Most of LPL proteins were totally endo H-sensitive, present as an aggregate, and had low affinity for heparin. LPL in cells treated with deoxymannojirimycin, an inhibitor of Golgi mannosidase I, was active, dimeric, and had high affinity for heparin as in normal cells. But LPL subunits were all endo H-sensitive. These results suggest that core glycosylation and subsequent removal of glucose residue is required, but processing after Golgi mannosidase I is not necessary for dimerization and acquisition of high heparin affinity of LPL.

Assay of ornithine aminotransferase with ninhydrin

We developed an assay system for ornithine aminotransferase (EC 2.6.1.13) using ninhydrin. Pyrroline 5-carboxylate, a product of enzymatic transamination, reacts with ninhydrin under hot acidic conditions to form a reddish pigment soluble in ethanol. The millimolar extinction coefficient of reaction product dissolved in ethanol was 16.5 at 510 nm. Acidification with perchloric acid effectively abolished the interfering color development by L-ornithine and L-glutamate. The paired activity measurement in mouse tissues by ninhydrin and o-aminobenzaldehyde methods showed a good correlation (gamma = 0.985). In our ninhydrin method, stable ninhydrin replaced unstable o-aminobenzaldehyde, and sensitivity was much higher than that with the conventional o-aminobenzaldehyde method.

A point mutation in the env gene of a murine leukemia virus induces syncytium formation and neurologic disease

TR1.3 is a Friend-related murine leukemia virus that has been shown to cause intracerebral hemorrhages and neurologic disease due to infection and subsequent cytopathology of cerebral vessel endothelium. A striking feature of this pathology is the formation of endothelial cell syncytia. The pathogenesis of this disease has now been mapped to a single amino acid substitution of tryptophan to glycine in the variable region of the envelope protein. This same mutation enabled TR1.3 to form syncytia and retard cell proliferation in vitro in the SC-1 mouse embryoblast line but did not affect the pH dependence of viral entry. These results demonstrate that subtle molecular changes in retroviral env genes can induce both syncytium formation and overt clinical disease.

Role of Ca2+ in alloxan-induced pancreatic beta-cell damage

Pretreatment of rats with verapamil, a Ca(2+)-antagonist, completely prevented alloxan-induced hyperglycemia. Verapamil also abolished the inhibition of insulin secretion by alloxan and H2O2 in isolated rat pancreatic islets. H2O2 generation from alloxan was not affected by verapamil, but alloxan- and H2O2-induced DNA strand breaks were completely prevented. Treatment of beta-cells with alloxan and H2O2 caused elevation of cytosolic free Ca2+, and this increase of Ca2+ was also abolished by verapamil. These results suggest that alloxan-derived oxygen radicals may disturb intracellular Ca2+ homeostasis by increasing Ca2+ influx, which results in secondary reactions ultimately leading to DNA strand breaks and cytotoxicity of beta-cells.

Intracerebral hemorrhages and infarction induced by a murine leukemia virus is influenced by host determinants within endothelial cells

The strain and developmental parameters that control susceptibility to murine leukemia virus (MuLV)-induced intracerebral hemorrhages and infarction were studied using the endothelial cell tropic MuLV TR1.3. Inoculated animals displayed an absolute age dependence on the development of intracerebral vascular disease; however, other genetic determinants affected the timing and magnitude of susceptibility to neurologic disease. BALB/c mice were susceptible to neurologic disease only when inoculated prior to Day 4 postpartum. In contrast, Swiss/NIH and C3H/HeN mice consistently showed a less virulent phenotype and were only susceptible when infected prior to Day 3 postpartum. These studies demonstrate that susceptibility to TR1.3 murine leukemia virus-induced neurologic disease is regulated by age- and strain-dependent factors encoded within cerebral endothelial cells.

Inhibitory mechanism of Ca2+ on the hemolysis caused by Vibrio vulnificus cytolysin

Calcium in millimolar concentrations protected mouse erythrocytes from hemolysis caused by Vibrio vulnificus cytolysin without affecting the release of intracellular K+ from the cells. This effect was maximal at 25 mM CaCl2. The protection was not absolute and could be partially overcome by increased concentrations of cytolysin. Calcium failed to block both the binding and oligomer formation of cytolysins on the erythrocyte membrane. After pore formation, the continued presence of calcium is required for the prevention of hemolysis. There was hardly any inflow of calcium into the erythrocytes through pores as measured by 45Ca2+ uptake. The presence of calcium after the abolition of Ca2+ gradient by ionomycin cannot inhibit the hemolysis caused by cytolysin. These results suggest that calcium exerts its major inhibitory effect on V. vulnificus cytolysin-induced hemolysis as an osmotic protectant, and that cytolysin may become an useful tool for permeabilizing cells selectively for small ions such as potassium or sodium while preventing the Ca2+ flow.

Molecular requirements for the inhibition of the tetracycline antiport protein and the effect of potent inhibitors on the growth of tetracycline-resistant bacteria

Forty-seven compounds and tetracycline (Tc) structural analogues were tested for their ability to interfere with [3H]Tc uptake in everted inner membrane vesicles derived from Tc-resistant Escherichia coli D1-209, bearing the class B tetracycline resistance efflux protein (Tet protein). For effective inhibition of Tc uptake, the molecule had to have an intact ABCD tetracyclic carbon skeleton and a conjugated phenolic beta-diketone substructure at positions 10-12a with the subsequent development of keto-enol tautomerization. Molecular variations at carbon positions 2, 4, 5, 6, 7, 8, and 9 did not decrease, and some increased, the inhibitory activity as compared to that of Tc. Among these compounds, the highest inhibition of uptake occurred with certain position 6 and 13 derivatives of 5-hydroxytetracycline. In a group of 13-(propylthio) derivatives of 5-OH-Tc [13-propyl, 13-(3-chloropropyl), and 13-(2-carboxyethyl)] there was a correlation between uptake inhibitory activity and antibacterial activity. The 13-(3-chloropropyl) derivative, with the best efflux inhibitory activity, exhibited synergistic activity when tested in combination with doxycycline against Tc-resistant E. coli bearing the class A or B determinant, against Staphylococcus aureus bearing class K, and against Enterococcus faecalis bearing the class L determinant. The 13-propyl analogue also showed high transport blocking activity and showed synergistic antibacterial activity against E. coli bearing the class A determinant and additive activity against the other Tc-resistant bacteria. The synergistic antibacterial activity of these compounds was not shown by the 13-[(2-carboxyethyl)thio] homologue, whose efflux blocking activity was 70-fold less. These findings suggest that multiple sites on the Tc molecule are available for synthetic modification toward the development of an effective Tc blocking agent. Such compounds, used alone or in combination with a standard tetracycline, show improved antibacterial activity.