Characterization of the murine macrophage mannose receptor: demonstration that the downregulation of receptor expression mediated by interferon-gamma occurs at the level of transcription

The macrophage mannose receptor (MMR) is a 175-Kd cell-surface transmembrane glycoprotein that is expressed on tissue macrophages where it functions both to mediate the uptake of mannose-rich glycoproteins and as a phagocytic receptor for bacteria, yeasts, and other pathogenic microorganisms. In this report we describe the cloning of the full-length cDNA of the mouse macrophage mannose receptor and we investigate the level at which interferon gamma (IFN-gamma) downregulates mannose receptor expression. The latter is a marker of the functional state of the cell as high levels are expressed on resident and inflammatory macrophages, whereas cells activated by treatment with IFN-gamma have decreased-to-absent cell-surface mannose receptor expression. The murine MMR cDNA contains an open reading frame that predicts a protein of 1,456 amino acids. Transient expression of the protein in heterologous cells shows that this cDNA encodes a functional mannose receptor. The deduced amino acid sequence of this protein has an overall 82% homology with the human mannose receptor and as such, the ectodomain contains an N-terminus that is cysteine-rich followed by a fibronectin type II domain and eight carbohydrate recognition domains (CRDs). The ectodomain is linked to a hydrophobic transmembrane region and a 46-amino acid cytoplasmic tail. All of the eight CRDs are particularly well conserved, especially CRD4, which shows 92% homology with the equivalent region of the human protein. Steady-state levels of murine MMR mRNA were measured in the macrophage cell line J774E, which is known to express the protein at the cell surface. These levels were decreased by a 4- to 8-hour incubation with IFN-gamma, but were almost abolished by overnight treatment with this cytokine. Nuclear run-on experiments showed that IFN-gamma inhibits MMR gene transcription. Therefore, the regulation of mannose receptor expression by IFN-gamma provides a novel system in which to study the mechanisms by which this cytokine represses gene expression.

Reporting alcohol-impaired drivers: results from a national survey of emergency physicians

STUDY OBJECTIVE

The purpose of this exploratory study was to learn of physicians' opinions on mandatory reporting of alcohol-impaired drivers they encounter in the course of their clinical work to the police or authorities from the Division of Motor Vehicles.

DESIGN AND PARTICIPANTS

Two thousand four hundred sixty-four physicians randomly selected from the American College of Emergency Physicians were sent an anonymous, one-time only, self-administered questionnaire seeking demographic information and assessing attitudes toward mandatory reporting and alcohol treatment.

MEASUREMENTS AND MAIN RESULTS

One thousand fifty-five physicians returned the survey. Seventy-eight percent of respondents agreed with mandatory reporting. More than half expressed strong agreement. Through canonical discriminant analysis we are able to identify the complex factors influencing attitude toward mandatory reporting.

CONCLUSION

Although our preliminary results must be interpreted with caution, it appears that with the appropriate legal safeguards, physicians are supportive of mandatory reporting of the alcohol-impaired driver encountered in the course of clinical work.

Systemic phospholipase A2 and cachectin levels in adult respiratory distress syndrome and multiple-organ failure

In this clinical study we have prospectively measured plasma phospholipase A2 (PLA2) activity and tumor necrosis factor (TNF) levels in ventilated intensive care unit (ICU) patients with (n = 9) and without (n = 12) evidence of respiratory distress syndrome (ARDS) and multiple-organ failure (MOF). The median peak TNF concentration in control patients was 40 ng/L (range less than 40-100 ng/L) and in ARDS patients 231 ng/L (range 100-2550 ng/L; p less than 0.001). All of the control patients were discharged alive from the ICU, whereas 6 of 9 ARDS patients died in the ICU. In 6 ARDS patients, it was possible to measure more than 4 consecutive plasma TNF levels. Of these 6 patients, the 3 with persistent elevations in systemic TNF above 230 ng/L succumbed (p less than 0.05, one-tailed). Patients with ARDS also had parallel elevations in plasma PLA2 activity above controls. These elevations were significant for arterial PLA2 activity but not for venous PLA2 activity. Our study suggests that serial measurement of plasma (arterial or venous) TNF levels may have (1) prognostic and (2) etiologic significance in ICU patients with ARDS and MOF.

Improving treatment outcome in pregnant opiate-dependent women

Outcomes for 6 pregnant methadone-maintained opiate-dependent subjects in enhanced treatment were compared to those of 6 women receiving conventional methadone maintenance. Enhanced treatment consisted of weekly prenatal care, relapse prevention groups, thrice weekly urine toxicology screening with positive contingency awards for abstinence, and therapeutic child care during treatment visits in addition to treatment as usual. Treatment as usual included daily methadone, group counseling, and random urine toxicology screening. Study patients differed from the comparison group in three important ways, having fewer urine toxicology screens positive for illicit substances (59% vs. 76%), three times as many prenatal visits (8.8 vs. 2.7), and heavier infants (median birth weight, 2959 vs. 2344 grams). These results suggest that enhanced drug treatment can improve pregnancy outcome and, in particular, reduce low birth weight for this high-risk population.

Lifetime psychiatric illness and premenstrual syndromes

The frequency of affective symptoms in most descriptions of premenstrual syndromes (PMS) suggests a potential etiologic link between menstrually-related mood changes and specific psychiatric disorders. The purpose of this study is to assess women presenting with "PMS" for lifetime psychiatric illness and PMS, according to rigorous diagnostic criteria comparable to those for "late luteal phase disorder," a proposed DSM-III-R diagnosis requiring further study. The women were interviewed with the Schedule for Affective Disorders and Schizophrenia Lifetime Version (SADS-L) and they kept prospective records of menstrual symptoms with the Moos Menstrual Distress Questionnaire - Form T (Moos MDQ-T). Of the 20 women evaluated, 85% had lifetime psychiatric illness and 30% had PMS. Careful psychiatric assessment is recommended in patients presenting with "PMS" as their chief complaint.

A clinically applicable method for long-term salvage of postischemic skeletal muscle

The clinical significance and applicability of interventions aimed at reducing reperfusion injury in postischemic skeletal muscle remain unproven, since long-term muscle salvage has not been demonstrated by most treatment protocols that attenuate early reperfusion injury. We have shown that reperfusion of ischemic skeletal muscle results in an early and prolonged sequestration of white blood cells and activation of the alternative complement cascade. The purpose of this study was to determine if 40 minutes of reperfusion with blood depleted of white blood cells and complement proteins, followed by 2 days of normal perfusion, would reduce muscle necrosis after 5 hours of ischemia. The isolated paired canine gracilis muscle model was used. The treatment muscle was initially reperfused with arterial blood that had been spun, washed, passed through a leukocyte removal filter, and resuspended in hydroxyethyl starch (greater than 99.9% removal of white blood cells and the complement proteins factor B and C4). The contralateral control muscle was subjected to unaltered reperfusion. Blood flow (ml/min/100 gm) was measured by timed collection of gracilis venous blood. Myeloperoxidase activity (absorbance at 655 nm/min/mg tissue protein) in muscle biopsies was used to monitor white blood cell sequestration. After 48 hours of reperfusion in vivo, necrosis was quantified by nitroblue tetrazolium staining. Initial reperfusion with white blood cell and complement depleted blood significantly reduced muscle necrosis (53% +/- 3% vs 29% +/- 8%, p less than 0.0025, paired t test). Early blood flow was improved, (p = 0.0025, repeated measure-ANOVA), but subsequent white blood cell sequestration was not altered (p = 0.33, repeated measure-ANOVA). This suggests that a significant amount of white blood cell mediated injury occurs during the first 40 minutes of reperfusion. Preventing early complement activation and white blood cell mediated reperfusion injury is an intervention that is feasible during surgery and may result in clinically significant salvage of postischemic skeletal muscle.

Mutational analysis of the C-terminus of human interferon-gamma

We have developed an expression/mutagenesis system and a series of screening procedures for the study of structure-function relationships in human interferon-gamma (HuIFN-gamma). Here we report a preliminary evaluation of the C-terminal portion of the molecule. An expression vector, p652trp gamma, was constructed which includes (i) the HuIFN-gamma gene under control of the trp promoter, (ii) elements controlling replication of both single- and double-stranded versions of the vector DNA; and (iii) the ampicillin resistance gene. (Other vectors using these same elements were constructed but proved to be unsatisfactory, being characterized by a rapid decline, as cells containing them were passaged, in their potential to achieve high expression levels.) A mutagenesis cassette was constructed by introduction of unique restriction sites flanking the nucleotides encoding the C-terminal 23 amino acids, and this cassette was replaced with chemically synthesized, degenerate oligonucleotides by ligation. Colonies from cells transformed with the reconstructed vector were stored in LB glycerol in microtiter plates, and these were screened by hybridization with synthetic oligonucleotides. Plates were grown in minimal medium to express the encoded interferon and lysed by an efficient, mild procedure. A polyclonal antibody specific for the C-terminal four amino acids of HuIFN-gamma was used to establish that the lysis procedure preserved the C-terminus, and to score for frame shift and nonsense mutations. Immunochemical assays also were used, with mixed results, to quantify IFN-gamma concentration in the lysate. An antiviral assay was employed to assess biological activity. Over 1000 isolates were screened and clones with properties representative of various classes of phenotypes were further characterized, in some cases after partial purification from the lysate. Three types of mutations were isolated: point mutations, nonsense mutations and frame shift mutations. The results from each type of mutation confirm earlier observations of the important role of basic residues in the 128-131 region of the molecule for biological activity. At the same time, the results suggest that most residues within the cassette can be altered without significant effects on biological activity. These results are discussed in the context of several possible mechanisms.

The emergency department surveillance of alcohol intoxication after motor vehicle accidents

To assess the surveillance of alcohol intoxication by surgical house staff, we examined the charts of 346 motor vehicle accident patients who presented to the trauma center of an urban teaching hospital emergency department. Half of the charts were reviewed before and half were reviewed after June 1986, when Connecticut enacted PA86-345, a law changing court rules of evidence so that the analysis by a hospital of a patient's blood could be used to establish probable cause for driving while under the influence of an intoxicant. We predicted and found no change in house-staff practice after passage of the law, since information about intoxication was obtained for immediate treatment. The rate of testing remained constant at 25%, with the median alcohol concentration at 200 mg/dL. Not one patient was referred for alcohol abuse evaluation or treatment. We recommend more vigorous attempts to evaluate, diagnose, and refer patients who abuse alcohol since they threaten the public health.

Patient characteristics in unplanned discharge from day hospital

In order to identify the distinguishing characteristics of patients with unplanned discharges from day hospital, the author reviewed 96% of all 1987 admissions. Unplanned discharges included precipitous in-patient hospitalization, discharge before 30-day program completion, and discharge against medical advice. Forty-three percent of reviewed admissions ended by unplanned discharge. Psychiatric patients with recent and/or remote substance abuse, and patients with multiple day-hospital admissions were especially vulnerable to unplanned discharge. Use of a backup bed during admission and being referred from the general-hospital emergency room or parent mental-health facility were associated with high rates of unplanned discharge. Patients with multiple admissions were more likely than those with a single admission to have personality disorders and to be female and white. Using logistic regression analysis, the author found that when patients had several characteristics increasing their risk for unplanned discharge, the odds of leaving before program completion were considerable.

Use of an established human hepatoma cell line with endogenous bioactivation for gene mutation studies

Genetic toxicology assays that rely on S9 microsomal mixes are subject to artifacts related to the generation of mutagenic metabolites by acidic pHs, variation in individual isolations of microsomes and the failure of subcellular fractions to faithfully produce metabolites generated in intact cells. We have developed a gene mutation assay utilizing the human hepatoma cell line HepG2, which has been shown to metabolize a broad spectrum of promutagens. Optimal conditions for assaying the induction of 6-thioguanine-resistant mutants in this cell line include: 1) growth of colonies for three weeks on lethally irradiated feeder layers of 10(6) thioguanine-resistant HepG2 cells (average plating efficiency = 60-80%); 2) a thioguanine concentration in selection dishes of 10(-4) M with a maximum seeding density of 2.5 x 10(5) cells per 100 mm culture dish; and 3) a minimum expression time of 6 days. In addition to ultraviolet light C (254 nm), a cytochrome P450 (cyclophosphamide)-dependent and a cytochrome P448 (aflatoxin B1)-dependent promutagen were shown to induce cytotoxicity and mutations in this test system. The present studies, therefore, suggest that the HepG2 cell line may be useful for a variety of assays in genetic toxicology.

Physicians' recognition of psychiatric disorders in children and adolescents

We examined the ability of physicians to recognize psychiatric and behavioral problems in the children and adolescents under their care. The report by 35 physicians of psychopathology in their patients was compared with the reports of parents and of children which were derived from direct and independent assessments of the children and of parents about their children. Physicians' reports of psychological problems were also compared with reports by a child psychiatrist who used all available data on the children and made a best estimate diagnosis. Agreement between the physicians and any of the three other sources of information--parents, children, or child psychiatrist--was poor, with kappa ranging from -15 to .11. Physicians tended to underreport both minor and serious psychiatric problems in children. These results are discussed in the context of the recent American Medical Association initiative to improve the health of children and adolescents.

Linear free energy relationship studies of enzyme active site binding: thymidylate synthase

The requirements for active-site binding of thymidylate synthase from three sources, Lactobacillus casei, murine leukemia L1210, and human lymphoblast (Molt/4F), were investigated by analyzing the binding of a series of 5-(p-substituted phenyl)-2'-deoxyuridylates (N1-substituted 5-aryl-2, 4-dioxopyrimidines) to the enzyme. Multiple regression analysis revealed that an increase in electron density of the heterocyclic ring and hydrophobic substituents enhance affinity. Correlations of biological results with spectral data indicated that higher electron densities at the oxygen atoms are responsible for increase in binding. These results support the presence of both a cationic binding site and a hydrophobic region. In addition, the results revealed an unusual reversal of electronic requirements for binding and catalysis. The formation of the binary complex is enhanced by electron-donating substituents, while the initial catalytic reaction, formation of the covalent ternary complex, is promoted and stabilized by electron-withdrawing substituents.

Endothelialization of polymer surfaces

The role that substrate surface properties play in influencing the extent of endothelialization of polymer surfaces has been investigated. For a wide range of polymer surfaces, the degree of endothelialization for both porcine and bovine endothelial cells is directly related to polymer surface tension: increased endothelialization occurring with increasing substrate surface tension. As a result of adsorption of the proteins in the culture media, the surface properties of the polymers are altered considerably. The protein-coated polymers were characterized by means of liquid-liquid contact angle measurements under non-denaturing conditions. A striking correlation is observed between the degree of endothelialization and the measured dextran contact angle. The degree of endothelial cell spreading is not related to polymer surface tension. Cell morphology and extracellular matrix production, however, are influenced by substrate surface properties.

Uptake and urinary excretion of aluminum among welders

The urinary excretion of aluminum was measured in 23 welders before and after an exposure-free interval of 16 to 37 days. In addition, the concentration of aluminum in the air was measured at the work site of 16 aluminum welders on the same workday as the first urine sample was taken. The concentration of aluminum in the urine depended on both the level and duration of exposure. The postshift urinary concentration of aluminum prior to an exposure-free interval was mainly related to the current air concentration, whereas the urinary concentration of aluminum determined after the exposure-free interval was related to total exposure duration (years). Among welders exposed for less than 1 year, the half-time for urinary concentration was about 9 days whereas welders exposed for more than 10 years had half-times calculated to be 6 months or longer. The results indicate that aluminum is retained and stored in at least two functional compartments of the body and is eliminated from these compartments at different rates.

Physical and hydrodynamic factors affecting erythrocyte adhesion to polymer surfaces

Using a flow cell design which ensures fully developed laminar flow, the influence of various hydrodynamic and physical factors in determining the extent of erythrocyte adhesion to various polymer surfaces has been examined. Specifically we have investigated the effect of exposure time, flow rate, erythrocyte concentration, and substrate surface tension on the extent of erythrocyte adhesion. The results indicate: (1) the extent of erythrocyte adhesion is most extensive on the more hydrophobic surfaces; (2) the rate of adhesion is higher on the more hydrophobic surfaces; (3) saturation coverage occurs after 7-10 min of exposure to the erythrocyte suspension for all substrates examined. No "lag-time" in the onset of adhesion was observed; (4) The level of saturation depends on the bulk erythrocyte concentration, increasing with increasing cell concentration; (5) the extent of adhesion decreases with an increase in flow rate; and (6) substrate surface defects such as roughness have a major effect on the pattern of erythrocyte adhesion.

Theoretical and experimental analysis of cellular adhesion to polymer surfaces

An exact discrete numerical solution to the Grabowski model for predicting cell adhesion to polymer surfaces is discussed. The solution technique allows the possibility of taking into account cell-cell interactions within the flow situation and the multistep process involved in thrombus formation. The proposed solution also allows modification of the wall reaction rate model into a two species reaction rate which distinguishes between the kinetics of contact adhesion and irreversible adhesion. The solution allows determination of effective diffusivity (De) and surface reaction rate (k) constants. Use of the model to examine available experimental data results in the following conclusions: (1) static or dynamic cell adhesion cannot be considered to be diffusion limited; (2) for flow conditions De is a monotonically increasing function of shear rate; (3) under static, i.e., zero flow conditions, De appears to be markedly larger than for flow conditions.

Effects of pyrimidine antagonists on sialic acid regeneration in HL-60 cells

Because alterations in cell membrane sialoglycoconjugates can affect the behavior of neoplastic cells, we investigated the effects of in vitro treatment with antimetabolites used in cancer therapy on the expression of membrane sialic acid in cultured HL-60 leukemic cells. In these studies, cells were incubated with Vibrio cholerae neuraminidase to remove surface sialic acid. Reappearance of membrane sialic acid during drug treatment was followed (a) by measuring changes in radioactive surface labeling of viable cells with sodium metaperiodate-sodium[3H]-borohydride, (b) by measuring the decline in accessible surface galactosyl receptor sites which occurred coincident with membrane sialic acid replacement, and (c) by measuring the incorporation of [3H]glucosamine into membrane-associated neuraminidase-labile sialic acid. We were especially interested in learning whether drugs that affect intracellular pools of cytidine triphosphate (CTP), an important nucleotide intermediate in sialylation reactions, could inhibit regeneration of membrane sialic acid. 3-Deazauridine, a competitive inhibitor of CTP synthetase, depleted CTP pools and curtailed surface membrane resialylation with little or no effect on synthesis of de novo sialic acid from precursor sugars. The addition of cytidine restored CTP pools and sialic acid regeneration. Acivicin, a glutamine antagonist, also depleted CTP pools and curtailed surface membrane resialylation. In addition, it retarded de novo synthesis of sialic acid. The addition of cytidine restored intracellular CTP pools and sialic acid regeneration. However, both cytidine and guanosine were required to restore sialic acid synthesis from precursor sugars. 1-beta-D-Arabinofuranosylcytosine, a competitive inhibitor of sialic acid synthetase and of sialyltransferase, inhibited both de novo sialic acid synthesis and membrane resialylation. Only the latter effect was reversed by the addition of exogenous cytidine. Hydroxyurea, an agent shown previously to inhibit glycoconjugate production in hamster fibroblasts, curtailed membrane resialylation and de novo synthesis of sialic acid without depleting CTP pools. Doxorubicin, at levels that caused marked arrest of cell proliferation, had no effect on sialic acid synthesis or expression on the membrane surface. These data suggest that antimetabolites, apart from their cytotoxic effects or effects on cellular growth, may directly inhibit the expression of membrane sialic acid.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of cholecystokinin-octapeptide and pentagastrin on electrical and motor activities of canine colonic circular muscle

The effects of cholecystokinin-octapeptide (CCK-OP) and pentagastrin on electrical and motor activities of circular muscle of the canine colon were studied with the sucrose gap technique. Additional organ bath experiments were performed to further characterize the motor response to the peptides and to elucidate their site of action. The electrical activity consisted of slow waves having an initial potential followed by a plateau potential, at a regular frequency of 4.5 cycles/min. Both peptides prolonged the duration and increased the amplitude of the plateau phase of the slow waves. Concomitantly, the slow wave frequency was reduced. In addition, CCK-OP increased spiking activity. Both spiking activity and the prolonged plateau potential generated contractile activity, prolonged phasic contraction occurring with slow waves with a prolonged plateau. In organ bath experiments, both CCK-OP and pentagastrin increased the basal tone of the muscle strips and prolonged the duration of the phasic contractions. The prolongation of the duration of the contractions was not antagonized by tetrodotoxin (TTX) and atropine. CCK-OP but not pentagastrin increased the force of contractions, this action was not affected by atropine but was reduced in the presence of TTX, suggesting that the increase in force may be partially mediated by noncholinergic excitatory nerves. The increase in basal tension by the peptides was enhanced in the presence of TTX indicating that myenteric inhibitory neurones were tonically active under our experimental conditions. The results provide the electrophysiological basis for CCK-OP and pentagastrin induced changes in colonic motility.

Electrophysiological basis of excitation of canine colonic circular muscle by cholinergic agents and substance P

The circular muscle layer of the canine colon exhibits omnipresent rhythmic periodic waves of depolarization (slow waves), acting as pacemaker activity. The electrophysiological and motor responses of this layer to the muscarinic agonists acetylcholine and carbachol, and to the excitatory peptide substance P, were studied using the sucrose-gap technique. In addition, changes in the contractile activity were examined in organ bath experiments. The slow waves consisted of an initial potential followed by a plateau potential. All substances depolarized the membrane and increased dramatically the duration of the plateau potential resulting in a decrease of the slow wave frequency. In addition, the amplitude of the plateau potential was often increased significantly. Carbachol and substance P readily evoked spiking activity whereas acetylcholine did not. Both spiking activity and the plateau potential generated contractile activity. The prolongation of the slow wave duration caused a profound alteration of the pattern of contractions. Long-lasting tachyphylaxis to the effect of substance P, but not to acetylcholine or carbachol, occurred. The electrophysiological and motor effects of the drugs were due to a direct action on the smooth muscle cell membrane. This study provides an electrophysiological basis for prolonged circular muscle contractions of the colon, and it emphasizes the pacemaker activity of gastrointestinal smooth muscle as an important site of drug action.

S-100-mediated inhibition of brain protein phosphorylation

The effects of the glial-specific, calcium-binding, S-100 protein on brain membrane and supernatant protein phosphorylation were assessed. S-100 concentrations as low as 5 micrograms/ml caused a marked inhibition of the phosphorylation of a soluble brain protein having a molecular weight of 73,000 daltons (73K). This protein was designated the S-100 protein-modulated phosphoprotein (SMP). Half-maximal inhibition of the phosphorylation of SMP by S-100 was obtained at concentrations of 12 micrograms/ml (0.57 microM). The inhibition of SMP phosphorylation by S-100 was calcium-dependent, with a calculated calcium Ka of 2.0 +/- 0.3 microM. SMP phosphorylation was also inhibited by calmodulin, but only partially and with a much lower potency. The inhibition of SMP phosphorylation by S-100 was not inhibited by fluphenazine, whereas the effect of calmodulin was. SMP was found in many brain areas, with the highest levels seen in the corpus callosum. Various peripheral tissues, such as kidney; liver; and pineal, pituitary, and adrenal glands, did not contain detectable SMP levels. At higher S-100 concentrations, greater than 10 micrograms/ml, the phosphorylation of several other soluble proteins was markedly inhibited. These proteins have molecular weights of 56K, 50K, and 47K. The phosphorylation of these proteins was enhanced by calmodulin. These data suggest that the S-100 protein may function to modulate the phosphorylation of brain proteins in a manner analogous to (although in a reciprocal fashion) that of calmodulin.

Mechanism of malic enzyme from pigeon liver. Magnetic resonance and kinetic studies of the role of Mn2+

As determined by EPR, malic enzyme from pigeon liver binds Mn2+ with a half-site stoichiometry of two tight binding sites (KD=6 to 10 mum) per enzyme tetramer and at two to four weak binding sites (KD=0.43 to 1.34 mM). The activation of malic enzyme by Mn2+ at high levels of L-malate shows biphasic kinetics yielding two activator constants for Mn2+. The dissociation constants of Mn2+ for both classes of sites are of the same order as the kinetically determined activator constants of Mn2+, indicating active site binding at both classes of binding sites. The binding of Mn2+ to the tight sites enhances the paramagnetic effect of Mn2+ on 1/T1 of water protons by a factor (epsilon) of 17, while binding at the weak sites yields a smaller epsilon of 11. The coenzymes TPN and TPNH have no effects on epsilon, while the carboxylic acid substrates L-malate and pyruvate and the inhibitors D-malate and oxalate significantly decrease epsilon. TPNH causes a 38-fold tightening of binding of the substrate L-malate to the enzyme-Mn2+ complex, consistent with the previously described highly ordered kinetic scheme, but only a 2-fold tightening of binding of the competitive inhibitor D-malate. The dissociation constant of L-malate from the quaternary E-Mn2+-TPNH-L-malate complex (32 muM) agrees with the Km of L-malate (25 muM), indicating active site binding. The dissociation constants of pyruvate from the ternary E-Mn2+-pyruvate complex (12 mM) and from the quaternary E-Mn2+-TPN-pyruvate complex (20 mM) are similar to the Km of pyruvate (5 mM), also indicating active site binding and a less highly ordered kinetic scheme for the reactions of pyruvate than for those of L-malate. Analysis of the frequency dependence of 1/T1 of water protons indicates that two fast exchanging water ligands remain coordinated to Mn2+ in the binary E-Mn2+ complex. The binding of the substrates L-malate and pyruvate and of the transition state analog oxalate to the E-Mn2+ complex decrease the number of fast exchanging water ligands on Mn2+ by approximately 1, but the binding of D-malate has no significant effect on this parameter, indicating the occlusion or replacement of a water ligand of the enzyme-bound Mn2+ by a properly oriented substituent on C-2 of the substrate. Occlusion rather than replacement of a water ligand by pyruvate is established by studies of 1/T1 of 13COO- and 13CO-enriched pyruvate which indicate second sphere Mn2+ to pyruvate distances of 4.6 A (COO-) and 4.8 A (CO) in the ternary enzyme-Mn2+-pyruvate complex. Formation of the quaternary complex with TPN increases these distances by 0.8 A, indicating the participation of a second sphere enzyme-Mn2+-(H2O)-pyruvate complex in catalysis. Thus, malic enzyme, like five other enzymes which utilize metals to polarize carbonyl groups, forms a second sphere complex with its substrate.