Identification of hBD-3 in respiratory tract and serum: the increase in pneumonia

Human beta-defensin (hBD)-3, a 45 amino acid antimicrobial peptide, was originally isolated from human skin. hBD-3 mRNA has also been detected in the airways by RT-PCR. While hBD-3 may be involved in antimicrobial defences within the respiratory tract, the presence of hBD-3 peptide in the respiratory system has not yet been confirmed. The antimicrobial activity of the synthesised hBD-3 peptide was measured by a radial diffusion assay and a colony count assay. The present authors confirmed the presence of hBD-3 peptide in homogenates of human lung and serum using reverse-phase HPLC coupled with a highly sensitive RIA. The localisation of the hBD-3 peptide was investigated by immunohistochemistry. In addition, the serum concentrations of hBD-3 were measured by RIA. hBD-3 exhibited a strong antimicrobial activity, which was unaffected by increasing salt concentrations. Immunohistochemically, the current authors observed the expression of hBD-3 in bronchial and bronchiolar epithelial cells. The mean+/-sd serum concentration of hBD-3 in patients with bacterial pneumonia was 239.4+/-17.8 pg x mL(-1) in the acute phase and, decreased to 159.3+/-20.1 pg x mL(-1) after the completion of therapy. In conclusion, these findings will help elucidate the role of human beta-defensin-3 in host immune responses and identify the pathophysiological significance of this molecule in respiratory infections.

Neuropeptide W is present in antral G cells of rat, mouse, and human stomach

Neuropeptide W (NPW) is a 30-amino-acid peptide initially isolated from the porcine hypothalamus as an endogenous ligand for the G protein-coupled receptors GPR7 and GPR8. An intracerebroventricular administration of NPW increased serum prolactin and corticosterone concentrations, decreased dark-phase feeding, raised energy expenditure, and lowered body weight. Peripherally, GPR7 receptors are abundantly expressed throughout the gastrointestinal tract; the presence of NPW in the gastrointestinal endocrine system, however, remains unstudied. Using monoclonal and polyclonal antibodies raised against rat NPW, we studied the localization of NPW in the rat, mouse, and human stomach by light and electron microscopy. NPW-immunoreactive cells were identified within the gastric antral glands in all three species. Double immunohistochemistry and electron-microscopic immunohistochemistry studies in rats demonstrated that NPW is present in antral gastrin (G) cells. NPW immunoreactivity localized to round, intermediate-to-high-density granules in G cells. NPW-immunoreactive cells accounted for 90% chromagranin A- and 85% gastrin-immunoreactive endocrine cells in the rat gastric antral glands. Using reversed-phase HPLC coupled with enzyme immunoassays specific for NPW, we detected NPW30 and its C-terminally truncated form, NPW23, in the gastric mucosa. Plasma NPW concentration of the gastric antrum was significantly higher than that of the systemic vein, suggesting that circulating NPW is derived from the stomach. Plasma NPW concentration of the gastric antrum decreased significantly after 15-h fast and increased after refeeding. This is the first report to clarify the presence of NPW peptide in the stomachs of rats, mice, and humans. In conclusion, NPW is produced in gastric antral G cells; our findings will provide clues to additional mechanisms of the regulation of gastric function by this novel brain/gut peptide.

Role for central ghrelin in food intake and secretion profile of stomach ghrelin in rats

Ghrelin, a 28-amino-acid peptide, has recently been isolated from the rat stomach as an endogenous ligand for the GH secretagogue receptor. We have reported previously that central or peripheral administration of ghrelin stimulates food intake, and the secretion of GH and gastric acid in rats. In the present study, we investigated how much endogenous centrally released ghrelin is involved in the control of food intake and body weight gain. We also examined the profile of ghrelin secretion from the stomach by RIA using two kinds of anti-ghrelin antiserum, one raised against the N-terminal ([Cys(12)]-ghrelin[1-11]) region and one raised against the C-terminal ([Cys(0)]-ghrelin [13-28]) region of the peptide. The former antibody recognizes specifically ghrelin with n- octanoylated Ser 3 (acyl ghrelin), and does not recognize des-acyl ghrelin. The latter also recognizes des-acyl ghrelin (i.e. total ghrelin). Intracerebroventricular treatment with the anti-ghrelin antiserum against the N-terminal region twice a day for 5 days decreased significantly both daily food intake and body weight. Des-acyl ghrelin levels were significantly higher in the gastric vein than in the trunk. Either fasting for 12 h, administration of gastrin or cholecystokinin resulted in increase of both acyl and des-acyl ghrelin levels. The ghrelin levels exhibited a diurnal pattern, with the bimodal peaks occurring before dark and light periods. These two peaks were consistent with maximum and minimum volumes of gastric content respectively. These results suggest that (1) endogenous centrally released ghrelin participates in the regulation of food intake and body weight, (2) acyl ghrelin is secreted from the stomach, (3) intestinal hormones stimulate ghrelin release from the stomach, and (4) regulation of the diurnal rhythm of ghrelin is complex, since ghrelin secretion is augmented under conditions of both gastric emptying and filling.

Ghrelin in neonatal rats: distribution in stomach and its possible role

Ghrelin, a 28 amino acid peptide, has recently been isolated from the rat stomach as an endogenous ligand for the GH secretagogue receptor. The fact that administration of ghrelin, centrally or peripherally, stimulates both food intake and GH secretion suggests that stomach ghrelin has an important role in the growth of rats. We used immunohistochemistry and radioimmunoassay to determine the age at which ghrelin-immunostained cells begin to appear in the rat stomach. Ghrelin-immunoreactive cells were found to be expressed in the fetal stomach from pregnancy day 18. The number of ghrelin-immunoreactive cells in the fetal stomach increased as the stomach grew. The amount of ghrelin in the glandular part of the rat stomach also increased, in an age-dependent manner, from the neonatal stage to adult. Eight hours of milk restriction significantly decreased the ghrelin concentration in the stomachs of 1-week-old rats, and increased the ghrelin concentration in their plasma. Administration of ghrelin to 1- and 3-week-old rats increased plasma GH concentrations. The daily subcutaneous administration of ghrelin to pregnant rats from day 15 to day 21 of pregnancy caused an increase in body weight of newborn rats. In addition, daily subcutaneous administration of ghrelin to neonatal rats from birth advanced the day of vaginal opening from day 30.7+/-0.94 to day 27.9+/-0.05. These results suggest that ghrelin may be involved in neonatal development.

Ghrelin in domestic animals: distribution in stomach and its possible role

Ghrelin, a novel growth-hormone-releasing acylated peptide, was recently isolated from rat and human stomachs. In rat, peripheral or central administration of ghrelin stimulates the secretion of growth hormone (GH) from the pituitary gland. Recent work suggests that ghrelin plays an important role in energy homeostasis, body weight, and food intake. We examined the distribution of cells immunoreactive to ghrelin in the stomachs of domestic animals and rats, using a polyclonal antibody for the N-terminal fragment of rat ghrelin [1-11]. We measured the plasma levels of ghrelin before and after feeding in cows, and GH levels after central administration of ghrelin in Shiba goats, to elucidate the possible role of ghrelin. Immunostained cells were widely distributed from the neck to the base of the oxyntic gland in all animals. The plasma ghrelin concentration in cows decreased significantly 1 h after feeding, and then recovered to pre-feeding levels. Administration of ghrelin into the third ventricle in Shiba goats dramatically increased the plasma GH concentration dose-dependently. These results suggest that ghrelin plays an important role in GH secretion and feeding regulation in domestic animals.

Upregulation of Ghrelin expression in the stomach upon fasting, insulin-induced hypoglycemia, and leptin administration

Ghrelin is a novel gut-brain peptide that binds to the growth hormone secretagogue receptor (GHS-R), thereby functioning in the regulation of growth hormone (GH) release and food intake. Ghrelin-producing cells are most abundant in the oxyntic glands of the stomach. The regulatory mechanism that governs the biosynthesis and secretion of ghrelin has not been clarified. We report that ghrelin mRNA expression in the gastric fundus was increased, but that ghrelin peptide content decreased after a 48-h fast. Both values returned to control levels after refeeding. The ghrelin plasma concentration in the gastric vein and systemic venous blood increased after 24- and 48-h fasts. Furthermore, des-octanoylated ghrelin and n-octanoylated ghrelin were found in rat stomach, with the ratio of des-octanoylated ghrelin to n-octanoylated ghrelin markedly increased after fasting. The ghrelin mRNA level in the stomach also increased after administration of insulin and leptin. Conversely, db/db mice, which are deficient in the leptin receptor, had lower ghrelin mRNA levels than control mice. These findings suggest that this novel gastrointestinal hormone plays a role in the regulation of energy balance.

Orexins (hypocretins): novel hypothalamic peptides with divergent functions

The hypothalamus is the most important region in the control of food intake and body weight. The ventromedial "satiety center" and lateral hypothalamic "feeding center" have been implicated in the regulation of feeding and energy homeostasis by various studies of brain lesions. The discovery of orexin peptides, whose neurons are localized in the lateral hypothalamus and adjacent areas, has given us new insight into the regulation of feeding. Dense fiber projections are found throughout the brain, especially in the raphe nucleus, locus coeruleus, paraventricular thalamic nucleus, arcuate nucleus, and central gray. Orexins mainly stimulate food intake, but by the virtue of wide immunoreactive projections throughout the brain and spinal cord, orexins interact with various neuronal pathways to potentiate divergent functions. In this review, we summarize recent progress in the physiological, neuroanatomical, and molecular studies of the novel neuropeptide orexins (hypocretins).

Two histidine residues are essential for catalysis by lecithin retinol acyl transferase

Lecithin retinol acyl transferase (LRAT) is a novel membrane bound enzyme that catalyzes the formation of retinyl esters from vitamin A and lecithin. The enzyme is both essential for vision and for the general mobilization of vitamin A. The sequence of LRAT defines it as a novel enzyme unrelated to any other protein of known function. LRAT possesses a catalytically essential active site cysteine residue. The enzyme also contains six histidine residues. It is shown here that two of these residues (H57 and H163) are essential for catalysis. A mechanistic hypothesis is presented to account for these observations.

Interaction of Cu2+ ion with milk xanthine oxidase

The interaction of Cu2+ ion with milk xanthine oxidase (XO) has been studied by optical spectroscopy, circular dichroism, ESR and transient kinetic techniques. It is observed that XO forms optically observable complexes with Cu2+ ion. The pH dependence studies of the formation of Cu2+-XO complex by optical spectroscopy and circular dichroism show that at least one ionizable group may be responsible for the formation of the complex. The EPR studies show that Cu2+ ion binds to XO with sulfur and nitrogenous ligands. The transient kinetic study of the interaction of Cu2+ with XO shows the existence of two Cu2+ bound XO complexes formed at two different time scales of the interaction, one at < or =5 ms and the other one at around 20 s. The complex formed at longer time scale may be responsible for the inhibition of the enzyme activity.

Ghrelin, a novel growth hormone-releasing acylated peptide, is synthesized in a distinct endocrine cell type in the gastrointestinal tracts of rats and humans

Ghrelin, a novel GH-releasing acylated peptide, was recently isolated from rat stomach. It stimulated the release of GH from the anterior pituitary through the GH secretagogue receptor (GHS-R). Ghrelin messenger RNA and the peptide are present in rat stomach, but its cellular source has yet to be determined. Using two different antibodies against the N- and C-terminal regions of rat ghrelin, we identified ghrelin-producing cells in the gastrointestinal tracts of rats and humans by light and electron microscopic immunohistochemistry and in situ hybridization combined with immunohistochemistry. Ghrelin-immunoreactive cells, which are not enterochromaffin-like cells, D cells, or enterochromaffin cells, accounted for about 20% of the endocrine cell population in rat and human oxyntic glands. Rat ghrelin was present in round, compact, electron-dense granules compatible with those of X/A-like cells whose hormonal product and physiological functions have not previously been clarified. The localization, population, and ultrastructural features of ghrelin-producing cells (Gr cells) indicate that they are X/A-like cells. Ghrelin also was found in enteric endocrine cells of rats and humans. Using two RIAs for the N- and C-terminal regions of ghrelin, we determined its content in the rat gastrointestinal tract. Rat ghrelin was present from the stomach to the colon, with the highest content being in the gastric fundus. Messenger RNAs of ghrelin and GHS-R also were found in these organs. Ghrelin probably functions not only in the control of GH secretion, but also in the regulation of diverse processes of the digestive system. Our findings provide clues to additional, as yet undefined, physiological functions of this novel gastrointestinal hormone.

Central effects of neuromedin U in the regulation of energy homeostasis

Neuromedin U (NMU) is a brain-gut peptide whose peripheral activities are well-understood but whose central actions have yet to be clarified. The recent identification of two NMU receptors in rat brain has provided a springboard for further investigation into its role in the central nervous system. Intracerebroventricular administration of NMU to free-feeding rats decreased food intake and body weight. Conversely, NMU increased gross locomotor activity, body temperature, and heat production. NMU, a potent endogenous anorectic peptide, serves as a catabolic signaling molecule in the brain. Further investigation of the biochemical and physiological functions of NMU will help our better understanding of the mechanisms of energy homeostasis.

Mechanism of the inhibition of milk xanthine oxidase activity by metal ions: a transient kinetic study

The nature and mechanism of the inhibition of the oxidoreductase activity of milk xanthine oxidase (XO) by Cu(2+), Hg(2+) and Ag(+) ions has been studied by steady state and stopped flow transient kinetic measurements. The results show that the nature of the inhibition is noncompetitive. The inhibition constants for Cu(2+) and Hg(2+) are in the micromolar and that for Ag(+) is in the nanomolar range. This suggests that the metal ions have strong affinity towards XO. pH dependence studies of the inhibition indicate that at least two ionisable groups of XO are involved in the binding of these metal ions. The effect of the interaction of the metal ions on the reductive and oxidative half reactions of XO has been investigated, and it is observed that the kinetic parameters of the reductive half reaction are not affected by these metal ions. However, the interaction of these metal ions with XO significantly affects the kinetic parameters of the oxidative half reaction. It is suggested that this may be the main cause for the inhibition of XO activity by the metal ions.

Distribution of orexin-A and orexin-B (hypocretins) in the rat spinal cord

Orexin-A and orexin-B (also known as hypocretin-1 and hypocretin-2) are hypothalamic peptides that regulate feeding behavior, energy metabolism, and sleep-wake cycle. We determined the distribution of orexin-A and -B in the rat spinal cord, using sensitive radioimmunoassays and an immunohistochemical technique with three antisera specific for these orexins. Orexins were distributed throughout the spinal cord, and their contents were highest in the cervical region. Orexin fibers were concentrated in lamina I of the dorsal horn and area X surrounding the central canal of the spinal cord. Abundant orexin fibers also were present in the preganglionic sympathetic and parasympathetic cell columns. These findings suggest that orexins in the spinal cord may be involved in the modulation of sensory informations and the autonomic system as neurotransmitters or neuromodulators, both.

Lecithin retinol acyltransferase contains cysteine residues essential for catalysis

Lecithin retinol acyltransferase (LRAT) is an essential enzyme in vitamin A metabolism and mobilization. The membrane-bound enzyme catalyzes the transfer of an acyl group from the sn-1 position of lecithin to vitamin A to generate retinyl esters. The sequence of LRAT is novel and hence does not suggest a mechanistic class to which the enzyme belongs. However, the activity of the enzyme is exceedingly sensitive to affinity labeling and group-specific reagents directed toward thiol groups. LRAT from human retinal pigment epithelium has cysteine residues at positions 161, 168, 182, and 208. Site-specific mutagenic studies show that C182 and C208 can be converted to alanines with little affect on activity. The activities of the C161A and C168A mutants are virtually nil. Moreover, while C168S is substantially active, C161S possesses only a few percent of the activity of wild-type (WT) LRAT. Also, pH-rate profiles show that C168S has virtually the same profile as WT LRAT, while C161S shows an aberrant profile quite unlike that of WT LRAT. Therefore, LRAT is a thiol acyltransferase and C161 may be the essential nucleophilic residue critical for catalysis.

Distribution of orexin/hypocretin in the rat median eminence and pituitary

We determined the distribution of orexin-A and orexin-B (also known as hypocretin-1 and hypocretin-2) and their receptors in the rat median eminence and pituitary using sensitive radioimmunoassays coupled with high-performance liquid chromatography, immunohistochemistry, and in situ hybridization. Orexin-A and -B were present in the median eminence, adenohypophysis, and neurohypophysis. Orexin fibers were abundant in the median eminence, and a few fibers projected to the neurohypophysis. Both the orexin(1)- and orexin(2)-receptor mRNAs were expressed robustly in the pituitary intermediate lobe, whereas in the anterior lobe, the orexin(1) receptor was more markedly expressed than the orexin(2) receptor. These two receptor mRNAs were also found in the posterior lobe. These findings may implicate orexin's involvement in additional as yet undefined physiological functions in the hypothalamo-hypophyseal tract.

The specific binding of small molecule isoprenoids to rhoGDP dissociation inhibitor (rhoGDI)

The activities of small G-proteins are in part regulated by their interactions with GDI proteins. This binding is thought to be dependent on the C-terminal isoprenoid modification (geranylgeranyl or farnesyl) of these proteins. G-proteins are generally isoprenylated/methylated at their C-terminal cysteine residues. A quantitative fluorescence assay is reported here to evaluate the specificity of binding of rhoGDI. A rhodamine-labeled geranylgeranylated/methylated cysteine derivative is used to measure its binding to rhoGDI. Saturable binding in the low micromolar range is found with various geranylgeranylated/farnesylated analogues. Interestingly, the carboxymethylated derivatives bound significantly better than their free acid counterparts, suggesting that the state of methylation of the analogues is important for binding. The binding is also selective with respect to isoprenoid. Analogues containing hydrophobic modifications other than geranylgeranyl or farnesyl do not bind with significant affinities. These data demonstrate a substantial degree of specificity in the binding of isoprenoids to a protein important in signal transduction.

Characterization of orexin-A and orexin-B in the microdissected rat brain nuclei and their contents in two obese rat models

Orexin-A and orexin-B (also known as hypocretins) are newly discovered hypothalamic peptides that stimulate food intake. Using separate radioimmunoassays for these rat orexins, we determined their distributions in microdissected nuclei of the diencephalon and brainstem which have accumulations of orexin fibers. High orexin contents (orexin-A: between 250 and 350 fmol/mg protein and orexin-B: between 650 and 900 fmol/mg protein) were present in the lateral hypothalamus; ventromedial hypothalamic, paraventricular thalamic and dorsal raphe nuclei; periaqueductal central gray and locus coeruleus. Moderate orexin contents (orexin-A: between 100 and 250 fmol/mg protein and orexin-B: between 300 and 500 fmol/mg protein) were found in the median eminence; suprachiasmatic, paraventricular hypothalamic, arcuate and supraoptic nuclei; substantia nigra and the nucleus of the solitary tract. Mature orexin-A and -B peptides were the major endogenous orexin molecules in these nuclei. The orexin-A and -B contents in the brains of obese Zucker rats that have disrupted leptin receptor were significantly higher than in their lean littermates, but in Otsuka Long-Evans Tokushima Fatty rats that have disrupted cholecystokinin type-A receptor the contents were similar to those of the controls. The widespread orexin distributions in the nuclei of diencephalon and brainstem suggest that orexins serve as neuromodulators, neurotransmitters, or both, in a wide variety of neural networks that regulate the autonomic and neuroendocrine systems.

Widespread distribution of orexin in rat brain and its regulation upon fasting

Orexins A and B, novel hypothalamic peptides encoded by a single mRNA transcript, stimulate food intake. Two antisera specific for the individual rat orexins were prepared and sensitive RIAs developed. Orexin-A and -B are abundant in the rat hypothalamus, medulla-pons, and midbrain-thalamus, and moderately abundant in the cerebral cortex. No orexins were found in the adipose tissues or visceral organs studied. The major endogenous molecule of orexin-A is a 33-amino-acid peptide, and that of orexin-B a 28-amino-acid peptide. After a 48 h fast, the orexin-A and -B contents of the lateral hypothalamus exhibited a trend to increase, but the contents of other brain tissues significantly decreased as compared with the fed control rats. No circadian variations in the orexin contents were found in the brain. The extensive and abundant distribution of orexins in the brain and changes in their contents upon fasting suggest that they serve as neuromodulators and/or neurotransmitters that regulate feeding behavior through interaction with diverse neural networks.

The role of distal histidine in peroxidase activity of myoglobin--transient-kinetics study of the reaction of H2O2 with wild-type and distal-histidine-mutanted recombinant human myoglobin

The distal histidine has been proposed to play a crucial role in the reaction of peroxidases with hydroperoxide. Myoglobin (Mb), due to its close similarity with peroxidases, also reacts with various peroxides. The effect of mutation of the distal histidine on the peroxidase activity of Mb has been investigated by stopped-flow kinetics of the reaction of hydrogen peroxide with wild-type Mb and [Gly64]Mb. The results indicate kinetic and mechanistic differences in the formation of peroxide compounds from these two forms of Mb. The rate of reaction of H2O2 with the wild-type Mb decreased 8-9-fold on mutation of the distal histidine to glycine ([Gly64]Mb). A second slow phase was observed for the reaction of H2O2 with [Gly64]Mb, but was not observed in the corresponding reaction with wild-type Mb. It is suggested that the decrease in the rate of the reaction on mutation is due to the absence of a general acid-base catalyst. The effect of pH on the rate of reaction of H2O2 with wild-type Mb and [Gly64]Mb is contrasting. While the rate of the Mb [corrected] reaction decreased for wild-type Mb at higher pH, probably due to the acid-alkaline transition of Mb at higher pH, the rate of reaction was found to increase at higher pH for mutant Mb. The increase in the rate of the reaction is suggested to be due to an increase in the ionization of H2O2 at higher pH, the rate-determining step being the formation of the intermediate Fe-O-O-H complex and not the subsequent step of oxo-ferryl complex formation. The thermodynamic parameters calculated from the temperature-dependent study showed that the enthalpy of binding of H2O2 with Mb is positive, indicating that the process is endothermic. The apparent energy of activation of the reaction of H2O2 with Mb was found to be higher than that of peroxidases, suggesting that this may be oue of the reasons for the slower rate of the reaction of H2O2 with Mb compared with peroxidases.

Kinetic studies on the oxidation of phenols by the horseradish peroxidase compound II

Oxidation of substituted phenols by horseradish peroxidase compound II were studied using stopped-flow technique. Dissociation constants (K(D)) of HRP-II-phenol complexes were deduced from the kinetic data. Magnitudes of K(D) fall in a relatively narrow range of 3-11 mM. These are comparable to 3-10 mM reported for the binding of substituted phenols to native HRP, suggesting that the mode of binding of phenols to native HRP and HRP compound II may be similar. pH dependence of the apparent second order rate constants (k(app)) of the reactions of all the phenols were determined. The k(app) values of reactions other than the reaction of tyrosine, were observed to increase in the acidic region but decreased in the alkaline region. The increase was attributed to the deprotonation of distal carboxylic acid residue on enzyme with pK(a) values of 4.2-5.2. For tyrosine, however, the apparent second-order rate constant was observed to further increase non linearly on increasing the pH in the alkaline region. Results were interpreted quantitatively on the basis that protonated form of the enzyme reacted with the protonated form of the phenol with different individual rate constants.

Kinetic studies of the two-step reactions of H2O2 with manganese-reconstituted myoglobin

Kinetics and thermodynamics of the reaction of manganese-reconstituted myoglobin (MnMb) with hydrogen peroxide have been investigated by the stopped-flow kinetic technique. The results show evidence of two-step formation of peroxide compound in MnMb. Detailed kinetic investigation provides the complete reaction mechanism of the formation of the peroxide compound. It is observed that the formation of the peroxide compound involves an equilibrium binding step with MnMb and H2O2. The H2O2 bound complex of MnMb undergoes irreversible transformation to the peroxide compound, MnMb-I. The microscopic rate constants, involved during these elementary transformation reactions, have been determined. The detailed thermodynamic investigation of the elementary transformation enables us to construct their energy diagram.

The inhibition of bovine xanthine oxidase activity by Hg2+ and other metal ions

The inhibition of the activity of bovine xanthine oxidase (XO) by divalent mercury and other metal ions has been investigated by optical spectroscopy and stop-flow kinetic measurements. The study shows that Hg2+ ion completely inhibits the activity of XO, while other metal ions such as Zn2+, Mg2+, Co2+, and Ni2+ inhibit the activity only marginally (approximately 10%). The inhibition by the Hg2+ ion was found to be monophasic and noncompetitive with strong affinity for binding to XO. The pH-dependent study of the inhibition indicates that at least two ionizing groups of XO are involved in the binding of the Hg2+ ion.

Kinetics and thermodynamics of the molecular mechanism of the reductive half-reaction of xanthine oxidase

The kinetics and thermodynamics of the reductive half-reaction of xanthine oxidase with xanthine as substrate have been investigated by stopped-flow kinetic measurements. The temperature dependence of the steady-state and transient kinetics of the reductive half-reaction reveals the existence of at least three molecular intermediates during this half-reaction. All the microscopic rate constants and the thermodynamic activation parameters of the elementary steps of the reductive half-reaction have been determined for the first time. The microscopic rate constants and the thermodynamic activation parameters of the individual steps show wide variations in their magnitudes. The present work provides the most detailed and incisive description of the reaction of xanthine oxidase with its physiological substrate xanthine.