Notch signalling pathway mediates hair cell development in mammalian cochlea

The mammalian cochlea contains an invariant mosaic of sensory hair cells and non-sensory supporting cells reminiscent of invertebrate structures such as the compound eye in Drosophila melanogaster. The sensory epithelium in the mammalian cochlea (the organ of Corti) contains four rows of mechanosensory hair cells: a single row of inner hair cells and three rows of outer hair cells. Each hair cell is separated from the next by an interceding supporting cell, forming an invariant and alternating mosaic that extends the length of the cochlear duct. Previous results suggest that determination of cell fates in the cochlear mosaic occurs via inhibitory interactions between adjacent progenitor cells (lateral inhibition). Cells populating the cochlear epithelium appear to constitute a developmental equivalence group in which developing hair cells suppress differentiation in their immediate neighbours through lateral inhibition. These interactions may be mediated through the Notch signalling pathway, a molecular mechanism that is involved in the determination of a variety of cell fates. Here we show that genes encoding the receptor protein Notch1 and its ligand, Jagged 2, are expressed in alternating cell types in the developing sensory epithelium. In addition, genetic deletion of Jag2 results in a significant increase in sensory hair cells, presumably as a result of a decrease in Notch activation. These results provide direct evidence for Notch-mediated lateral inhibition in a mammalian system and support a role for Notch in the development of the cochlear mosaic.

Carotenoid status of pregnant women with and without HIV infection in Malawi

OBJECTIVE

To characterise the major plasma carotenoids in pregnant women with and without HIV infection attending antenatal clinic in Blantyre, Malawi.

DESIGN

A cross sectional study.

SETTING

Antenatal clinic of Queen Elizabeth Central Hospital, Blantyre, Malawi.

SUBJECTS

Nine hundred women (697 HIV-positive and 203 HIV-negative women) in their second trimester of pregnancy.

MAIN OUTCOME MEASURES

Plasma carotenoid levels as related to HIV status and level of disease progression.

RESULTS

There were no significant differences in plasma carotenoid levels between HIV-positive and HIV-negative women. Median (25th, 75th percentiles) plasma levels of carotenoids for all women in the study were alpha-carotene, 0.040 (0.23, 0.071) mumol/L; beta-carotene, 0.350 (0.192, 0.595) mumol/L; beta-cryptoxanthin, 0.050 (0.029, 0.091) mumol/L; lutein/zeaxanthin 0.646 (0.426, 0.976) mumol/L; lycopene, 0.088 (0.055, 0.138) mumol/L, and total carotenoids 1.321 (0.884, 1.874) mumol/L. Mothers had higher mean plasma concentrations of alpha-carotene (p < 0.04), beta-carotene (p < 0.0001), lutein/zeaxanthin (p < 0.0001), and total carotenoids (p < 0.0001) in the wet season than the dry season. No seasonality was observed for beta-cryptoxanthin, lycopene, or retinol.

CONCLUSION

This study suggests that pregnant women with and without HIV infection in Blantyre, Malawi have relatively low plasma carotenoid levels and poor dietary intake of provitamin A carotenoids.

[The significance of combination of electrophysiology and automated perimetry tests on the early diagnosis of primary open angle glaucoma]

OBJECTIVE

The study was designed to analyze the relationship among the three electrophysiological tests and visual fields in primary open angle glaucoma (POAG) in order to find some more sensitive and specific parameters in earlier and early diagnosis of patients with POAG.

METHODS

Thirty-six(70 eyes) patients with POAG, 8(12 eyes) glaucoma suspects and 30 (60 eyes) normal subjects underwent pattern electroretinogram (PERG), oscillatory potentials (OPs), pattern visual evoked potentials (PVEP) and Humphery automated perimetry tests respectively.

RESULTS

The abnormal rate of PERG was the highest among the three electrophysiological tests in the glaucoma suspects and those of PERG and automated perimetry results were higher than others after early stage. In the correlation analysis of all objects, the amplitude of PERG had negative correlated with IOP and visual indices, but no correlation with C/D and VA. OPs had negative correlated with C/D, and no correlation with visual fields indices, intraocular pressure (IOP) and visual acuity (VA). The latency of PVEP had positive correlation with visual fields indices and C/D, but no correlation with IOP.

CONCLUSION

Statistical result showed that different parameters could be selected according to different appearance of POAG. In the earlier stage before visual field defects could be found, PERG would be a more sensitive indicator if IOP were high; once C/D was abnormal, OPs and PVEP would be the better signs even though IOP was normal. PERG and Humphery automated perimetry were very useful indicators after early stage. It was significant in combination of many visual function tests on earlier or early diagnosis of POAG.

Construction of phosphorylatable chimeric monoclonal antibody CC49

Phosphorylation sites were introduced into chimeric monoclonal antibody CC49 (MAb-chCC49) by inserting synthetic fragments encoding two and six phosphorylation sites into an expression vector, pdHL7. The phosphorylation sites were created by using the predicted consensus sequences for phosphorylation by the cAMP-dependent protein kinase to the carboxyl terminus of the heavy chain constant region of the MAb-chCC49. The resultant modified antibodies (MAb-chCC49K1 and MAb-chCC49-6P) were expressed in NS0 cells and purified. The MAb-chCC49K1 protein contains two phosphorylation sites per heavy chain whereas the MAb-chCC49-6P protein contains six sites per heavy chain. Both MAb-chCC49K1 and MAb-chCC49-6P proteins can be phosphorylated by the catalytic subunit of cAMP-dependent protein kinase with [gamma-32P]ATP to high specific activity. The 32P-labeled MAb-chCC49K1 and MAb-chCC49-6P proteins bind to cells expressing TAG-72 antigens. The introduction of phosphorylation sites into a monoclonal antibody provides a reagent for the diagnosis and treatment of cancer. The use of multiple phosphorylation sites provides antibodies with very high specific radioactivity and demonstrates that cassettes of phosphorylation sites can be introduced into proteins without altering their functional activity.

The Slug gene is not essential for mesoderm or neural crest development in mice

The Slug gene encodes a zinc finger protein, homologous to the product of the Drosophila Snail gene, that is implicated in the generation and migration of both mesoderm and neural crest cells in several vertebrate species. We describe here the cloning and genetic analysis of the mouse Slug (Slugh) gene. Slugh encodes a 269-amino-acid protein the shares 92% amino acid identity with the product of the chicken Slug gene. We have characterized Slugh gene expression during early mouse embryogenesis by whole mount in situ hybridization of Slugh mRNA and through detection of beta-galactosidase expression from an in-frame SlughIacZ allele generated through homologous recombination. Slugh expression is first detected in extraembryonic mesoderm and is later detected in many mesodermal subsets, although it is not detected in the primitive streak. In contrast to many other vertebrates, the mouse Slug gene is not expressed in premigratory neural crest cells but is expressed in migratory neural crest cells. Analysis of a targeted null mutation that deleted all Slugh coding sequences revealed that Slugh is not required for mesoderm formation or for neural crest generation, migration, or development in mice. These results indicate that neither the expression pattern nor the biological function of the Slug gene is conserved among all vertebrates. These data also raise interesting questions about the regulation of neural crest generation, which is one of the distinguishing characteristics of the vertebrate subphylum.

Antibody-IL-12 fusion proteins are effective in SCID mouse models of prostate and colon carcinoma metastases

IL-12 is a complex cytokine in both its structure and its range of biologic activities. Fusions of this heterodimeric molecule with an intact antitumor Ab were made to test the feasibility and efficacy of targeting IL-12 to tumors to elicit a local immune response. Fusion proteins composed of the human p35 and p40 subunits had IL-12 bioactivities that were nearly as potent on human immune cells as the rIL-12 standard, but were inactive on mouse cells. Hybrid IL-12 fusion proteins composed of mouse p35 and human p40, fused to Ab, were capable of inducing IFN-gamma, but were much less active on mouse spleen cells than a mouse IL-12 standard. Despite this relatively low activity, the hybrid fusion protein was as effective in a SCID mouse model as a fully active Ab-IL-2 fusion protein in eliminating established pulmonary metastases of CT26 colon carcinoma. Specific targeting of a human IL-12 fusion protein to metastatic prostate carcinoma xenografts was also shown to be effective in SCID mice transplanted with human lymphocyte-activated killer cells. These results demonstrate the importance of directing this potent cytokine to the tumor microenvironment and suggest an important alternative to systemic IL-12 administration or gene therapy for increasing its therapeutic index.

[Study on the relationship between occupational factors and work ability of middle aged workers]

Based on the assessment of work ability index (WAI) of middle aged workers, associations between occupational factors and work ability decline of middle aged workers were examined in a case-control study. 1037 workers (age ranged from 40 to 60) were included in the study. The study group comprised 180 workers of mean age 47.71 years representing those who's work ability decreased. The control group comprised 857 workers of mean age 47.22 years representing those who's work ability was on a normal level. Results showed that some common occupation hazards such as dusts, noise, vibration, humidity, and high temperature were more frequently occurred in work environment of the study group than that of the control group. High risks were found in workers with these hazards. Another significant risk factor of work ability decline was heavy physical load. Significantly elevated risks were observed for physical load perception(OR = 2.52), repetitive work (OR = 1.36), poor work postures(OR = 1.48), and carrying heavy loads(OR = 1.59). As compared to physical loads, mental loads were not in relation to work ability decline. The information about occupational factors affecting work ability will be helpful for redesiging the work for the middle aged workers.

High level expression and secretion of Fc-X fusion proteins in mammalian cells

We have developed a general expression system that enhances the production and secretion of proteins in mammalian cells. The protein of interest is expressed as a fusion to a signal peptide and the Fc fragment of immunoglobulin as the N-terminal fusion partner, which can direct the cellular processes into expressing and secreting high levels of many different types of proteins. These include secretory proteins, enzymes and soluble domains of membrane proteins, as well as nuclear and regulatory proteins. Typical expression levels of these proteins from stable cell lines ranged from several to 100 microg/ml in conditioned media. The Fc domain helps to solubilize hydrophobic proteins and provides a handle for easy detection and purification of the fusion proteins; and it can be cleaved off by treatment with protease if desired.

Defects in limb, craniofacial, and thymic development in Jagged2 mutant mice

The Notch signaling pathway is a conserved intercellular signaling mechanism that is essential for proper embryonic development in numerous metazoan organisms. We have examined the in vivo role of the Jagged2 (Jag2) gene, which encodes a ligand for the Notch family of transmembrane receptors, by making a targeted mutation that removes a domain of the Jagged2 protein required for receptor interaction. Mice homozygous for this deletion die perinatally because of defects in craniofacial morphogenesis. The mutant homozygotes exhibit cleft palate and fusion of the tongue with the palatal shelves. The mutant mice also exhibit syndactyly (digit fusions) of the fore- and hindlimbs. The apical ectodermal ridge (AER) of the limb buds of the mutant homozygotes is hyperplastic, and we observe an expanded domain of Fgf8 expression in the AER. In the foot plates of the mutant homozygotes, both Bmp2 and Bmp7 expression and apoptotic interdigital cell death are reduced. Mutant homozygotes also display defects in thymic development, exhibiting altered thymic morphology and impaired differentiation of gamma delta lineage T cells. These results demonstrate that Notch signaling mediated by Jag2 plays an essential role during limb, craniofacial, and thymic development in mice.

Elimination of established murine colon carcinoma metastases by antibody-interleukin 2 fusion protein therapy

A recombinant humanized antibody-interleukin 2 fusion protein (huKS1/4-IL-2) was used to direct IL-2 to the tumor microenvironment and elicit a T cell-mediated eradication of established pulmonary and hepatic CT26-KSA colon carcinoma metastases in syngeneic BALB/c mice. This antitumor effect was specific because a fusion protein, which was nonreactive with these tumor cells, failed to exert any such effect. The efficacy of the huKS1/4-IL-2 fusion protein in eliminating metastases was documented because mixtures of monoclonal antibody huKS1/4 with recombinant human IL-2 were ineffective and, at best, only partially reduced tumor load. Two lines of evidence indicated the eradication of metastases and the absence of minimal residual disease in animals treated with the fusion protein: first, the lack of detection of CT26-KSA cells by reverse transcription-PCR, which can detect one tumor cell in 10(6) liver cells; and second, the tripling of life span. The effector mechanism involved in this tumor eradication is dependent on T cells because the IL-2-directed therapy is ineffective in T cell-deficient SCID mice. The essential effector cells were further characterized as CD8+ T cells by in vivo depletion studies. Such T cells, isolated from tumor-bearing mice after fusion protein therapy, elicited MHC class I-restricted cytotoxicity in vitro against colon carcinoma target cells. Taken together, these data indicate that fusion protein-directed IL-2 therapy induces a T cell-dependent host immune response capable of eradicating established colon cancer metastases in an animal tumor model.

Effects of changes in calorie intake on intestinal nutrient uptake and transporter mRNA levels in aged mice

In aged, chronically calorie-restricted (CR) mice, intestinal nutrient uptake is significantly higher than in same-age ad libitum controls. Can this chronic restriction-induced enhancement of uptake be reversed by ad libitum feeding? We addressed this question by switching 32-mo-old chronically CR mice to ad libitum feeding for 4 wk (CRAL). Intestinal transport rate and total intestinal absorptive capacity for D-sugars and several nonessential L-amino acids decreased significantly in CRAL mice. In contrast, switching CR mice to an ad libitum regimen for only 3 d had no effect on intestinal nutrient transport, indicating that the negative effects of ad libitum feeding require a duration longer than the 3-d lifetime of most enterocytes. Permeability of the intestinal mucosa to L-glucose was independent of the switches in diet. Levels of the brushborder glucose transporter SGLT1, brushborder fructose transporter GLUT5, and basolateral sugar transporter GLUT2 mRNA as determined by reverse transcriptase-polymerase chain reaction in 6-, 24-, and 32-mo-old mice were each apparently independent of caloric restriction and age. We conclude that the high rates of intestinal nutrient uptake exhibited by chronically CR mice can be reversed by ad libitum feeding of only 1 mo duration. These decreases in uptake were due mainly to specific decreases in transport per unit weight of intestine and not to nonspecific decreases in intestinal mass. Changes in rates of sugar uptake induced by chronic CR and age are apparently not accompanied by changes in steady-state levels of mRNA coding for those transporters.

The identification and characterization of a G4-DNA resolvase activity

There is increasing evidence that four-stranded Hoogsteen-bonded DNA structures, G4-DNA, play an important role in cellular processes such as meiosis and recombination. The Hoogsteen-bonded G4-DNA is thermodynamically more stable than duplex DNA, and many guanine-rich genomic DNA sequences with the ability to form G4-DNA have been identified. A protein-dependent activity that resolves G4-DNA into single-stranded DNA has been identified in human placental tissue. The resolvase activity was purified from any apparent nuclease activity and is dependent on NTP hydrolysis and MgCl2. Resolvase activity is optimal with 5 mM MgCl2. The Vmax/Km of ATP is 0. 055%/min/microM, higher than the Vmax/Km of the other dNTPs. The products of the resolvase reaction are unmodified single-stranded DNA. The resolvase is not a duplex DNA helicase or a topoisomerase II activity and does not unwind Hoogsteen-bonded triplex DNA. Resolvase is a novel activity that unwinds stable G4-DNA structures using a dNTP-dependent mechanism producing unmodified single-stranded DNA. Potential in vivo roles for this G4-DNA resolvase activity are discussed.

[The residue and degradation of herbicide ammonium 2,4-dichloro-6-nitrophenate in the system of wheat and soil]

To study the residue and degradation of ammonium 2, 4-dichloro-6-nitrophenate (a herbicide) in wheat and soil, a field test was made in Wuxi Jiangsu of East China and Chengdu Sichuan of West Chain during 1985-1992. The results showed that ammonium 2,4-dichloro-6-nitrophenate was unstable with a half-life of 12-18 days in soil. The residue in wheat was less than 0.05 mg/kg on the field doses recommended. These results may be used as a basis for the hygienic evaluation of ammonium, 2,4-dichloro-6-nitrophenate.

[The relationship between pulmonary function and work ability of aging workers]

To explore the relationship between pulmonary function and work ability, a field study was conducted in 285 aging workers (aged 40 to 60) of different occupations in Chengdu area. The pulmonary function indices included FVC, FEV1, FEV1%, MMEF, and the pulmonary function index (PFI) which was the sum of the rating values from the first three indices. The work ability index (WAI) was also measured. Between work types, the indices, including FVC and FEV1%, showed significant difference (P < 0.05); FVC also showed significant difference between male and female workers (P < 0.05). There was a significant relationship between PFI (including FEV1% and MMEF) and WAI (P < 0.05), but the relationship between FVC (or FEV1) and WAI was not statistically significant (P > 0.05). Moreover, the agreement percentage between the classifications of PFI and WAI was 43.8%; the disagreement percentage was 4.56%. The results suggest that FEV1% and MMEF may be beneficial to the objective evaluation of the work ability of aging workers.

High prevalence of hantavirus infection in a group of Chinese patients with acute hepatitis of unknown aetiology

In southwestern China, small but substantial numbers of patients with acute hepatitis were found without known hepatropic viral infections (hepatitis A, B, C, D or E, cytomegalovirus, Epstein-Barr virus) and were receiving no hepatotoxic drugs. Prevalence of antibodies, both neutralizing and specific immunoglobulin (Ig)M and IgG, to Hantaan virus were evaluated in a cohort of 136 such patients: 83 were of unknown aetiology, 53 had known viral hepatitis and 59 healthy subjects acted as controls. The results showed that the incidence of neutralizing antibody to Hantaan virus in acute hepatitis patients with non-hepatitis A-E virus infections (13 of 83) was significantly higher than in those with A-E infections (0 of 53, P<0.01). Furthermore, the incidence of specific IgM antibody to Hantaan virus in acute hepatitis patients with non-hepatitis A-E virus infections (6 of 83) was significantly higher than in those with A-E infections (0 of 53, P<0.05) and in healthy subjects (0 of 59, P < 0.05). These findings suggest that Hantaan virus may be an important agent, contributing, at least in southwestern China, to a significant number of the cases of acute hepatitis of unknown aetiology. This hantavirus infection resulted in an acute hepatitis, differing from the typical diseases: haemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS).

Both the paired domain and homeodomain are required for in vivo function of Drosophila Paired

Drosophila paired, a homolog of mammalian Pax-3, is key to the coordinated regulation of segment-polarity genes during embryogenesis. The paired gene and its homologs are unusual in encoding proteins with two DNA-binding domains, a paired domain and a homeodomain. We are using an in vivo assay to dissect the functions of the domains of this type of molecule. In particular, we are interested in determining whether one or both DNA-binding activities are required for individual in vivo functions of Paired. We constructed point mutants in each domain designed to disrupt DNA binding and tested the mutants with ectopic expression assays in Drosophila embryos. Mutations in either domain abolished the normal regulation of the target genes engrailed, hedgehog, gooseberry and even-skipped, suggesting that these in vivo functions of Paired require DNA binding through both domains rather than either domain alone. However, when the two mutant proteins were placed in the same embryo, Paired function was restored, indicating that the two DNA-binding activities need not be present in the same molecule. Quantitation of this effect shows that the paired domain mutant has a dominant-negative effect consistent with the observations that Paired protein can bind DNA as a dimer.

Reduced hydrogen peroxide production in neutrophils from patients with diabetes

We evaluated the effects of granulocyte colony-stimulating factors (G-CSF), granulocyte-macrophage colony-stimulating factors (GM-CSF) and phorbol myristate acetate (PMA) on H2O2 production in purified neutrophils from patients with diabetes mellitus (DM), using flow cytometry. Twenty-two age-matched male subjects were selected: 11 were normal volunteers and the remainder had DM. No significant differences in intracellular H2O2 production per 60 min was observed in the "resting' neutrophils from DM patients (37.2 +/- 20.4 A.U.) compared with those from normal volunteers (24.9 +/- 8.4 A.U.). The PMA-stimulated neutrophils from normal volunteers generated approximately 4-fold increases in H2O2 per 60 min compared with those from DM patients. Under similar culture conditions, G-CSF caused 1.6-fold increases of H2O2 in neutrophils from normal volunteers compared with those of DM patients. Increases after GM-CSF stimulation were 2-fold higher in volunteer neutrophils compared with those from DM patients. The levels of G-CSF- or GM-CSF-stimulated H2O2 production in neutrophils from DM patients were low and were little different from non-stimulated resting cells. These data showed that H2O2 production in neutrophils induced by PMA is impaired in patients with DM, and neither G-CSF nor GM-CSF enhances its production.

[The evaluation of oscillatory potentials of the electroretinogram in the early diagnosis of the patients with primary open angle glaucoma]

PURPOSE

To study the changes in the oscillatory potentials (OPs) of the electroretinogram in the earlier or early stage of primary open angle glaucoma.

METHODS

OPs were recorded in thirty-three normal subjects (66 eyes) and 30 patients with primary open angle glaucoma (POAG) (58 eyes). The abnormal rates of OPs in the different stages of POAG were analyzed and the sensitivity and the specificity of OPs were also determined.

RESULTS

The overall OPs amplitudes (sigma o) were reduced in the patients with POAG and the amounts of small wavelets were also decreased. The total abnormal rate of sigma o was 67.24%, and 42.86% in the earlier stage, 65.38% in the early stage and 76.00% in the advanced and late stage. The sensitivity and the specificity of OPs were 67.24% and 92.42%. OPs had correlation with C/D.

CONCLUSION

OPs can be considered as an indicator of earlier or early diagnosis of patients with POAG before visual field loss is detected.

The changes of pattern reversal visual evoked potentials in normal infants

PURPOSE

To study pattern reversal visual evoked potential (PVEPs) and determine the developmental character and mature time of visual function in normal infants at different months of age.

METHODS

PVEPs were recorded from 115 normal infants at 3, 6, 9, 12 months age. P1 latency for different checks (1 degree 40', 25', 6') was analyzed and compared to those of normal adults. Changes of N1, N2 latency of PVEPs were also examined.

RESULTS

P1 latency for all checks (1 degree 40', 25', 6') was significantly longer at 3 months than at 6 months of age (P < 0.05), but no significant differences can be seen after 6 months of age for larger (1 degree 40') and intermediate (25') checks (P > 0.05). P1 latency for larger checks (1 degree 40') reached adult level after 3 months of age, but not for the intermediate check (25'), while P1 latency for small check (6') presented the character of fluctuation.

CONCLUSION

The visual system continued to develop after birth and appeared a certain regularity. Our results showed that P1 latency for larger check (1 degree 40') reached adult levels after 3 months of age. But P1 latency for intermediate check still has not reached adult levels after 3 months of age. To determine the age at which adult levels are finally reached, infants of 12 months and older must be tested. The reason why P1 latency for smaller check (6') presented the character of fluctuation should be the temporal tuning function developing much more slowly.

A study on national differences in identification standards for Chinese skull-image superimposition

This study selected 15 nationalities as test subjects among 56 nationalities in China, each having a population of > 1,000,000 million and characterized by significant features in physical type. The test nationalities include Yi, Bai, Dai, Yao, Korean, Hui, Man, Zhuang, Miao, Dong, Bouyei, Mongol, Zang, Uygur and Kazak nationalities. Three thousand test subjects (100 males and 100 females for each nationality) were randomly selected to be photographed, tested and observed by means of somatometry of faces, photographs of facial landmarks and X-ray photography. In accordance with this result, an identification standard for skull-image superimposition was established for different nationalities and sexes. On the basis of this, we then made a comparison of identification indices between Han and 15 nationalities as well as a T-value test on the significance of national differences. The result of this study shows that the test values of indices for 15 test nationalities are all different from those for Han nationality and the difference greatly reveals regionality. For example, Han in the Northeast of China is similar to Man and Korean nationalities in the Northeast, but much different from Zhuang, Miao, Dai and Dong nationalities in the Southwest. The main differences are in head length, head breadth, alae nasi, nasal tip, mouth and zygoma, etc.

Evidence for a (triosephosphate isomerase-like) "catalytic loop" near the active site of glyoxalase I

The conformational mobility of glyoxalase I (Glx I) during catalysis has been probed using stable analogs of the enediol intermediate that forms along the reaction pathway: GSC(O)N(OH)R, where GS = glutathionyl and R = CH3 (1), C6H5 (2), C6H4Cl (3), or C6H4Br (4). For human erythrocyte Glx I, catalysis is unlikely to be coupled to major changes in protein secondary structure, as the circular dichroism spectrum of the enzyme (190-260 nm) is insensitive to saturating concentrations of either enediol analog or S-D-lactoylglutathione, the product of the Glx I reaction. However, a small conformational change is indicated by the fact that binding of enediol analog to the active site decreases intrinsic protein fluorescence by 11%, and protects the enzyme from proteolytic cleavage by Pronase E at the C-side of Ala-92 and Leu-93. In contrast, binding of S-D-lactoylglutathione does not affect protein fluorescence, and increases the rate of proteolytic cleavage by 1.5-fold. These observations are consistent with a model of catalysis in which a flexible peptide loop folds over and stabilizes the enediol intermediate bound to the active site. Indeed, a highly conserved sequence of amino acid residues is found near the proteolytic cleavage sites, for human Glx I (100-111) and Pseudomonas putida Glx I (93-105), that shows significant sequence homology to the "catalytic loop" of chicken muscle triosephosphate isomerase (TIM) (165-176). The active site base (Glu-165) of TIM, which catalyzes the proton transfer reaction during isomerization, corresponds in position to Glu-93 of P. putida Glx I. Consistent with a functional role for Glu-93, a mutant enzyme in which Glu-93 is replaced by Asp shows no detectable catalytic activity.