Fat digestion using RELiZORB in children with short bowel syndrome who are dependent on parenteral nutrition: Protocol for a 90-day, phase 3, open labeled study.

BACKGROUND: Short bowel syndrome (SBS) is a leading cause of intestinal failure resulting in parenteral nutrition (PN) dependence and nutritional deficiencies. Long-term PN use is associated with the development of sepsis and intestinal failure-associated liver disease. Achieving enteral autonomy is the optimal way to prevent these complications. In SBS, the decreased intestinal length, bile acid deficiency, and rapid transit time contribute to fat malabsorption and continued PN dependence. We propose the use of an immobilized lipase cartridge (ILC; RELiZORB) that connects in-line with enteral feed tubing sets and is designed to breakdown the majority of fats provided in enteral nutrition (EN). Preclinical studies have demonstrated both improved fat and fat-soluble vitamin absorption with ILC use in a porcine model of SBS. To evaluate the clinical applicability of these findings, we designed a phase 3, open labeled, single center, clinical trial to determine the safety, tolerability, and efficacy of the RELiZORB enzyme cartridge when used daily with EN for 90 days. METHODS: The patient population will include PN dependent children with SBS, aged 2-18 years. The primary outcome is the change in PN calories from baseline, assessed weekly throughout the study. Changes in growth Z-scores, 72-hour fecal fat and coefficient of fat absorption, plasma fatty acids and fat-soluble vitamins will also be evaluated. Assessment of change in continuous outcomes will be made using the area under the curve, expressed as a percent change relative to baseline, calculated over study day 7 to 90 (AUC7-90). The incidence of adverse events will be monitored and summarized by system organ class. DISCUSSION: If successful, RELiZORB may offer a safe alternative to reducing PN dependence and achieving enteral autonomy in pediatric intestinal failure. These results would be clinically significant given the clear association between long-term PN use and complications in SBS. TRIAL REGISTRATION: ClinicalTrials.gov NCT03530852; registered on May 21st, 2018, last update posted on September 14th, 2022.

A Digestive Cartridge Reduces Parenteral Nutrition Dependence and Increases Bowel Growth in a Piglet Short Bowel Model.

OBJECTIVE: To determine whether the use of an immobilized lipase cartridge (ILC) to hydrolyze fats in enteral nutrition (EN) reduces parenteral nutrition (PN) dependence in a porcine model of short bowel syndrome with intestinal failure (SBS-IF). BACKGROUND: SBS-IF occurs after intestinal loss resulting in malabsorption and PN dependence. Limited therapeutic options are available for achieving enteral autonomy. METHODS: Eleven Yorkshire piglets underwent 75% jejunoileal resection and were randomized into control (n=6) and treatment (n = 5) groups. PN was initiated postoperatively and reduced as EN advanced if predefined clinical criteria were fulfilled. Animals were studied for 14 days and changes in PN/EN calories were assessed. Intestinal adaptation, absorption, and nutrition were evaluated at the end of the study (day 15). Comparisons between groups were performed using analysis of covariance adjusted for baseline. RESULTS: ILC animals demonstrated a 19% greater reduction in PN calories ( P < 0.0001) and higher mean EN advancement (66% vs 47% of total calories, P < 0.0001) during the 14-day experiment. Treatment animals had increased intestinal length (19.5 vs 0.7%, P =0.03) and 1.9-fold higher crypt cell proliferation ( P =0.02) compared with controls. By day 15, ILC treatment resulted in higher plasma concentrations of glucagon-like peptide-2 ( P = 0.02), eicosapentaenoic acid ( P < 0.0001), docosahexaenoic acid ( P = 0.004), vitamin A ( P = 0.02), low-density lipoprotein ( P = 0.02), and high-density lipoprotein ( P = 0.04). There were no differences in liver enzymes or total bilirubin between the two groups. CONCLUSIONS: ILC use in conjunction with enteral feeding reduced PN dependence, improved nutrient absorption, and increased bowel growth in a porcine SBS-IF model. These results support a potential role for the ILC in clinical SBS-IF.

Improvements in anthropometric measures and gastrointestinal tolerance in patients with cystic fibrosis by using a digestive enzyme cartridge with overnight enteral nutrition.

BACKGROUND: Patients with cystic fibrosis (CF) and pancreatic insufficiency are at risk for suboptimal fat absorption, inability to maintain weight, poor growth, and increased gastrointestinal (GI) symptoms due to malabsorption. Enteral nutrition (EN) is used to supplement caloric intake and requires pancreatic enzyme replacement for effective digestion. We evaluated the relationship between long-term use of an in-line digestive enzyme cartridge with EN and changes in anthropometric measures and GI symptoms in patients with CF. METHODS: This is a single-center, retrospective case review of patients with CF using a digestive enzyme cartridge with EN. Data were collected from the patient medical records and included weight, height, body mass index, EN regimen, and reported GI symptoms. RESULTS: Thirteen pediatric and five adult patients with a mean age of 12.6 years used a digestive enzyme cartridge with EN for a period of 3-27 months. Most patients (n = 14) had been using oral digestive enzymes with EN before using the digestive enzyme cartridge, whereas four started from the onset of EN. The indications to convert from oral enzymes to the digestive enzyme cartridge included poor growth (72.2%) and poor tolerance of EN (69.2%). There was a reduction in reported GI symptoms after initiating use of the digestive enzyme cartridge. After 12 months of digestive cartridge use, there were improvements in anthropometrics. CONCLUSIONS: This real-world experience with prolonged use of a digestive enzyme cartridge with EN demonstrated improved clinical outcomes and a reduction in GI symptoms in patients with CF.

Outcomes and Perioperative Nutritional Management in a Porcine Model of Short Bowel Syndrome.

INTRODUCTION: Short bowel syndrome (SBS) results from significant intestinal loss and is characterized by insufficient absorption of nutrients and fluids. Preclinical large animal SBS models typically require parenteral nutrition (PN) support and may not be appropriate for studying interventions to improve intestinal absorption or adaptation. Here, we describe the development of a porcine SBS model that does not require PN support. METHODS: Eight male Yorkshire piglets underwent either a 75% or 90% jejunoileal resection (n = 5) or no resection (n = 3). Continuous enteral nutrition (EN) was provided via a gastrostomy tube. The final SBS model consisted of a 75% resection and nutrition provided via combination EN (60%) and per oral pig chow (40%). Body weight and concentration of fat-soluble vitamins were assessed on postoperative days (POD) 7, 14, and 21. For assessing fat malabsorption, the coefficient of fat absorption (CFA) was calculated following a 72-h stool collection. RESULTS: Resected animals had decreased weight gain compared to unresected controls (POD21 + 8.3% versus +28.8%, P = 0.048). Vitamin D concentration was significantly lower in resected animals compared to controls on POD 7, POD 14, and POD 21. Serum vitamin E concentration was also lower on POD 21. Resected animals developed fat malabsorption with lower CFA (76.5% versus 95.3%, P = 0.014). CONCLUSIONS: We describe the development of a porcine SBS model that does not require PN support. Piglets in this model gain less weight, demonstrate fat malabsorption, and develop fat-soluble vitamin deficiencies. This model will benefit investigations of intestinal absorption or adaptation while potentially decreasing costs and confounding complications related to PN administration.

Evaluation of the Effectiveness of In-line Immobilized Lipase Cartridge in Enterally Fed Patients With Cystic Fibrosis.

BACKGROUND: Pancreatic insufficiency occurs in most patients with cystic fibrosis (CF) contributing to malnutrition. In the United States, 3600 patients with CF require enteral feeding (EF). Oral pancreatic enzymes are commonly used with EF, despite not being designed or approved for this use. An immobilized lipase cartridge (ILC) for extracorporeal digestion of enteral feedings was developed. The sponsor provided it to patients via a structured program, which we evaluated to assess the effectiveness of the ILC on nutritional status. METHODS: The program provided the ILC to patients prescribed the device while reimbursement efforts were ongoing. Baseline anthropometric data were obtained and subsequent measurements of height, weight, and body mass index (BMI) were collected at 6 and 12 months. RESULTS: Inclusion criteria were met by 100 patients (age = 0--45 years). Over 12 months of use in patients >2 years of age (n = 93), there were significant improvements seen in height and weight z-scores with improvement trend seen in BMI. The frequency of achieving the 50th percentile increased steadily for weight and BMI from baseline to 12 months but not for height. CONCLUSIONS: This evaluation of a program to assist patient access to ILC demonstrates that better growth is possible over standard of care. The association of ILC use with significant improvements in anthropometric parameters over a 12-month period in people with CF demonstrates the effectiveness of ILC as rational enzyme therapy during enteral feedings.

Spectrophotometric assays for monitoring tRNA aminoacylation and aminoacyl-tRNA hydrolysis reactions.

Aminoacyl-tRNA synthetases play a central role in protein synthesis, catalyzing the attachment of amino acids to their cognate tRNAs. Here, we describe a spectrophotometric assay for tyrosyl-tRNA synthetase in which the Tyr-tRNA product is cleaved, regenerating the tRNA substrate. As tRNA is the limiting substrate in the assay, recycling it substantially increases the sensitivity of the assay while simultaneously reducing its cost. The tRNA aminoacylation reaction is monitored spectrophotometrically by coupling the production of AMP to the conversion of NAD+ to NADH. We have adapted the tyrosyl-tRNA synthetase assay to monitor: (1) aminoacylation of tRNA by l- or d-tyrosine, (2) cyclodipeptide formation by cyclodipeptide synthases, (3) hydrolysis of d-aminoacyl-tRNAs by d-tyrosyl-tRNA deacylase, and (4) post-transfer editing by aminoacyl-tRNA synthetases. All of these assays are continuous and homogenous, making them amenable for use in high-throughput screens of chemical libraries. In the case of the cyclodipeptide synthase, d-tyrosyl-tRNA deacylase, and post-transfer editing assays, the aminoacyl-tRNAs are generated in situ, avoiding the need to synthesize and purify aminoacyl-tRNA substrates prior to performing the assays. Lastly, we describe how the tyrosyl-tRNA synthetase assay can be adapted to monitor the activity of other aminoacyl-tRNA synthetases and how the approach to regenerating the tRNA substrate can be used to increase the sensitivity and decrease the cost of commercially available aminoacyl-tRNA synthetase assays.

Hyperactive Editing Domain Variants Switch the Stereospecificity of Tyrosyl-tRNA Synthetase.

d-Amino acids are excluded at three different steps during protein synthesis: the aminoacylation of tRNA, binding of aminoacyl-tRNAs to EF-Tu, and selection of the aminoacyl-tRNA by the ribosome. We previously altered the enantioselectivity of tyrosyl-tRNA synthetase (TyrRS) by inserting the editing domain from phenylalanyl-tRNA synthetase (FRSed) between Gly 161 and Ile 162 in tyrosyl-tRNA synthetase (the editing domain hydrolyzes l-Tyr-tRNA but not d-Tyr-tRNA). In this paper, we test the hypothesis that the enantioselectivity of this TyrRS-FRSed chimera can be shifted further toward the formation of d-Tyr-tRNA by introducing activating mutations into the editing site. Yokoyama and colleagues previously identified six alanine substitutions in phenylalanyl-tRNA synthetase that increase its editing activity.1 We have introduced these alanine substitutions into TyrRS-FRSed in various combinations, generating 14 different variants. To analyze their editing activity, we developed a continuous, spectrophotometric, steady-state post-transfer editing assay in which l-Tyr-tRNA is generated in situ, resulting in the release of one molecule of AMP during each editing cycle. Post-transfer editing is monitored by coupling the release of AMP to the reduction of NAD(+) (via the actions of AMP deaminase and IMP dehydrogenase), resulting in an increase in absorbance at 340 nm. In general, TyrRS-FRSed variants containing two activating mutations are the most active, with additional alanine substitutions decreasing the activity of the editing domain. Linear free energy relationships indicate that high kcat values are correlated with high binding affinities for l-Tyr-tRNA. Lastly, competition assays indicate that at least one TyrRS-FRSed variant (F145A/S211A) preferentially aminoacylates tRNA with d-tyrosine, demonstrating that the enantioselectivity of tyrosyl-tRNA synthetase can be inverted using hyperactive editing domains.

Altering the Enantioselectivity of Tyrosyl-tRNA Synthetase by Insertion of a Stereospecific Editing Domain.

Translation of mRNAs by the ribosome is stereospecific, with only l-amino acids being incorporated into the nascent polypeptide chain. This stereospecificity results from the exclusion of d-amino acids at three steps during protein synthesis: (1) the aminoacylation of tRNA by aminoacyl-tRNA synthetases, (2) binding of aminoacyl-tRNAs to EF-Tu, and (3) recognition of aminoacyl-tRNAs by the ribosome. As a first step toward incorporating d-amino acids during protein synthesis, we have altered the enantioselectivity of tyrosyl-tRNA synthetase. This enzyme is unusual among aminoacyl-tRNA synthetases, as it can aminoacylate tRNA with d-tyrosine (albeit at a reduced rate compared to l-tyrosine). To change the enantioselectivity of tyrosyl-tRNA synthetase, we introduced the post-transfer editing domain from Pyrococcus horikoshii phenylalanyl-tRNA synthetase into the connective polypeptide 1 (CP1) domain of Geobacillus stearothermophilus tyrosyl-tRNA synthetase (henceforth designated TyrRS-FRSed). We show that the phenylalanyl-tRNA synthetase editing domain is stereospecific, hydrolyzing l-Tyr-tRNA(Tyr), but not d-Tyr-tRNA(Tyr). We further show that inserting the phenylalanyl-tRNA synthetase editing domain into the CP1 domain of tyrosyl-tRNA synthetase decreases the activity of the synthetic site in tyrosyl-tRNA synthetase. This decrease in activity is critical, as it prevents the rate of synthesis from overwhelming the ability of the editing domain to hydrolyze the l-Tyr-tRNA(Tyr) product. Overall, inserting the phenylalanyl-tRNA synthetase editing domain results in a 2-fold shift in the enantioselectivity of tyrosyl-tRNA synthetase toward the d-Tyr-tRNA(Tyr) product. When a 4-fold excess of d-tyrosine is used, approximately 40% of the tRNA(Tyr) is aminoacylated with d-tyrosine.

Expanding a tyrosyl-tRNA synthetase assay to other aminoacyl-tRNA synthetases.

Aminoacyl-tRNA synthetases catalyze the attachment of amino acids to their cognate tRNAs. In general, aminoacyl-tRNA synthetase assays require stoichiometric amounts of tRNA, which limits their sensitivity while increasing their cost. This requirement for stoichiometric amounts of tRNA can be alleviated if the aminoacyl-tRNA product is cleaved following the tRNA aminoacylation reaction, regenerating the free tRNA substrate. This data article is related to the research article entitled "A continuous tyrosyl-tRNA synthetase assay that regenerates the tRNA substrate" in which this approach is used to develop a continuous spectrophotometric assay for tyrosyl-tRNA synthetase [1]. Here we present enzymes that can be used to cleave the aminoacyl-tRNA product for at least 16 of the 20 naturally occurring amino acids. These enzymes can be used to extend the tyrosyl-tRNA synthetase assay to other aminoacyl-tRNA synthetases.

A continuous spectrophotometric assay for monitoring adenosine 5'-monophosphate production.

A number of biologically important enzymes release adenosine 5'-monophosphate (AMP) as a product, including aminoacyl-tRNA synthetases, cyclic AMP (cAMP) phosphodiesterases, ubiquitin and ubiquitin-like ligases, DNA ligases, coenzyme A (CoA) ligases, polyA deadenylases, and ribonucleases. In contrast to the abundance of assays available for monitoring the conversion of adenosine 5'-triphosphate (ATP) to ADP, there are relatively few assays for monitoring the conversion of ATP (or cAMP) to AMP. In this article, we describe a homogeneous assay that continuously monitors the production of AMP. Specifically, we have coupled the conversion of AMP to inosine 5'-monophosphate (IMP) (by AMP deaminase) to the oxidation of IMP (by IMP dehydrogenase). This results in the reduction of oxidized nicotine adenine dinucleotide (NAD(+)) to reduced nicotine adenine dinucleotide (NADH), allowing AMP formation to be monitored by the change in the absorbance at 340 nm. Changes in AMP concentrations of 5 µM or more can be reliably detected. The ease of use and relatively low expense make the AMP assay suitable for both high-throughput screening and kinetic analyses.

A continuous tyrosyl-tRNA synthetase assay that regenerates the tRNA substrate.

Tyrosyl-tRNA synthetase catalyzes the attachment of tyrosine to the 3' end of tRNA(Tyr), releasing AMP, pyrophosphate, and l-tyrosyl-tRNA as products. Because this enzyme plays a central role in protein synthesis, it has garnered attention as a potential target for the development of novel antimicrobial agents. Although high-throughput assays that monitor tyrosyl-tRNA synthetase activity have been described, these assays generally use stoichiometric amounts of tRNA, limiting their sensitivity and increasing their cost. Here, we describe an alternate approach in which the Tyr-tRNA product is cleaved, regenerating the free tRNA substrate. We show that cyclodityrosine synthase from Mycobacterium tuberculosis can be used to cleave the l-Tyr-tRNA product, regenerating the tRNA(Tyr) substrate. Because tyrosyl-tRNA synthetase can use both l- and d-tyrosine as substrates, we replaced the cyclodityrosine synthase in the assay with d-tyrosyl-tRNA deacylase, which cleaves d-Tyr-tRNA. This substitution allowed us to use the tyrosyl-tRNA synthetase assay to monitor the aminoacylation of tRNA(Tyr) by d-tyrosine. Furthermore, by making Tyr-tRNA cleavage the rate-limiting step, we are able to use the assay to monitor the activities of cyclodityrosine synthetase and d-tyrosyl-tRNA deacylase. Specific methods to extend the tyrosyl-tRNA synthetase assay to monitor both the aminoacylation and post-transfer editing activities in other aminoacyl-tRNA synthetases are discussed.

Identification and characterization of mutations conferring resistance to D-amino acids in Bacillus subtilis.

UNLABELLED: Bacteria produce d-amino acids for incorporation into the peptidoglycan and certain nonribosomally produced peptides. However, D-amino acids are toxic if mischarged on tRNAs or misincorporated into protein. Common strains of the Gram-positive bacterium Bacillus subtilis are particularly sensitive to the growth-inhibitory effects of D-tyrosine due to the absence of D-aminoacyl-tRNA deacylase, an enzyme that prevents misincorporation of D-tyrosine and other D-amino acids into nascent proteins. We isolated spontaneous mutants of B. subtilis that survive in the presence of a mixture of D-leucine, D-methionine, D-tryptophan, and D-tyrosine. Whole-genome sequencing revealed that these strains harbored mutations affecting tRNA(Tyr) charging. Three of the most potent mutations enhanced the expression of the gene (tyrS) for tyrosyl-tRNA synthetase. In particular, resistance was conferred by mutations that destabilized the terminator hairpin of the tyrS riboswitch, as well as by a mutation that transformed a tRNA(Phe) into a tyrS riboswitch ligand. The most potent mutation, a substitution near the tyrosine recognition site of tyrosyl-tRNA synthetase, improved enzyme stereoselectivity. We conclude that these mutations promote the proper charging of tRNA(Tyr), thus facilitating the exclusion of D-tyrosine from protein biosynthesis in cells that lack D-aminoacyl-tRNA deacylase. IMPORTANCE: Proteins are composed of L-amino acids. Mischarging of tRNAs with D-amino acids or the misincorporation of D-amino acids into proteins causes toxicity. This work reports on mutations that confer resistance to D-amino acids and their mechanisms of action.

Estimation of diffusion anisotropy in microporous crystalline materials and optimization of crystal orientation in membranes.

The complex nature of the porous networks in microporous materials is primarily responsible for a high degree of intracrystalline diffusion anisotropy. Although this is a well-understood phenomenon, little attention has been paid in the literature with regards to classifying such anisotropy and elucidating its effect on the performance of membrane-based separation systems. In this paper, we develop a novel methodology to estimate full diffusion tensors based on the detailed description of the porous network geometry through our recent advances for the characterization of such networks. The proposed approach explicitly accounts for the tortuosity and complex connectivity of the porous framework, as well as for the variety of diffusion regimes that may be experienced by a guest molecule while it travels through the different localities of the crystal. Results on the diffusion of light gases in silicalite demonstrate good agreement with results from experiments and other computational techniques that have been reported in the literature. A comprehensive computational study involving 183 zeolite frameworks classifies these structures in terms of a number of anisotropy metrics. Finally, we utilize the computed diffusion tensors in a membrane optimization model that determines optimal crystal orientations. Application of the model in the context of separating carbon dioxide from nitrogen demonstrates that optimizing crystal orientation can offer significant benefit to membrane-based separation processes.

Cost-effective CO2 capture based on in silico screening of zeolites and process optimization.

A hierarchical computational approach is introduced that combines materials screening with process optimization. This approach leads to novel materials for cost-effective CO2 capture. Zeolites are screened using shape, size, and adsorption selectivities. Next, process optimization is introduced to generate a rank-ordered list based on total cost of capture and compression. We not only select the most cost-effective materials, but we also attain the optimal process conditions while satisfying purity, recovery, and other process constraints. The top ten zeolites (AHT, NAB, MVY, ABW, AWO, WEI, VNI, TON, OFF and ITW) can capture and compress CO2 to 150 bar from a mixture of 14% CO2 and 86% N2 at less than $30 per ton of CO2 captured. Several zeolites have moderate selectivities, yet they cost-effectively capture CO2 with 90% purity and 90% recovery using a 4-step adsorption process. Such nonintuitive selection demonstrates the necessity of combining materials-centric and process-centric viewpoints.

Predictive framework for shape-selective separations in three-dimensional zeolites and metal-organic frameworks.

With the growing number of zeolites and metal-organic frameworks (MOFs) available, computational methods are needed to screen databases of structures to identify those most suitable for applications of interest. We have developed novel methods based on mathematical optimization to predict the shape selectivity of zeolites and MOFs in three dimensions by considering the energy costs of transport through possible pathways. Our approach is applied to databases of over 1800 microporous materials including zeolites, MOFs, zeolitic imidazolate frameworks, and hypothetical MOFs. New materials are identified for applications in gas separations (CO2/N2, CO2/CH4, and CO2/H2), air separation (O2/N2), and chemicals (propane/propylene, ethane/ethylene, styrene/ethylbenzene, and xylenes).

Stereochemically consistent reaction mapping and identification of multiple reaction mechanisms through integer linear optimization.

Reaction mappings are of fundamental importance to researchers studying the mechanisms of chemical reactions and analyzing biochemical pathways. We have developed an automated method based on integer linear optimization, ILP, to identify optimal reaction mappings that minimize the number of bond changes. An alternate objective function is also proposed that minimizes the number of bond order changes. In contrast to previous approaches, our method produces mappings that respect stereochemistry. We also show how to locate multiple reaction mappings efficiently and determine which of those mappings correspond to distinct reaction mechanisms by automatically detecting molecular symmetries. We demonstrate our techniques through a number of computational studies on the GRI-Mech, KEGG LIGAND, and BioPath databases. The computational studies indicate that 99% of the 8078 reactions tested can be addressed within 1 CPU hour. The proposed framework has been incorporated into the Web tool DREAM ( http://selene.princeton.edu/dream/ ), which is freely available to the scientific community.

L-DOPA ropes in tRNA(Phe).

L-DOPA is the most commonly prescribed drug for the treatment of Parkinson's disease. Here, Moor et al. (2011) report that phenylalanyl-tRNA synthetase catalyzes the misacylation of tRNA(Phe) by L-DOPA, suggesting that it may contribute to the elevated levels of L-DOPA-containing proteins found in patients treated with this drug.

Computational characterization of zeolite porous networks: an automated approach.

An automated method has been developed to fully characterize the three-dimensional structure of zeolite porous networks. The proposed optimization-based approach starts with the crystallographic coordinates of a structure and identifies all portals, channels, and cages in a unit cell, as well as their connectivity. We apply our algorithms to known zeolites, hypothetical zeolites, and zeolite-like structures and use the characterizations to calculate important quantities such as pore size distribution, accessible volume, surface area, and largest cavity and pore limiting diameters. We aggregate this data over many framework types to gain insights about zeolite selectivity. Finally, we develop a continuous-time Markov chain model to estimate the probability of occupancy of adsorption sites throughout the porous network. ZEOMICS, an online database of structure characterizations and web tool for the automated approach is freely available to the scientific community (http://helios.princeton.edu/zeomics/).

Dominant Intermediate Charcot-Marie-Tooth disorder is not due to a catalytic defect in tyrosyl-tRNA synthetase.

Charcot-Marie-Tooth disorder (CMT) is the most common inherited peripheral neuropathy, afflicting 1 in every 2500 Americans. One form of this disease, Dominant Intermediate Charcot-Marie-Tooth disorder type C (DI-CMTC), is due to mutation of the gene encoding the cytoplasmic tyrosyl-tRNA synthetase (TyrRS). Three different TyrRS variants have been found to give rise to DI-CMTC: replacing glycine at position 41 by arginine (G41R), replacing glutamic acid at position 196 by lysine (E196K), and deleting amino acids 153-156 (Δ(153-156)). To test the hypothesis that DI-CMTC is due to a defect in the ability of tyrosyl-tRNA synthetase to catalyze the aminoacylation of tRNA(Tyr), we have expressed each of these variants as recombinant proteins and used single turnover kinetics to characterize their abilities to catalyze the activation of tyrosine and its subsequent transfer to the 3' end of tRNA(Tyr). Two of the variants, G41R and Δ(153-156), display a substantial decrease in their ability to bind tyrosine (>100-fold). In contrast, the E196K substitution does not significantly affect the kinetics for formation of the tyrosyl-adenylate intermediate and actually increases the rate at which the tyrosyl moiety is transferred to tRNA(Tyr). The observation that the E196K substitution does not decrease the rate of catalysis indicates that DI-CMTC is not due to a catalytic defect in tyrosyl-tRNA synthetase.