Vitreous body glutamate concentration in dogs with glaucoma

OBJECTIVE

To analyze the vitreal amino acid concentrations in dogs with breed-related primary glaucoma to determine whether excitotoxic amino acids associated with retinal genglion cell death in other species were present in affected dogs.

SAMPLES

11 normal control and 10 glaucomatous canine eyes.

PROCEDURE

Amino acid analyses were performed by high-pressure liquid chromatography in masked manner.

RESULTS

Eyes from dogs with primary glaucoma had significantly high vitreal glutamate concentration, compared with values for eyes of clinically normal control dogs. Mean (+/-SD) glutamate concentrations were 31.7 +/- 12.4 and 6.9 +/- 6.3 microM in glaucomatous and normal eyes, respectively (P < 0.0001). Eyes from dogs with glaucoma also had lower vitreal glycine (37.0 +/- 17.0 vs 59.4 +/- 28.2 microM; P < 0.043) and higher of vitreal tryptophan (39.0 +/- 22.8 vs 17.5 +/- 11.2 microM; P < 0.012) concentrations, compared with values for normal eyes.

CONCLUSION

Glutamate concentration potentially toxic to retinal ganglion cells is associated with the pathogenesis of primary glaucoma in dogs. Increased glutamate concentration provides evidence of an ischemic mechanism for retinal ganglion cell death and optic nerve atrophy in dogs with glaucoma.

CLINICAL RELEVANCE

The emphasis on reduction and normalization of high intraocular pressure as the primary focus of treatment for glaucoma in dogs should be augmented by other therapeutic approaches.

Cysteine 265 is in the active site of, but is not essential for catalysis by tRNA-guanine transglycosylase (TGT) from Escherichia coli

Site-directed mutagenesis and X-ray absorption spectroscopy studies have previously shown that the tRNA-guanine transglycosylase (TGT) from Escherichia coli is a zinc metalloprotein and identified the enzymic ligands to the zinc [Chong et al. (1995), Biochemistry 34, 3694-3701; Garcia et al. (1966), Biochemistry 35, 3133-3139]. During these studies one mutant, TGT (C265A), was found to exhibit a significantly lower specific activity, but was not found to be involved in the zinc site. The present report demonstrates that TGT is inactivated by treatment with thiol reagents (e.g., DTNB, MMTS, and N-ethylmaleimide). Further, this inactivation is shown to be due to modification of cysteine 265. The kinetic parameters for the mutants TGT (C265A) and TGT (C265S), however, suggest that this residue is not performing a critical role in the TGT reaction. We conclude that cysteine 265 is in the active site of TGT, but is not performing a critical catalytic function. This conclusion is supported by the recent determination of the X-ray crystal structure of the TGT from Zymomonas mobilis [Romier et al. (1966), EMBO J. 15, 2850-2857], which reveals that the residue corresponding to cysteine 265 is distant from the putative catalytic site, but is in the middle of a region of the enzyme surface proposed to bind tRNA.

X-ray absorption spectroscopy of the zinc site in tRNA-guanine transglycosylase from Escherichia coli

A key step in the post-transcriptional modification of tRNA with queuine in Escherichia coli is the exchange of the queuine precursor, preQ1 into tRNA. This reaction is catalyzed by tRNA-guanine transglycosylase (TGT). We have previously shown that the E. coli TGT is a zinc metalloprotein [Chong et al. (1995) Biochemistry 34, 3694-3701]. Site-directed mutagenesis studies indicated that cysteines 302, 304, 307 and histidine 317 constitute the four ligands to the zinc. The involvement of histidine 317 is somewhat confounded by the presence of histidine 316. We have examined the zinc site in TGT (wt) and TGT (H317C) by X-ray absorption spectroscopy. The TGT (wt) data are most consistent with a tetracoordinate zinc with one nitrogen and three sulfur ligands. Interestingly, the data for TGT (H317C) are also consistent with a tetracoordinate zinc with one nitrogen and three sulfur ligands. The outer shell imidazole scattering for TGT (H317C) appears to be somewhat more ordered than that for TGT (wt), consistent with our previous suggestion that the wild-type enzyme may exist in two conformations the predominant one involving histidine 317 liganding to the zinc and the minor conformer involving histidine 316 liganding to the zinc. The minor conformer, with histidine 316 coordinating the zinc, appears to have an overall conformation that is subtly different from that of the wild-type enzyme. While TGT (H317C) has kinetic parameters very similar to the wild-type, it does not form the homotrimer quaternary structure of the wild-type. TGT (H317A) has previously [Chong et al. (1995) Biochemistry 34, 3694-3701] been found to contain a significant amount of zinc, but is essentially inactive. This suggests that careful analysis of EXAFS data can reveal subtle conformational changes in metal binding sites that are not observed in more common probes of protein conformation such as CD spectroscopy.

Mechanism-based inactivation of tRNA-guanine transglycosylase from Escherichia coli by 2-amino-5-(fluoromethyl)pyrrolo[2,3-d]pyrimidin-4 (3H)-one

In Escherichia coli, tRNA-guanine transglycosylase (TGT) catalyzes the incorporation of the queuine precursor preQ1 [2-amino-5-(aminomethyl)pyrrolo[2,3-d]pyrimidin-4(3H)-one] into tRNA. This precursor is further elaborated to queuine by two subsequent enzymic reactions [Slany, R. K., & Kersten, H. (1994) Biochimie 76, 1178-1182]. Our previous studies [Hoops, G. C., Townsend, L. B., & Garcia, G. A., (1995) Biochemistry (in press)] on a series of synthetic 5- and 6-substituted 2-aminopyrrolo[2,3-d]pyrimidin-4(3H) -ones have revealed that the E. coli TGT tolerates a wide diversity of substituents (isosteric, or nearly so, to the aminomethyl group of preQ1) at the 5 position. We report here that 2-amino-5-(fluoromethyl)pyrrolo[2,3-d]pyrimidin-4 (3H)-one (FMPP) inactivates TGT in a time- and concentration-dependent manner with k(inact) = 0.074 min-1 and KI = 136 microM. A competitive inhibitor (7-methyl-preQ1), with respect to preQ1, of TGT [Hoops, G.C., Townsend, L.B., & Garcia, G.A. (1995) Biochemistry (in press)] protects the enzyme from inactivation by FMPP. FMPP also acts as a competitive inhibitor (KI = 114 microM) of TGT under initial velocity conditions. The rate of fluoride release from FMPP is slightly faster (0.064 min-1) than the k(inact) (0.053 min-1) at 300 microM FMPP, consistent with fluoride release preceding inactivation. FMPP appears to partition between "normal" turnover (kcat = 0.461 min-1 and Km = 152 microM), inactivation, and an alternative processing to an unidentified, fluoride-released product.(ABSTRACT TRUNCATED AT 250 WORDS)

tRNA-guanine transglycosylase from Escherichia coli: structure-activity studies investigating the role of the aminomethyl substituent of the heterocyclic substrate PreQ1

A series of 5-substituted 2-aminopyrrolo[2,3-d]pyrimidin-4(3H)-ones have been synthesized in order to study the substrate specificity of the tRNA-guanine transglycosylase (TGT) from Escherichia coli. A number of these compounds were initially examined as inhibitors of radiolabeled guanine incorporation into tRNA catalyzed by TGT [Hoops, G. C., Garcia, G. A., & Townsend, L. B. (1992) 204th National Meeting of the American Chemical Society, Washington, DC, August 23-28, 1992, Division of Medicinal Chemistry, Abstract 113]. The kinetic parameters of these analogues as substrates in the TGT reaction have been determined by monitoring the loss of radiolabeled guanine from 8-[14C]G34-tRNA. This study reveals that the tRNA-guanine transglycosylase from E. coli will tolerate a wide variety of substituents at the 5-position. The role of the 5-substituent appears to be entirely in binding/recognition with no apparent effects upon catalysis. A correlation between N7 pKa and Vmax suggests the deprotonation of N7 during the reaction, which must occur prior to subsequent glycosidic bond formation, appears to be partially rate-determining for the natural substrate. Comparison of the Kis of 7-methyl-substituted competitive inhibitors to the Kms of their corresponding substrates suggests that some substrates (including preQ1) are kinetically "sticky" (i.e., Km is equivalent to Kd) and other substrates have Kms that reflect catalytic rates as well as binding.

tRNA-guanine transglycosylase from Escherichia coli. Minimal tRNA structure and sequence requirements for recognition

Previously, we have demonstrated that the tRNA-guanine transglycosylase (TGT) from Escherichia coli is capable of utilizing an in vitro generated minihelix consisting of the anticodon stem and loop sequence of E. coli tRNA(Tyr) (Curnow, A. W., Kung, F. L., Koch, K. A., and Garcia, G. A. (1993) Biochemistry 32, 5239-5246). This suggests that the tRNA structural motifs necessary for recognition comprise a loop at the end of a short helix. To gain further insight into the structural requirements for TGT recognition, we have investigated the conformation of this minimal substrate. Thermal denaturation studies and kinetic analyses at 20 and 37 degrees C indicate that this minihelix is predominantly melted at 37 degrees C and that the melted conformation is not a substrate for TGT. This is confirmed by the determination that a non-helical analogue of the minihelix is not a substrate for TGT at either temperature. Two additional minihelices designed to be stable at 37 degrees C, ECYMH (a 4-base pair extension of the previous minihelix) and SCDMH (a yeast tRNA(Asp) analogue of ECYMH), were generated and characterized. Finally, several sequence mutants of SCDMH, focusing on the G30U40 base pair and U33G34U35 loop sequence, have been produced, and kinetic parameter determinations have been performed at 37 degrees C. Our results are consistent with a recent report (Nakanishi, S., Ueda, T., Hori, H., Yamazaki, N., Okada, N., and Watanabe, K. (1994) J. Biol. Chem. 269, 32221-32225) indicating that a UGU sequence in a 7-base loop is the minimal requirement for TGT recognition.

tRNA-guanine transglycosylase from Escherichia coli is a zinc metalloprotein. Site-directed mutagenesis studies to identify the zinc ligands

tRNA-guanine transglycosylase (TGT) from Escherichia coli catalyzes the exchange of the queuine precursor, preQ1, into tRNA as part of the biosynthetic pathway for the posttranscriptionally modified base, queuine. No significant sequence homologies exist between TGT and any of the proteins in the GenBank database. However, an unusual arrangement of cysteine residues was observed upon manual examination of the TGT sequence. Comparison of this sequence (residues 302-321) revealed similarities to structural zinc-binding motifs in proteins of known structure [Jaffe (1993) Comments Inorg. Chem. 15, 67-93]. Within this region of the TGT sequence, there are six residues (four cysteines and two histidines), any four of which could serve as the ligands to the zinc. We report here that wild-type TGT contains ca. 0.8 mol of zinc/mol of subunit, determined by atomic emission spectrometry. In order to determine which enzyme residues are serving as the ligands to the zinc, site-directed mutagenesis studies have been performed. Gross structural probes (native PAGE and CD spectra), enzyme activity assays, and tRNA-binding assays indicate that cysteines 302, 304, and 307 and histidine 317 are the ligands to the zinc. These results also suggest that the zinc site is necessary for TGT homotrimer formation and for tRNA binding.

Molecular mechanisms of the antihormonal and antiimplantation effects of norethisterone and its A-ring reduced metabolites

Norethisterone (NET) has been used as a contragestational postcoital agent. It is biotransformed to 5 alpha dihydro-NET (5 alpha-NET) and 3 beta,5 alpha tetrahydro-NET (3 beta,5 alpha-NET) in target tissues. The participation of these metabolites in NET effects is unknown. We have examined the antiimplantation and antiprogestational effects of NET and its metabolites, in adult mated female rabbits, by assessing the number of implantation sites and the expression products of the uteroglobin (UTG) gene in the uterus, and by comparing them with those of RU-486 and estradiol. Steroids were daily administered s.c. at several doses for 7 consecutive days, starting 24 hr after coitus. To assure that fertilization occurred in all animals, the presence of early pregnancy factor was determined. The results demonstrated that high doses (5 mg/kg) of NET reduced both implantation and the expression of the UTG gene. On the other hand, lower doses (1.5 mg/kg) of 5 alpha-NET produced an antiimplantation effect and suppressed UTG synthesis and its mRNA. These effects were similar to those of RU-486. At lower doses (1 mg/kg), both estradiol and the estrogenic metabolite 3 beta,5 alpha-NET were also effective in inhibiting implantation and UTG gene expression. The overall results suggest that NET metabolites exert antiimplantation and antiprogestational effects through their interaction with progesterone and estrogen receptors, and provide an explanation for the molecular mechanisms involved in the postcoital contraceptive action of NET.

Norethisterone metabolites modulate the uteroglobin and progesterone receptor gene expression in prepubertal rabbits

Norethisterone (NET) is a synthetic progestin, used as a contraceptive agent, that is biotransformed at target tissues into 5 alpha-NET and 3 beta,5 alpha-NET, which possess different pharmacological properties. The effects of these metabolites on the expression of uteroglobin (UG) and progesterone receptor (PR) genes, both regulated by progesterone (P4), were evaluated in the uterus of prepubertal female rabbits that were simultaneously treated with P4 (1.0 mg) for 5 consecutive days. As determined by Western and Northern blot analyses, 5 alpha-NET inhibited the P4-induced UG gene expression in a dose-dependent manner. A similar inhibition was observed with the administration of RU-486. The estrogenic agent 3 beta,5 alpha-NET and estradiol at a dose of 1.0 mg also inhibited the UG gene expression induced by P4. Both 5 alpha-NET and 3 beta,5 alpha-NET blocked the PR down-regulation induced by P4 as assessed by Western and Northern blot methods. The inhibition of UG synthesis and PR down-regulation by 5 alpha-NET and 3 beta,5 alpha-NET indicates that these NET metabolites possess antiprogestational properties.

A prospective evaluation of thoracoscopy for the diagnosis of penetrating thoracoabdominal trauma

Penetrating thoracoabdominal trauma presents a difficult diagnostic dilemma. Violation of the diaphragm may be very difficult to establish. Conventional diagnostic procedures such as chest radiography, computed tomography, and diagnostic peritoneal lavage have been shown to be unreliable. Mandatory exploratory celiotomy carries a 20%-30% negative rate. Twenty-eight patients with penetrating thoracoabdominal trauma over a 6-month period were prospectively evaluated by thoracoscopy at a major urban trauma center. All patients were hemodynamically stable, had no indications for immediate celiotomy, and demonstrated thoracic injury on chest radiography or physical examination. All thoracoscopy was performed in the operating room under general anesthesia. Patients consisted of 25 males and 3 females with an age range of 15-48 years. Mechanism of injury consisted of 24 stab wounds and 4 gunshot wounds. Twelve of the procedures were for right chest wounds and 16 involved the left hemithorax. Diaphragmatic injury was identified at thoracoscopy in 9 patients (32%), with all confirmed and repaired at celiotomy. Eight of 9 patients (89%) undergoing celiotomy were found to have significant intra-abdominal injuries requiring surgical repair. Thoracoscopy was also useful for evacuation of blood from the pleural space. There were no procedure-related complications. Thoracoscopy is a safe, accurate, reliable diagnostic technique for evaluating thoracoabdominal penetrating trauma. It is less invasive than celiotomy and has the added benefit of diagnosis and therapy of the intrathoracic injuries.

An oligonucleotide-directed, in vitro mutagenesis method using ssDNA and preferential DNA amplification of the mutated strand

The sequential addition of primers in a PCR enables one to preferentially amplify one of the two strands of a heteroduplex DNA template. This serves as the basis for a novel site-directed mutagenesis technique involving a heteroduplex DNA template that has been generated from a single-stranded, wild-type template and one or more mutagenic oligonucleotides. This preferential PCR method yields a mutation efficiency greater than 90% (consistent with the theoretical estimate for the method). The ability to generate multiple mutants also enables the screening of potential mutants by restriction endonuclease digestion.

A versatile and general prokaryotic expression vector, pLACT7

We have previously reported the constitutive over-expression of the tRNA-guanine transglycosylase (TGT) from plasmid pTGT1 in Escherichia coli. To obtain a controllable expression system for TGT, we have subsequently cloned the tgt gene into pET21b. Though the overexpression of TGT is inducible in pET21b, the plasmid has a low copy number, a poor yield of single-stranded DNA and relies on an E. coli strain that produces T7 RNA polymerase for protein expression. We have combined the features of pTZ18U and pET21b and have constructed a versatile plasmid pLACT7 that has a high copy number, a high yield of single-stranded DNA and both the T7 and lac promoters for protein expression in a wide variety of E. coli strains.

Serine 90 is required for enzymic activity by tRNA-guanine transglycosylase from Escherichia coli

An Escherichia coli mutant described by Noguchi et al. [Noguchi, S., et al. (1982) J. Biol. Chem. 275, 6544-6550] contains tRNA lacking the hypermodified wobble nucleoside queuosine (Q) due to an inactive tRNA-guanine transglycosylase (TGT). TGT catalyzes the posttranscriptional base exchange of the Q precursor preQ1 with the genetically encoded guanine in tRNA(Asp,Asn,His,Tyr). The mutant tgt gene was cloned and sequenced; it contained a single point mutation resulting in the change of serine 90 to phenylalanine. Overexpression of the mutant gene yielded TGT(S90F) that showed a reduced solubility and did not purify in the same fashion as the wild-type enzyme. TGT(S90F) has no detectable enzymic activity. To determine whether serine 90 performs a catalytic role in the TGT reaction or whether the loss of activity was caused solely by a conformational change of the enzyme, we used site-specific mutagenesis to construct serine-to-alanine (S90A) and serine-to-cysteine (S90C) mutants. Both S90A and S90C mutants were purified in a manner identical to that used for the wild-type enzyme. SDS-PAGE of dimethyl suberimidate-cross-linked mutants showed a pattern identical to that of the wild-type TGT, indicative of a trimeric quaternary structure. Native PAGE of wild-type and mutant TGTs in the absence and presence of substrate tRNA exhibited band shifts indicating that both mutants retain the ability to bind tRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

tRNA-guanine transglycosylase from Escherichia coli: recognition of dimeric, unmodified tRNA(Tyr)

In order to probe the interaction between tRNA and the tRNA hypermodifying enzyme, tRNA-guanine transglycosylase (TGT) from Escherichia coli, we have undertaken the generation of E coli tRNA(Tyr) and analogues. During efforts to adapt currently available in vitro transcription techniques we encountered difficulties attributable to dimerization of the tRNA products. E coli tRNA(Tyr) has previously been characterized for its ability to form a dimer in solutions of suitable salt concentrations at appropriate temperatures (Yang SK, Söll DG, Crothers DM (1972) Biochemistry 11, 2311-2320; Rordorff BF, Kearns DR (1976) Biochemistry 15, 3320-3330). We have applied similar techniques to our unmodified analogue of E coli tRNA(Tyr) and produced both monomeric and dimeric forms of E coli tRNA(Tyr). In this report we find that the dimer does serve as a substrate for modification by TGT. While both the conformers are equal in terms of Vmax (within experimental error) a 2.5-fold increase in KM occurs when going from monomer to dimer. This suggests that TGT preferentially binds the monomer but once either conformer is bound will catalyze the modification reaction equally well. We have also compared the results for the two conformers to our previous data of an RNA minihelix corresponding to the anticodon arm of E coli tRNA(Tyr). Here we find that our earlier conclusion, that the recognition elements for TGT are localized within the anticodon arm of cognate tRNAs, is supported.

Reaction of modified and unmodified tRNA(Tyr) substrates with tyrosyl-tRNA synthetase (Bacillus stearothermophilus)

Three species of tRNA(Tyr) have been examined as substrates for the transfer reaction of the tyrosyl-tRNA synthetase (TyrRS) from Bacillus stearothermophilus: Escherichia coli tRNA(Tyr), B. stearothermophilus tRNA(Tyr) expressed in E. coli, and B. stearothermophilus tRNA(Tyr) that has been transcribed in vitro. The binding of the first two substrates to TyrRS may be readily monitored by stopped-flow studies of tryptophan fluorescence to give the rate and equilibrium constants. The in vitro-transcribed tRNA(Tyr), which lacks the modified bases queuosine and 2-(methylthio)-N6-isopentenyladenosine in the anticodon loop, does not cause a significant change in tryptophan fluorescence upon binding. The three tRNA(Tyr) substrates exhibit very similar steady-state kinetics in the charging reaction. Pre-steady-state kinetics of the transfer reaction, monitored by stopped-flow measurements of the change in protein fluorescence on the addition of tRNA(Tyr) to the E.Tyr-AMP complex, show two exponential changes for the modified tRNA(Tyr) substrates. The first is that due to substrate binding. The second has an identical rate to the single change observed for the reaction with the in vitro-transcribed tRNA(Tyr) and to that monitored by quenched-flow measurements on the formation of Tyr-tRNA(Tyr). Hence, the transfer reaction can be observed by stopped-flow. The dissociation constants (KtRNA) of tRNA from the enzyme and rates of tyrosine transfer (k4) show that all three tRNA molecules are kinetically equivalent substrates for TyrRS. The value of k4 is also similar to that found for authentic tRNA(Tyr) from B. stearothermophilus.(ABSTRACT TRUNCATED AT 250 WORDS)

tRNA-guanine transglycosylase from Escherichia coli. Overexpression, purification and quaternary structure

tRNA-guanine transglycosylase (TGT) is the enzyme responsible for the posttranscriptional modification of specific tRNAs (Asn, Asp, His and Tyr) with queuine. In E. coli this modification occurs via a two-step reaction: (1) TGT-catalyzed base exchange of guanosine-34 with preQ1 (7-aminomethyl-7-deazaguanine) and (2) addition of a cyclopentenediol moiety to the preQ1-34 tRNA. E. coli TGT is normally expressed at very low levels (approximately 1 mg from 500 g cells). The sequence of the queuine operon of E. coli has recently been reported by Reuter et al. (1991). We have cloned the tgt gene into an overexpressing vector in order to provide a more efficient preparation of TGT. A simple, four-step purification scheme yields 78 mg of homogeneous TGT per liter of cell culture (A600 = 5 to 6). Amino-terminal protein sequencing confirms the identity of the recombinant protein and indicates that the initiator methionine is retained in the mature form. Native-PAGE of TGT and SDS-PAGE of cross-linked TGT are most consistent with a hexameric quaternary structure for the enzyme. The cross-linking data also suggests that the enzyme exists as a dimer of trimers of identical 42.5 kDa subunits (total M(r) = 255 kDa. The enzyme is inactivated by cross-linking with the bisimidoester, dimethylsuberimidate. Substrate (tRNA) protects the enzyme against cross-linking and inactivation by dimethylsuberimidate and against inactivation by modification with ethylacetimidate, a monofunctional, imidoester. This indicates that the enzymic residues (presumably lysines) that are involved in cross-linking and the inactivation are in the active site of the enzyme.

tRNA-guanine transglycosylase from Escherichia coli: gross tRNA structural requirements for recognition

tRNA-guanine transglycosylase (TGT) is the enzyme responsible for the post-transcriptional modification of specific tRNAs (those for Asn, Asp, His, and Tyr) with the hypermodified base, queuine. In Escherichia coli this enzyme catalyzes the exchange of guanine-34 in the anticodon with preQ1, which is subsequently further modified to queuine. There is evidence that such hypermodified tRNA molecules may play a role in the control of cell proliferation and differentiation. In order to perform detailed, in vitro mechanistic studies and to probe the tRNA-enzyme interaction, we have generated unmodified E. coli tRNA(Tyr) and truncated analogues using an in vitro RNA synthesis system suggested by Milligan and Uhlenbeck [Milligan, J. F., & Uhlenbeck, O. C. (1989) Methods Enzymol. 180, 51-62]. From this system we have generated three tRNA analogues totally devoid of any post-transcriptional modifications. In order to compare the unmodified tRNA with the true physiological substrate for TGT, that is, tRNA that contains all modified bases except queuine, we have isolated E. coli tRNA(Tyr) from an overexpressing clone in a TGT-deficient strain of E. coli. We report here that unmodified, full-length tRNA(Tyr) serves as a substrate for TGT with kinetic parameters that are, within experimental error, the same as those for in vivo isolated tRNA(Tyr). This indicates that other post-transcriptional modifications have negligible effects upon TGT recognition of tRNA. A 17-base oligoribonucleotide, corresponding to the anticodon loop and stem, is also a substrate for TGT with only a 20-fold loss in Vmax/KM, versus the full-length tRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Metal ion dependence of phosphorothioate ATP analogues in the Bacillus stearothermophilus tyrosyl-tRNA synthetase reaction

Pre-steady-state kinetic analyses on the formation of tyrosyl adenylate from tyrosine and each of the four diastereomers of alpha- and beta-phosphorothioate adenosine triphosphates [ATP alpha S and ATP beta S; Eckstein, F., & Goody, R. (1976) Biochemistry 15, 1685-1691; Yee, D., Armstrong, V. W., & Eckstein, F. (1979) Biochemistry 18, 4116-4123] were performed in the presence of Mg2+, Co2+, and Cd2+ as the divalent metal ion cofactor. A modest preference of 5.5-fold in kappa 3/KA' (where kappa 3 is the rate constant for tyrosyl adenylate formation and KA' is the dissociation constant for ATP, or phosphorothioate ATP, from the E.Tyr.metal.ATP complex) for the Sp ATP alpha S diastereomer and the absence of an inversion of preference when the metal ion is changed suggest that there is a stereospecific enzyme-alpha-phosphate interaction and that there is no direct metal ion interaction with the alpha-phosphate. The extent of reaction of the ATP alpha S diastereomers (30-50%) implies that these analogues are more susceptible to the hydrolytic site reaction previously reported for this enzyme [Wells, T. N. C., & Fersht, A. R. (1986) Biochemistry 25, 1881-1886]. The strong preference in kappa 3/KA' for the RP ATP beta S diastereomer (16-fold for Mg2+ and 50-fold for Co2+) is indicative of a stereospecific interaction with the pro SP beta oxygen of ATP.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-acting estrogenic responses of estradiol fatty acid esters

Estradiol esters at C-17 and C-3 with palmitic, stearic and oleic acids were chemically synthesized and then evaluated for their long-acting estrogenic responses in ovariectomized rats. The duration of the biological effects was measured after a single subcutaneous dose of 0.1 mumol of each ester and compared with those observed with 17 beta-estradiol, estradiol 3-benzoate and estradiol 17-enanthate. Vaginal citology, uterophyc action, serum gonadotropins inhibition and 17 beta-estradiol levels were measured 0, 5, 10, 20, 30 and 60 days after injection. The results disclosed that most of the estradiol derivatives evaluated exhibited a long-acting estrogenic action. However, the monoesters at C-17 showed longer effects that monoesters at C-3, while the estradiol diesters exhibited the shortest effects. In addition as shown by its low serum levels, all estradiol esters with unsaturated fatty acids show a decreased E2 absorption. The overall results indicated that esterification of E2 with long chain fatty acids provided long-acting properties to it, being higher with C-17 esters. Whether some of these compounds could be employed in substitutive endocrine therapy remains to be established.

Differential reactivity in the processing of [p-(halomethyl)benzoyl] formates by benzoylformate decarboxylase, a thiamin pyrophosphate dependent enzyme

A series of [p-(halomethyl)benzoyl]formates have been investigated as substrates for benzoylformate decarboxylase. These analogues vary from acting as normal substrates to acting as potent competitive inhibitors. The fluoro analogue is a substrate with Km (190 microM) and turnover number (20 s-1) similar to those of benzoylformate (Km = 340 microM; 81 s-1). The bromo analogue is a competitive inhibitor (Ki = 0.3 microM) and exhibits processing to eliminate bromide and form (p-methylbenzoyl)thiamin pyrophosphate. This modified cofactor hydrolyzes to form the p-methylbenzoate in quantitative yield. The chloro analogue [Km(app) = 21 microM] partitions between these two pathways such that 0.6% of the analogue ultimately forms p-methylbenzoate. These data are consistent with the interpretation that the leaving group potential of the halogen determines the enzymic fate of the analogue and that the potent inhibition observed for the bromo analogue is due to covalent modification of the cofactor.

Kinetics and mechanism of benzoylformate decarboxylase using 13C and solvent deuterium isotope effects on benzoylformate and benzoylformate analogues

Benzoylformate decarboxylase (benzoylformate carboxy-lyase, BFD; EC 4.1.1.7) from Pseudomonas putida is a thiamine pyrophosphate (TPP) dependent enzyme which converts benzoylformate to benzaldehyde and carbon dioxide. The kinetics and mechanism of the benzoylformate decarboxylase reaction were studied by solvent deuterium and 13C kinetic isotope effects with benzoylformate and a series of substituted benzoylformates (pCH3O, pCH3, pCl, and mF). The reaction was found to have two partially rate-determining steps: initial tetrahedral adduct formation (D2O sensitive) and decarboxylation (13C sensitive). Solvent deuterium and 13C isotope effects indicate that electron-withdrawing substituents (pCl and mF) reduce the rate dependence upon decarboxylation such that decreased 13(V/K) effects are observed. Conversely, electron-donating substituents increase the rate dependence upon decarboxylation such that a larger 13(V/K) is seen while the D2O effects on V and V/K are not dramatically different from those for benzoylformate. All of the data are consistent with substituent stabilization or destabilization of the carbanionic intermediate (or carbanion-like transition state) formed during decarboxylation. Additional information regarding the mechanism of the enzymic reaction was obtained from pH studies on the reaction of benzoylformate and the binding of competitive inhibitors. These studies suggest that two enzymic bases are required to be in the correct protonation state (one protonated and one unprotonated) for optimal binding of substrate (or inhibitors).

Survival of Listeria monocytogenes in milk during high-temperature, short-time pasteurization

Milk from cows inoculated with Listeria monocytogenes was pooled for 2 to 4 days and then heated at 71.7 to 73.9 degrees C for 16.4 s or at 76.4 to 77.8 degrees C for 15.4 s in a high-temperature, short-time plate heat exchanger pasteurization unit. L. monocytogenes was isolated from milk after heat treatment in six of nine pasteurization trials done at 71.7 to 73.9 degrees C and in none of three trials done at 76.4 to 77.8 degrees C. An average of 1.5 to 9.2 L. monocytogenes cells was seen in each milk polymorphonuclear leukocyte before heat treatment in 11 of 12 pasteurization trials. Noticeable degradation of leukocytes with intracellular listeria was detected in unpasteurized milk after 3 days of storage at 4 degrees C, and by 4 days of storage leukocytes had deteriorated to cellular debris, suggesting that holding unpasteurized milk refrigerated for 4 or more days would eliminate a protective effect leukocytes may provide for increasing heat resistance of L. monocytogenes. Results indicate that under the conditions of this study, L. monocytogenes can survive the minimum high-temperature, short-time treatment (71.7 degrees C, 15 s) required by the U.S. Food and Drug Administration for pasteurizing milk.