Discovery and characterization of l-DOPA 2,3-dioxygenase from Streptomyces hygroscopicus jingganensis

The largest natural reservoir of untapped carbon can be found in the cell-wall strengthening, plant woody-tissue polymer, lignin - a polymer of catechols or 1,2-dihydroxybenzene monomers. The catecholic carbon of lignin could be valorized into feedstocks and natural products by way of catabolic and biosynthetic transformations, including the oxygen-dependent cleavage reaction of extradiol dioxygenase (EDX) enzymes. The EDX l-DOPA 2,3-dioxygenase was first discovered as part of a biosynthetic gene cluster to the natural product antibiotic, lincomycin, and also contributes to the biosyntheses of anthramycin, sibiromycin, tomaymycin, porothramycin and hormaomycin. Using these l-DOPA 2,3-dioxygenases as a starting point, we searched sequence space in order to identify new sources of dioxygenase driven natural product diversity. A "vicinal-oxygen-chelate (VOC) family protein" from Streptomyces hygroscopicus jingganensis was identified using bioinformatic methods and biochemically investigated for dioxygenase activity against a suite of natural and synthetic catechols. Steady-state oxygen consumption assays were used to screen and identify substrates, and a steady-state kinetic model of oxygen consumption was developed to evaluate activity of the S. hygroscopicus jingganensis VOC-family-protein with respect to activity of l-DOPA 2,3-dioxygenases from Streptomyces lincolnensis and Streptomyces sclerotialus. Lastly, these data were integrated with steady-state kinetic methods to observe the formation of the EDX cleavage product with UV-visible spectroscopy. The genomic context and enzymatic activity of the S. hygroscopicus jingganensis VOC family protein are consistent with a l-DOPA 2,3-dioxygenase contained within a cryptic biosynthetic pathway.

The pivotal role of neuronal nitric oxide synthase in the release of 6-nitrodopamine from mouse isolated vas deferens

6-Nitrodopamine (6-ND) is released from rat and human vas deferens and is considered a major mediator of both tissues contractility. The contractions induced by 6-ND are selectively blocked by both tricyclic antidepressants and α1-adrenoceptor antagonists. Endothelial nitric oxide synthase (eNOS) is the major isoform responsible for 6-ND release in mouse isolated heart, however the origin of 6-ND in the vas deferens is unknown. Here it was investigated by LC-MS/MS the basal release of 6-ND from isolated vas deferens obtained from control, eNOS-/-, nNOS-/-, and iNOS-/- mice. In addition, it was evaluated in vitro vas deferens contractility following electric field stimulation (EFS). Basal release of 6-ND was significantly reduced in nNOS-/- mice compared to control mice, but not decreased when the vas deferens were obtained from either eNOS-/- or iNOS-/- mice. Pre-incubation of the vas deferens with tetrodotoxin (1 μM) significantly reduced the basal release of 6-ND from control, eNOS-/-, and iNOS-/- mice but had no effect on the basal release of 6-ND from nNOS-/- mice. EFS-induced frequency-dependent contractions of the vas deferens, which were significantly reduced when the tissues obtained from control, eNOS-/- and iNOS-/- mice, were pre-incubated with l-NAME, but unaltered when the vas deferens was obtained from nNOS-/- mice. In addition, the EFS-induced contractions were significantly smaller when the vas deferens were obtained from nNOS-/- mice. The results clearly demonstrate that nNOS is the main NO isoform responsible for 6-ND release in mouse vas deferens and reinforces the concept of 6-ND as a major modulator of vas deferens contractility.

Measurement of 6-cyanodopamine, 6-nitrodopa, 6-nitrodopamine and 6-nitroadrenaline by LC-MS/MS in Krebs-Henseleit solution. Assessment of basal release from rabbit isolated right atrium and ventricles

This study presents the validation of a sensitive method for the determination of 6-nitrodopa, 6-nitrodopamine, 6-nitroadrenaline and 6-cyanodopamine in Krebs-Henseleit solution by LC-MS/MS with ESI+ . HRMS was used to precisely characterize the structures of the fragment ions. The method was applied to investigate the catecholamine basal release from rabbit isolated atria and ventricles. The atria and ventricles were suspended separately in a 5 ml organ bath containing Krebs-Henseleit solution with ascorbic acid (3 mM), gassed (95%O2 /5%CO2 ) at 37°C for 30 min. Strata-X 33 μm SPE cartridges were employed for the extraction of the catecholamines and the internal standard 6-nitrodopamine-d4 . The catecholamines were separated employing a 150 × 3 mm Shim-pack GIST C18-AQ (3 mm particle size), placed in an oven at 40°C and perfused by 65% of mobile phase A (MeCN/H2 O; 90/10, v/v) + 0.4% CH3 COOH and 35% mobile phase B (deionized H2 O) + 0.2% CH2 O2 at 320 μl/min in isocratic mode. The method was linear at 0.1-20 ng/ml. The method was used to identify for the first-time basal release of the three nitrocatecholamines mentioned above and a member of a novel class of catecholamines, the cyanocatecholamines.

Synthesis and analysis of novel catecholic ligands as inhibitors of catechol-O-methyltransferase

L-DOPA, a dopamine precursor, is commonly used as a treatment for patients with conditions such as Parkinson's disease. This therapeutic L-DOPA, as well as the dopamine derived from L-DOPA, can be deactivated via metabolism by catechol-O-methyltransferase (COMT). Targeted inhibition of COMT prolongs the effectiveness of L-DOPA and dopamine, resulting in a net increase in pharmacological efficiency of the treatment strategy. Following the completion of a previous ab initio computational analysis of 6-substituted dopamine derivatives, several novel catecholic ligands with a previously unexplored neutral tail functionality were synthesized in good yields and their structures were confirmed. The ability of the catecholic nitriles and 6-substituted dopamine analogues to inhibit COMT was tested. The nitrile derivatives inhibited COMT most effectively, in agreement with our previous computational work. pKa values were used to further examine the factors involved with the inhibition and molecular docking studies were performed to support the ab initio and experimental work. The nitrile derivatives with a nitro substituent show the most promise as inhibitors, confirming that both the neutral tail and the electron withdrawing group are essential on this class of inhibitors.

L-DOPA Dioxygenase Activity on 6-Substituted Dopamine Analogues

Dioxygenase enzymes are essential protein catalysts for the breakdown of catecholic rings, structural components of plant woody tissue. This powerful chemistry is used in nature to make antibiotics and other bioactive materials or degrade plant material, but we have a limited understanding of the breadth and depth of substrate space for these potent catalysts. Here we report steady-state and pre-steady-state kinetic analysis of dopamine derivatives substituted at the 6-position as substrates of L-DOPA dioxygenase, and an analysis of that activity as a function of the electron-withdrawing nature of the substituent. Steady-state and pre-steady-state kinetic data demonstrate the dopamines are impaired in binding and catalysis with respect to the cosubstrate molecular oxygen, which likely afforded spectroscopic observation of an early reaction intermediate, the semiquinone of dopamine. The reaction pathway of dopamine in the pre-steady state is consistent with a nonproductive mode of binding of oxygen at the active site. Despite these limitations, L-DOPA dioxygenase is capable of binding all of the dopamine derivatives and catalyzing multiple turnovers of ring cleavage for dopamine, 6-bromodopamine, 6-carboxydopamine, and 6-cyanodopamine. 6-Nitrodopamine was a single-turnover substrate. The variety of substrates accepted by the enzyme is consistent with an interplay of factors, including the capacity of the active site to bind large, negatively charged groups at the 6-position and the overall oxidizability of each catecholamine, and is indicative of the utility of extradiol cleavage in semisynthetic and bioremediation applications.

Design, Modeling and Synthesis of 1,2,3-Triazole-Linked Nucleoside-Amino Acid Conjugates as Potential Antibacterial Agents

Copper-catalyzed azide-alkyne cycloadditions (CuAAC or click chemistry) are convenient methods to easily couple various pharmacophores or bioactive molecules. A new series of 1,2,3-triazole-linked nucleoside-amino acid conjugates have been designed and synthesized in 57–76% yields using CuAAC. The azido group was introduced on the 5′-position of uridine or the acyclic analogue using the tosyl-azide exchange method and alkylated serine or proparylglycine was the alkyne. Modeling studies of the conjugates in the active site of LpxC indicate they have promise as antibacterial agents.

Steric restrictions of RISC in RNA interference identified with size-expanded RNA nucleobases

Understanding the interactions between small interfering RNAs (siRNAs) and the RNA-induced silencing complex (RISC), the key protein complex of RNA interference (RNAi), is of great importance to the development of siRNAs with improved biological and potentially therapeutic function. Although various chemically modified siRNAs have been reported, relatively few studies with modified nucleobases exist. Here we describe the synthesis and hybridization properties of siRNAs bearing size-expanded RNA (xRNA) nucleobases and their use as a novel and systematic set of steric probes in RNAi. xRNA nucleobases are expanded by 2.4 Å using benzo-homologation and retain canonical Watson-Crick base-pairing groups. Our data show that the modified siRNA duplexes display small changes in melting temperature (+1.4 to -5.0 °C); substitutions near the center are somewhat destabilizing to the RNA duplex, while substitutions near the ends are stabilizing. RNAi studies in a dual-reporter luciferase assay in HeLa cells revealed that xRNA nucleobases in the antisense strand reduce activity at some central positions near the seed region but are generally well tolerated near the ends. Most importantly, we observed that xRNA substitutions near the 3'-end increased activity over that of wild-type siRNAs. The data are analyzed in terms of site-dependent steric effects in RISC. Circular dichroism experiments show that single xRNA substitutions do not significantly distort the native A-form helical structure of the siRNA duplex, and serum stability studies demonstrated that xRNA substitutions protect siRNAs against nuclease degradation.

Encoding phenotype in bacteria with an alternative genetic set

An unnatural base-pair architecture with base pairs 2.4 Å larger than the natural DNA-based genetic system (xDNA) is evaluated for its ability to function like DNA, encoding amino acids in the context of living cells. xDNA bases are structurally analogous to natural bases but homologated by the width of a benzene ring, increasing their sizes and resulting in a duplex that is wider than native B-DNA. Plasmids encoding green fluorescent protein were constructed to contain single and multiple xDNA bases (as many as eight) in both strands and were transformed into Escherichia coli. Although they yielded fewer colonies than the natural control plasmid, in all cases in which a modified plasmid (containing one, two, three, or four consecutive size-expanded base pairs) was used, the correct codon bases were substituted, yielding green colonies. All four xDNA bases (xA, xC, xG, and xT) were found to encode the correct partners in the replicated plasmid DNA, both alone and in longer segments of xDNA. Controls with mutant cell lines having repair functions deleted were found to express the gene correctly, ruling out repair of xDNA and confirming polymerase reading of the unnatural bases. Preliminary experiments with polymerase deletion mutants suggested combined roles of replicative and lesion-bypass polymerases in inserting correct bases opposite xDNA bases and in bypassing the xDNA segments. These experiments demonstrate a biologically functioning synthetic genetic set with larger-than-natural architecture.

Tyrosine-based 1-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine and -adenine ((S)-HPMPC and (S)-HPMPA) prodrugs: synthesis, stability, antiviral activity, and in vivo transport studies

Eight novel single amino acid (6-11) and dipeptide (12, 13) tyrosine P-O esters of cyclic cidofovir ((S)-cHPMPC, 4) and its cyclic adenine analogue ((S)-cHPMPA, 3) were synthesized and evaluated as prodrugs. In vitro IC(50) values for the prodrugs (<0.1-50 μM) vs vaccinia, cowpox, human cytomegalovirus, and herpes simplex type 1 virus were compared to those for the parent drugs ((S)-HPMPC, 2; (S)-HPMPA, 1; IC(50) 0.3-35 μM); there was no cytoxicity with KB or HFF cells at ≤100 μM. The prodrugs exhibited a wide range of half-lives in rat intestinal homogenate at pH 6.5 (<30-1732 min) with differences of 3-10× between phostonate diastereomers. The tyrosine alkylamide derivatives of 3 and 4 were the most stable. (l)-Tyr-NH-i-Bu cHPMPA (11) was converted in rat or mouse plasma solely to two active metabolites and had significantly enhanced oral bioavailability vs parent drug 1 in a mouse model (39% vs <5%).

Serine side chain-linked peptidomimetic conjugates of cyclic HPMPC and HPMPA: synthesis and interaction with hPEPT1

Cidofovir (HPMPC), a broad spectrum antiviral agent, cannot be administered orally due to ionization of its phosphonic acid group at physiological pH. One prodrug approach involves conversion to the cyclic form (cHPMPC, 1) and esterification by the side chain hydroxyl group of a peptidomimetic serine. Transport studies in a rat model have shown enhanced levels of total cidofovir species in the plasma after oral dosing with L-Val-L-Ser-OMe cHPMPC, 2a. To explore the possibility that 2a and its three L/D stereoisomers 2b-d undergo active transport mediated by the peptide-specific intestinal transporter PEPT1, we performed radiotracer uptake and electrophysiology experiments applying the two-electrode voltage clamp technique in Xenopus laevis oocytes overexpressing human PEPT1 (hPEPT1, SLC15A1). 2a-d did not induce inward currents, indicating that they are not transported, but the stereoisomers with an L-configuration at the N-terminal valine (2a and 2b) potently inhibited transport of the hPEPT1 substrate glycylsarcosine (Gly-Sar). A "reversed" dipeptide conjugate, L-Ser-L-Ala-OiPr cHPMPC (4), also did not exhibit detectable transport, but completely abolished the Gly-Sar signal, suggesting that affinity of the transporter for these prodrugs is not impaired by a proximate linkage to the drug in the N-terminal amino acid of the dipeptide. Single amino acid conjugates of cHPMPC (3a and 3b) or cHPMPA (5, 6a and 6b) were not transported and only weakly inhibited Gly-Sar transport. The known hPEPT1 prodrug substrate valacyclovir (7) and its L-Val-L-Val dipeptide analogue (8) were used to verify coupled transport by the oocyte model. The results indicate that the previously observed enhanced oral bioavailability of 2a relative to the parent drug is unlikely to be due to active transport by hPEPT1. Syntheses of the novel compounds 2b-d and 3-6 are described, including a convenient solid-phase method to prepare 5, 6a and 6b.

Modifications to the dNTP triphosphate moiety: from mechanistic probes for DNA polymerases to antiviral and anti-cancer drug design

Abnormal replication of DNA is associated with many important human diseases, most notably viral infections and neoplasms. Existing approaches to chemotherapeutics for diseases associated with dysfunctional DNA replication classically involve nucleoside analogues that inhibit polymerase activity due to modification in the nucleobase and/or ribose moieties. These compounds must undergo multiple phosphorylation steps in vivo, converting them into triphosphosphates, in order to inhibit their targeted DNA polymerase. Nucleotide monophosphonates enable bypassing the initial phosphorylation step at the cost of decreased bioavailability. Relatively little attention has been paid to higher nucleotides (corresponding to the natural di- and triphosphate DNA polymerase substrates) as drug platforms due to their expected poor deliverability. However, a better understanding of DNA polymerase mechanism and fidelity dependence on the triphosphate moiety is beginning to emerge, aided by systematic incorporation into this group of substituted methylenebisphosphonate probes. Meanwhile, other bridging, as well as non-bridging, modifications have revealed intriguing possibilities for new drug design. We briefly survey some of this recent work, and argue that the potential of nucleotide-based drugs, and intriguing preliminary progress in this area, warrant acceptance of the challenges that they present with respect to bioavailability and metabolic stability.

Puromycin-sensitive aminopeptidase: an antiviral prodrug activating enzyme

Cidofovir (HPMPC) is a broad-spectrum antiviral agent, currently used to treat AIDS-related human cytomegalovirus retinitis. Cidofovir has recognized therapeutic potential for orthopox virus infections, although its use is hampered by its inherent low oral bioavailability. Val-Ser-cyclic HPMPC (Val-Ser-cHPMPC) is a promising peptide prodrug which has previously been shown by us to improve the permeability and bioavailability of the parent compound in rodent models (Eriksson et al., 2008. Molecular Pharmaceutics 5, 598-609). Puromycin-sensitive aminopeptidase was partially purified from Caco-2 cell homogenates and identified as a prodrug activating enzyme for Val-Ser-cHPMPC. The prodrug activation process initially involves an enzymatic step where the l-Valine residue is removed by puromycin-sensitive aminopeptidase, a step that is bestatin-sensitive. Subsequent chemical hydrolysis results in the generation of cHPMPC. A recombinant puromycin-sensitive aminopeptidase was generated and its substrate specificity investigated. The k(cat) for Val-pNA was significantly lower than that for Ala-pNA, suggesting that some amino acids are preferred over others. Furthermore, the three-fold higher k(cat) for Val-Ser-cHPMPC as compared to Val-pNA suggests that the leaving group may play an important role in determining hydrolytic activity. In addition to its ability to hydrolyze a variety of substrates, these observations strongly suggest that puromycin-sensitive aminopeptidase is an important enzyme for activating Val-Ser-cHPMPC in vivo. Taken together, our data suggest that puromycin-sensitive aminopeptidase makes an attractive target for future prodrug design.

Prodrug approaches to improving the oral absorption of antiviral nucleotide analogues

Nucleotide analogues have been well accepted as therapeutic agents active against a number of viruses. However, their use as antiviral agents is limited by the need for phosphorylation by endogenous enzymes, and if the analogue is orally administered, by low bioavailability due to the presence of an ionizable diacid group. To circumvent these limitations, a number of prodrug approaches have been proposed. The ideal prodrug achieves delivery of a parent drug by attachment of a non-toxic moiety that is stable during transport and delivery, but is readily cleaved to release the parent drug once at the target. Here, a brief overview of several promising prodrug strategies currently under development is given.

Serine peptide phosphoester prodrugs of cyclic cidofovir: synthesis, transport, and antiviral activity

Cidofovir (HPMPC, 1), a broad-spectrum antiviral agent, is currently used to treat AIDS-related human cytomegalovirus (HCMV) retinitis and has recognized therapeutic potential for orthopox virus infections, but is limited by its low oral bioavailability. Cyclic cidofovir (2) displays decreased nephrotoxicity compared to 1, while also exhibiting potent antiviral activity. Here we describe in detail the synthesis and evaluation as prodrugs of four cHPMPC dipeptide conjugates in which the free POH of 2 is esterified by the Ser side chain alcohol group of an X-L-Ser(OMe) dipeptide: 3 (X=L-Ala), 4 (X=L-Val), 5 (X=L-Leu), and 6 (X=L-Phe). Perfusion studies in the rat establish that the mesenteric permeability to 4 is more than 20-fold greater than to 1, and the bioavailability of 4 is increased 6-fold relative to 1 in an in vivo murine model. In gastrointestinal and liver homogenates, the cHPMPC prodrugs are rapidly hydrolyzed to 2. Prodrugs 3, 4, and 5 are nontoxic at 100 microM in HFF and KB cells and in cell-based plaque reduction assays had IC 50 values of 0.1-0.5 microM for HCMV and 10 microM for two orthopox viruses (vaccinia and cowpox). The enhanced transport properties of 3-6, conferred by incorporation of a biologically benign dipeptide moiety, and the facile cleavage of the Ser-O-P linkage suggest that these prodrugs represent a promising new approach to enhancing the bioavailability of 2.

Assess your quality of care quotient

The upsurge of large, single/multi-specialty group practices in contemporary health care has precipitated a corresponding surge in marketing strategies aimed at promoting group speed, efficiency, and/or productivity. Subsequently, the quality of care is often overlooked or redefined in financial terms. However, assessing quality of care--above and beyond strictly economic factors--can provide crucial information for executives concerned with making informed managerial decisions. This article explores one prototypical physician group's quest to assess their "quality care quotient," and explicates the process by which they obtained important data from patients, as well as how they used the assessment to improve their practice. The process may serve as an exemplar to physician groups concerned with conducting such analyses.

Communicating with staff nurses: support or hostility?

Communication researchers specializing in social support determine factors that influence and make up supportive encounters in the hospital environment. Effective communication strategies provide a vital dyad's link between nurse managers and staff nurses. Thus, supportive relationships are formed that can ease stressful situations and encourage innovative ideas.

The use of children's drawings in the evaluation and treatment of child sexual, emotional, and physical abuse

Primary care physicians can be instrumental in the initial identification of potential sexual, emotional, and physical abuse of children. We reviewed the use of children's artwork as a method of communicating individual and family functioning. A quantitative method of analyzing children's artwork provides more reliability and validity than some methods used previously. A new scoring system was developed that uses individual human figure drawings and kinetic family drawings. This scoring system was based on research with 842 children (341 positively identified as sexually molested, 252 positively not sexually molested but having emotional or behavioral problems, and 249 "normal" public school children). This system is more comprehensive than previous systems of assessment of potential abuse.

The stress of child sexual abuse examinations

The objective of this paper was to determine if the medical evaluation of sexual abuse victims is stressful by heart rate variability monitoring. The design of the study was a case series of children referred for sexual abuse examination to determine child stress response monitored by heart rate variability during baseline, disclosure, and anogenital examination with photographs. The setting was a referral center for the investigation of child sexual abuse in two sites (Reno and Las Vegas, Nevada, USA). Patients included a consecutive sample of 30 children; 15 in Reno ranging in age from 3-10 years with a mean age of 7.1 (1 male and 14 females); 15 in Las Vegas ranging in age from 6-10 years with a mean age of 7.8 (3 males and 12 females). Measurements and results were: each subject's heart rate was obtained during the first 3 minutes of the baseline, disclosure, and the anogenital examination with photographs, using a telemetric Vantage Performance Heart Watch consisting of a 5.5 x 1.25 inch transmitter attached to a chest strap and a microcomputer wristwatch attached to the subject's non-dominant wrist. Heart rates were taken every 5 or 15 seconds throughout the entire examination. A follow-up home visit was made 6 weeks after the examination to determine the replicability of clinic baseline measures and child behavioral sequelae. Physiologic measurements did not show statistically significant stress responses when children were prepared according to the clinic protocol in Reno and Las Vegas. There was a trend toward more responsivity among some of the older subjects during the anogenital examination with photographs, and when clinic procedures inadvertently overlapped several procedures at the same time (anogenital examination, photographs and disclosure). The conclusion of the study was that child sexual abuse investigations may be accomplished without undue psychophysiological stress when recommended precautions are taken.

Maternal sexual abuse of male children. When to suspect and how to uncover it

Physicians need to be aware that women and mothers are capable of sexually abusing children and need to watch for signs that a mother may be at risk for perpetrating abuse or that a child may have been abused. Posttraumatic stress disorder, attention-deficit hyperactivity disorder, secondary enuresis and encopresis, nightmares, and inappropriate sexual behavior are often symptoms of sexual abuse. It is important for physicians who are attempting to elicit a report of abuse from a child to keep in mind that (1) children express themselves most easily by nonverbal means, (2) the waiting room, school, and home are settings where information might be retrieved, and (3) the child's statements need to be evaluated using established criteria for analyzing their content. Physicians also need to remember that there is often an absence of physical findings in boys who have been sexually abused, although physical abuse and sexual abuse present simultaneously in some cases.

Post-traumatic stress disorder in a child following an automobile accident

A 3-year-old girl was diagnosed with post-traumatic stress disorder (PTSD) following a minor automobile accident. The child presented with nightmares, violent play, and trauma-specific fears. The common symptoms of PTSD in childhood are reviewed, and the importance of careful history-taking and prompt referral to a mental health professional are discussed.

Using family life drawings in medical student education

A life drawing technique was used to educate third-year medical students in a diagnostic and therapeutic approach to young children. Students were asked to produce their own early family life drawings. A sample of 48 drawings were collected. A major goal was to sensitize students as to how their early family experiences likely shaped their current perceptions regarding children and families in the practice of medicine. Six of these student drawings were chosen to illustrate this goal. A few students even found that their drawings helped them resolve personal early life stresses.

Loss of Ribulose 1,5-Diphosphate Carboxylase and Increase in Proteolytic Activity during Senescence of Detached Primary Barley Leaves

Symptoms typical of senescence occurred in green detached primary barley (Hordeum vulgare L.) leaves placed in darkness and in light. Chlorophyll, total soluble protein, ribulose 1,5-diphosphate carboxylase protein and activity each progressively decreased in darkness and to a lesser extent in light. In all treatments most of the total soluble protein lost was accounted for by a decrease in ribulose 1,5-diphosphate carboxylase protein, suggesting that the chloroplast was a major site of degradation early in senescence.Loss of ribulose 1,5-diphosphate carboxylase protein was negatively correlated with an increase in proteolytic activity measured against azocasein. Both rates were exponential, with about a 30% difference in apparent rate constants. Cycloheximide essentially prevented the loss of chlorophyll, ribulose 1,5-diphosphate carboxylase protein, and activity and completely inhibited the increase in proteolytic activity against azocasein. Since chloramphenicol had little effect on the loss of ribulose 1,5-diphosphate carboxylase protein or chlorophyll, or on proteolytic activity against azocasein, it is suggested that the proteolytic activity was developed on cytoplasmic 80 S ribosomes.Kinetin greatly retarded the onset of such symptoms of senescence by inhibiting the losses of chlorophyll and ribulose 1,5-diphosphate carboxylase protein and protected against inactivation of enzymic activity. It also prevented the increase in proteolytic activity measured against azocasein. Incorporation of labeled amino acids into ribulose 1,5-diphosphate carboxylase during its rapid degradation showed that the enzyme was under turnover. The changes in ribulose 1,5-diphosphate carboxylase protein and activity, chlorophyll, soluble protein other than ribulose 1,5-diphosphate carboxylase, proteolytic and esterolytic activity during senescence indicate that senescence is a selective, sequential process.

Evidence for lack of turnover of ribulose 1,5-diphosphate carboxylase in barley leaves

Turnover of ribulose 1,5-diphosphate carboxylase in barley leaves (Hordeum vulgare L.) was followed over time in light and dark. The enzyme was degraded in prolonged darkness and was resynthesized after the plants were returned to light. Labeling with (14)C showed that simultaneous synthesis and degradation (turnover) did not occur in light. In contrast, the remaining soluble protein was turned over rapidly in light. Although ribulose 1,5-diP carboxylase can be both degraded and synthesized, these processes seem not to occur simultaneously, but can be induced independently by changing environmental conditions.