Probing the Vibrio harveyi luciferase beta subunit functionality and the intersubunit domain by site-directed mutagenesis

While the critical role of the bacterial luciferase alpha subunit in catalysis has been amply documented, the beta subunit was only known to be involved in thermal stability and substrate binding. Two conserved histidyl residues at position 81 and 82 of the beta subunit of Vibrio harveyi luciferase were each mutated to an alanine, aspartate, or lysine to probe further the beta functionality. These mutations resulted in higher Km values for reduced riboflavin 5'-phosphate, less efficient oxidations of the aldehyde substrate, and decreased light-emitting activities. beta His82 appears to be significantly more critical than beta His81. For the beta His82-mutated luciferases, the maximal light intensities and total light outputs were reduced to 19-4% of that for the wild-type enzyme, and the values of Vmax/Km,flavin were decreased by 2-3 orders of magnitude. The reduced light emission activities for these mutated luciferases can be correlated to lower yields of the flavin 4a-hydroperoxide intermediate, reduced productions of the excited flavin emitter, and/or enhanced quenching of the emitter. The beta subunit and the conserved beta His82 in particular have thus been shown to be critical not only to flavin binding but also to catalytic characteristics of luciferase. The dimeric structure of luciferase is essential to its high catalytic efficiency. To characterize the intersubunit domain, three sets of single/double mutants were constructed, and the additivities of mutational effects were tested to screen for residues that could interact across the subunit interface.(ABSTRACT TRUNCATED AT 250 WORDS)

A volume-dependent apneic threshold during NREM sleep in the dog

We determined the causes of central apnea that commonly follow the hyperpnea resulting from brief airway occlusion during non-rapid-eye-movement (NREM) sleep. Ventilation and end-tidal gases were measured before, during, and after 214 trials of 15-20 s of tracheal occlusion in three dogs during NREM sleep. Airway occlusion was accompanied by progressive increases in inspiratory effort and was followed by transient one- to four-breath hyperapneas, with subsequent central apnea [3-15 times eupneic control expiratory duration (TE)] in 62% of occlusion trials. Significant TE prolongation after hyperventilation did not occur until tidal volume (VT) was three times greater than control; i.e., there was a volume-dependent apneic threshold. Transient electroencephalogram arousal at the end of the occlusion often augmented VT, thereby contributing to the subsequent central apnea; however, arousal was not required for the apnea to occur. Significant transient hypocapnia (up to -12 Torr arterial PCO2) commonly occurred after release of airway occlusion but was not closely correlated with the length of central apnea. During vagal blockade, after release of airway occlusion, significant transient hyperventilation occurred but at VT < 40% greater than control, and TE prolongation was markedly reduced. In summary, after release of airway occlusion in NREM sleep, 1) VT greater than three times eupnea was necessary to cause central apnea, 2) transient arousal at the termination of airway occlusion caused longer apneas by augmenting VT, and 3) transient hypocapnia per se made a significant but minor contribution to the postocclusion central apnea.

Apnoea following normocapnic mechanical ventilation in awake mammals: a demonstration of control system inertia

1. Inhibition of inspiratory muscle activity from volume-related feedback during mechanical ventilation has been shown previously. To determine if this neuromechanical inhibition displays a memory effect, the duration of expiration immediately following cessation of mechanical ventilation was assessed in eight normal subjects. The subjects were passively mechanically ventilated via a nasal mask until the end-tidal CO2 (PET,CO2) was a minimum of 30 mmHg and inspiratory effort was no longer detected, as evidenced by stabilization of mouth pressure and disappearance of surface diaphragm EMG activity. The ventilator output was held constant at a mean tidal volume (VT) of 1.0 l and breath duration of 4.6 s and PET,CO2 was increased 1-1.5 mmHg/min (via increased inspired CO2 fraction, FI,CO2) until inspiratory muscle activity returned. The PET,CO2 at which activation first occurred was defined as the CO2 recruitment threshold (PCO2,RT). The mechanical ventilation protocol was repeated and the PET,CO2 increased 1-1.5 mmHg/min until it was a mean of 1.1 mmHg above spontaneous PET,CO2 and 3.6 mmHg below PCO2,RT. After 4-6 min of mildly hypercapnic mechanical ventilation, the mechanical ventilation was terminated. 2. Following termination of mechanical ventilation, the duration of the subsequent apnoea was 14.6 +/- 2.8 s (mean +/- S.E.M.) or 453 +/- 123% > spontaneous TE and 178 +/- 62% > the TE chosen by the subject during 'assist control' ventilation at VT = 1.0 l. 3. To test the hypothesis that the apnoea following cessation of mechanical ventilation was due to a vagally mediated memory effect, the study was repeated in five double-lung transplant patients with similar PCO2,RT to normal subjects. These pulmonary vagally denervated patients also displayed an apnoea (14.5 +/- 4.0 s) upon cessation of mechanical ventilation (at a PET,CO2 2.0 mmHg > eupnoea and 2.4 mmHg < PCO2,RT), that was 367 +/- 162% > spontaneous TE. 4. We also found significant apnoea in the awake dog immediately following mildly hypercapnic passive mechanical ventilation, and this was similar before and after bilateral vagal blockade (15.7 +/- 1.3 and 19.7 +/- 4.7 s, respectively). 5. We conclude that neuromechanical inhibition of inspiratory muscle activity, produced by passive mechanical ventilation at high VT, exhibits a memory effect reflected in TE prolongation, which persists in the face of substantial increases in chemoreceptor stimuli. This effect is not dependent on vagal feedback from lung receptors. 6. We hypothesize that this persistent apnoea represents an inherent 'inertia', characteristic of the ventilatory control system.(ABSTRACT TRUNCATED AT 400 WORDS)

Sequence and antigenic diversity in two immunodominant regions of the L2 protein of human papillomavirus types 6 and 16

To assess the extent of sequence and antigenic diversity in the minor capsid proteins (L2) of human papillomavirus (HPV) types 6 and 16, 24 clinical samples were obtained, and the regions encoding the immunodominant epitopes 6U3 and 16REx were amplified by polymerase chain reaction, sequenced, cloned into pATH plasmids, and tested for reactivity with human sera. Two of 11 HPV-6 DNAs were identical to the prototype strain in the 6U3 region, while 9 variants had a G to A transition at nt5020, changing a valine residue to isoleucine. Of 16 sera that did not react with the prototype HPV-6L2 fusion protein, 2 reacted with the 6U3-isoleucine variant, and all 8 sera that reacted with the prototype also reacted with the variant. Twelve of 13 HPV-16 DNAs were identical to the prototype strain in the 16REx region, while 1 variant had a C to G transversion at nt4825, changing a proline to an arginine, but not affecting antigenicity.

Effects of rapid-eye-movement sleep on the apneic threshold in dogs

We determined whether the apneic threshold after active hyperventilation was different in rapid-eye-movement (REM) vs. non-REM (NREM) sleep. Sleeping dogs were repeatedly exposed to 35-45 s of hypoxia of varying severity (end-tidal PO2 40-60 Torr) that was abruptly terminated with 100% O2. Changes in breathing pattern after brief hypoxia were compared with those after a normoxia-to-hyperoxia transition, i.e., control conditions. In NREM sleep, hypoxic hyperventilation was consistently followed by central apnea, the duration of which was linearly related to the corresponding hypocapnia and/or increase in tidal volume (VT) during hypoxia. After hypoxia, expiratory duration averaged 3.5 x control value at -5-Torr change in end-tidal PCO2 and twofold increase in VT; mean expiratory duration was 5 x control value at -10-Torr change in end-tidal PCO2 and fourfold increase in VT. In REM sleep, central apnea of varying duration did occur on occasion after brief hypoxic hyperventilation, but there was no systematic relationship with magnitude of hypocapnia or increase in VT. Breathing pattern during or after hypoxia in REM was not related to temporal changes in either eye movement density or electroencephalogram frequency. Thus, in contrast to NREM sleep, in REM sleep ("phasic" or "tonic") a posthyperventilation apneic threshold was not present. We attribute this effect of REM to 1) a reduced VT response to hypoxia that would minimize inhibitory "memory" effect from lung stretch and 2) attenuated inhibitory response to any given magnitude of hypocapnia or increased VT. Active hyperventilation-induced apneic threshold may be "masked" by actions of nonchemoreceptor and nonmechanoreceptor inputs affecting respiratory motor output in REM sleep. These data are consistent with the relative absence of central apnea and periodic breathing in humans in REM sleep.

Ventilatory response to exercise after heart and lung denervation in humans

This study, aimed at investigating some aspects of breathing control at work, was conducted on 8 heart and lung transplant recipients (HLTR) (age 33 +/- 13 years, mean +/- SD; 10 +/- 6 months post-transplantation) and on two control groups, i.e. 11 heart transplant recipients (HTR) and 11 healthy untrained subjects (C). The patients performed a series of 2 to 6 1-min exercise bouts (at 25 or 50 W, corresponding to about 50% of their VO2max) on a bicycle ergometer, followed by a 5 min 25 or 50 W constant load. C exercised both at 50 W (C1) and at 50% of their VO2max (C2). Inspiratory (VI) and expiratory (VE) ventilation, tidal volume (VT), respiratory frequency (fR), end-tidal O2 and CO2 partial pressures (PETO2 and PETCO2 and gas exchange (VO2 and VCO) were measured breath-by-breath. "Phase I" ventilatory response (ph I) was determined as the mean changes of VI, VE, VT, fR, PETO2 and PETCO2, compared to rest, during the first two respiratory cycles following exercise onset. In HLTR ph I did not significantly differ from that of C1 and C2, whereas the response was lower in HTR. VE, VO2 and VCO2 responses during "phase II" (t 1/2 on-) and "phase III" (steady state exercise) were similar in HLTR and in HTR. t 1/2 on- were longer in HLTR and in HTR compared to C1. In 3 HLTR the ventilatory pattern during the 5 min constant loads was similar to that of HTR and C, whereas 4 HLTR presented higher VT and lower fR values. It is concluded that: 1) The ventilatory response to exercise, in all its phases, is substantially preserved despite lung denervation. When slight alterations are found (i.e. the slower phase II), they are presumably of peripheral origin. 2) The normal ph I in HLTR indicates that cardiac and/or pulmonary inputs to the respiratory centers are not involved in its regulation, or that their role can be subserved by other ventilatory control mechanisms.

Construction and characterization of hybrid luciferases coded by lux genes from Xenorhabdus luminescens and Vibrio fischeri

Molecular cloning techniques were employed to obtain hybrid luciferases with their alpha and beta subunits encoded by luxA and luxB genes, respectively, from Xenorhabdus luminescens strain HW or Vibrio fischeri. Although the two wild-type luminous bacteria are phylogenetically diverged, the hybrid luciferase Xf comprising an alpha from X. luminescens HW and a beta from V. fischeri and the hybrid luciferase VI comprising an alpha from V. fischeri and a beta from X. luminescens HW were both functional in bioluminescence. Their general kinetic properties were close to the wild-type enzymes from which the alpha subunit was derived. The X. luminescens HW enzyme is distinct in having a high optimal temperature for in vitro bioluminescence, a high thermal stability and a sensitivity to aldehyde substrate inhibition. Comparisons of the Xf and VI hybrid luciferases with the two wild-type enzymes indicated that these unusual properties of the X. luminescens HW luciferase originated primarily from the alpha subunit.

Effects of memory from vagal feedback on short-term potentiation of ventilation in conscious dogs

1. We assessed short-term potentiation of ventilation in response to brief systemic normocapnic hypoxaemia in conscious dogs. Four recumbent dogs were exposed to Pa, O2 35-55 mmHg with Pa, CO2 maintained normocapnic for forty to fifty seconds and then abruptly returned to normoxia. Minute ventilation (VI) increased 4- to 5-fold during hypoxia due to both increased tidal volume (VT) and frequency (f). Several trials of hypoxic exposure with normoxic restoration were conducted with animals intact and following bilateral cold blockade of the cervical vagus nerves sufficient to block completely the Hering-Breuer reflex. 2. In the vagally intact dog, when normoxia was restored immediately following normocapnic hypoxia (PET, O2 = 40 Torr), expiratory time (TE) was prolonged to 190 +/- 68% of control (mean +/- S.E.M., range 53-350%) on the second or third breath and then returned slowly to control values on subsequent breaths. The prolongation of TE following removal of the hypoxic stimulus was positively correlated with the magnitude of the peak VT reached during hypoxic exposure. However, VT and VI remained significantly greater than control over a twenty second or four-breath period following hypoxia. 3. In the vagally blocked dog, no prolongation of TE was observed following isocapnic hypoxia; nor was TE following hypoxia correlated with the magnitude of the VT during hypoxia. The time constants of decay of VI, VT and f back to control, following hypoxia averaged 16, 19 and 9 s, respectively. 4. We conclude that short-term potentiation of ventilatory output following peripheral chemoreceptor hypoxic stimulation does exist in the awake dog, but the stimulatory after-effect is masked and TE is prolonged by a 'memory' of inhibitory vagal feedback. The magnitude of this inhibitory after-effect on TE prolongation increases in proportion to the increase in tidal volume achieved during the hypoxaemia. 5. This inhibitory effect of vagal memory may contribute to instability of breathing pattern and apnoea following transient disturbances in ventilatory output.

[Study on L-form induction of C. albicans]

After culturing C. albicans with ketoconazole on high osmosing medium containing thioglycollate, pig serum and sucrose, a cell wall-deficient strain of C. albicans was obtained. We compared its physiologic and biochemical characteristics with those of the parent strain. This cell wall-deficient strain not only changed its morphology but also the chemical components of the cell wall. The cell wall-deficient strain lacked 4 characteristic protein bands and had cell wall mannan decreased to 44% of that of the parent strain. Animal models revealed differences between the toxicities of the two strains: the mean survival time of mice inoculated intravenously with 10(6) deficient strain cells was 23.5 days (70% of the mice died within 30 days), but that of mice injected with 10(6) parent strain cells was 15.8 days (100% of the mice died in 30 days).

Functional consequences of site-directed mutation of conserved histidyl residues of the bacterial luciferase alpha subunit

The available sequences for the different bacterial luciferases reveal five conserved histidyl residues at positions 44, 45, 82, 224, and 285 of the alpha subunit. Ten variants of Vibrio harveyi luciferase were obtained by selective site-directed mutations of these five histidines. The essentiality of alpha His44 and alpha His45 was indicated by 4-7 orders of magnitude of bioluminescence activity reductions resulting from the substitution of either histidine by alanine (alpha H44A or alpha H45A), aspartate (alpha H44D or alpha H45D), or lysine (alpha H45K). Moreover, alpha H44A and alpha H45A were distinct from the native luciferase in thermal stabilities. Mutations at the other three positions also resulted in activity reductions ranging from a fewfold to 3 orders of magnitude. Despite these widely different bioluminescence light outputs, mutated luciferases exhibited, in nonturnover in vitro assays, light emission decay rates mostly similar to that of the native luciferase using octanal, decanal, or dodecanal as a substrate. This is attributed to a similarity in the catalytic rate constants of the light-emitting pathway for the native and mutated luciferases, but various mutated luciferases suffer in different degrees from competing dark reaction(s). In accord with this interpretation, the bioluminescence activities of mutated luciferases showed a general parallel with the relative stabilities of their 4a-hydroperoxyflavin intermediate species. Furthermore, the drastically reduced bioluminescence activities for luciferases with the alpha His44 or alpha His45 substituted by aspartate, alanine, or lysine were accompanied by little or no activities for consuming the aldehyde substrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Elicitation of an oxidase activity in bacterial luciferase by site-directed mutation of a noncatalytic residue

Flavin-dependent external monooxygenases and oxidases could catalyze the same flavin oxidation reaction involving distinct mechanisms. To gain insights into enzyme structure-function relationship, site-directed mutagenesis was carried out for Vibrio harveyi luciferase, a monooxygenase. The substitution of the alpha subunit cysteine 106 by alanine shows unambiguously that the alphaCys106 is not essential to catalysis. The corresponding substitution by valine resulted in a substantial reduction of the bioluminescence activity correlatable with the induction of a new flavin oxidation activity typical for oxidases. These findings indicate that mutation of a single noncatalytic residue at the active center of a flavoenzyme could transform one enzyme type to another, thus highlighting the subtlety of enzyme active site structure in relation to catalysis and the versatility of enzyme evolution.

Effects of acute hypoxia on ventilatory response at the onset of submaximal exercise

In order to investigate the effects of acute hypoxia and accompanying hypocapnia on the ventilatory response at the onset of dynamic exercise, four healthy adult men performed 50W rectangular loads on a cycle-ergometer in normoxic (FIO2 = 0.21) and hypoxic (FIO2 = 0.11) conditions. No statistically significant differences in the initial ventilatory responses to exercise (both delta VI and delta VE assessed on a breath-by-breath basis) were found between eucapnic normoxia (PETO2 approximately 95, PETCO2 approximately 42 Torr) and hypocapnic hypoxia (PETO2 approximately 45, PETCO2 approximately 35 Torr). The present findings support the contention that the neurogenic ventilatory drive at the onset of early exercise is independent from PO2 and PCO2.