Contribution to catalysis and stability of the five cysteines in Escherichia coli aspartate aminotransferase. Preparation and properties of a cysteine-free enzyme.

The five cysteines, at positions 82, 191, 192, 270, and 401, of Escherichia coli aspartate aminotransferase (AATase) were, individually and in some combinations, converted to alanine by site-directed mutagenesis (C82A, C191A, C192A, C270A, C401A). Cys-191, which is conserved in all AATase isozymes, was mutated to serine as well (C191S). A quintuple mutant, with all cysteines converted to alanines (Quint), was also constructed. The effects of these single and multiple mutations were examined by steady-state kinetics and urea denaturation. The thermal stabilities of Quint and of the wild-type enzyme (WT) were determined by differential scanning calorimetry. The mutants had kcat values up to 50% greater than that of WT and KMAsp and KM alpha-KG values up to 1.5- and 3.3-fold higher than that of WT. The mutants C82A and C191A exhibit nearly the same CM in urea denaturation experiments as WT, while the other single mutants and Quint are less stable, with CM differences of up to 0.7 M urea. Quint is also less thermostable than WT, with a delta TM of 3.3-4.4 degrees C. Thus the five cysteine replacements yield small, but significant, changes in catalytic and denaturation parameters, but none of the cysteines was found to be essential. The changes manifested in the mutation of the conserved Cys-191 to alanine are no greater than those observed with the four nonconserved cysteines. We consider the evolutionary implications of these findings.     

Influence of exogenous NO donator and variations in the Ca<Superscript>2+</Superscript> content on transport activity related to tonoplast proton pumps in ontogenesis and under hyperosmotic stress

The changes in transport activity of tonoplast proton pumps under the influence of exogenous NO donator and modulation of Ca²⁺ concentration jointly and separately were investigated at different stages of ontogenesis and under hyperosmotic stress. The results suggest that both exogenous NO donator and Ca²⁺ ions can influence the activity of transport processes related to tonoplast and this influence is especially evident in the period of growth and accumulation of metabolites. Under hyperosmotic stress, H⁺-pyrophosphatase plays a more important role than H⁺-ATPase: the activity of the former increases 2.3-fold compared to the control osmotic conditions, whereas the activity of H⁺-ATPase is practically unchanged. H⁺-pyrophosphatase was more responsive to the presence of exogenous NO donator and to variations in Ca²⁺ concentration. The effects of exogenous NO donator on tonoplast proton pumps depended on the concentration of Ca²⁺, which apparently can mediate NO action.     

Stream drift,size-specific predation,and the evolution of ovum size in an amphibian

Summary A stream-breeding race of small-mouthed salamanders (Ambystoma texanum) in central Kentucky produces ova that are twice as large as those of a pond-breeding race found nearby. Embryos of stream-breeders also hatch at a more advanced developmental stage than those of pond-breeders. Morphological evidence indicates that stream-breeders were derived from pond-breeding stock. Assuming that differences between stream and pond-breeders reflect evolutionary change, and that the ancestral pond stock that invaded streams was similar to extant pond-breeders, we examined three hypotheses that might explain changes in ovum size and stage at hatching following the invasion of streams. (1) Larger ovum size evolved indirectly as a consequence of selection for rapid development which minimizes mortality risk from stream drying. (2) Increased ovum (hatchling) size and stage at hatching of stream-breeders are adaptations to resist stream current. (3) Increased ovum (hatchling) size and stage at hatching are adaptations to reduce predation on hatchlings from stream invertebrates. The results of field and laboratory studies only support hypotheses (2) and (3). Hatchlings that were relatively large or at a more advanced developmental stage had slower drift rates and were less vulnerable to predation by Phagocata gracilis, a flatworm abundant in streams in central Kentucky. Developmental and growth parameters were not correlated significantly with ovum size in populations of either geographic race. Differences in degree of parental care among races also cannot explain variation in ovum size since both races abandon their eggs immediately after oviposition.     

Antagonism by theophylline of respiratory inhibition induced by adenosine

The effects on respiration of an analogue of adenosine, L-2-N6-(phenylisopropyl)adenosine (PIA), and of the methylxanthine, theophylline, were determined in 19 vagotomized glomectomized cats whose end-tidal PCO2 was kept constant by means of a servo-controlled ventilator. Integrated phrenic nerve activity was used to represent respiratory output. Our results show that PIA, whether given systemically or into the third cerebral ventricle, depressed respiration. Systemically administered theophylline stimulated respiration. Theophylline given intravenously, or into the third ventricle not only reversed the depressive effects of previously administered PIA but caused further increases of respiration above the control level. Prior systemic administration of theophylline blocked both respiratory and hypotensive effects of subsequently administered PIA. Effects of either agent on medullary extracellular fluid pH did not explain the results. We conclude that the adenosine analogue PIA, acts to inhibit neurons in the brain that are involved in the control of respiration and that its effects are blocked by theophylline. We suggest that adenosine acts as a tonic modulator of respiration and that theophylline stimulates breathing by competitive antagonism of adenosine at neuronal receptor sites.     

PATTERNS OF REPRODUCTION IN THREE SANDY BEACH WHELKS OF THE GENUS BULLIA GRIFFITH

The breeding cycles of two species of sandy beach whelk (Bulliadigitalis and B. pura) are presented and compared with a thirdspecies (B. rhodostoma). In all three species, egg maturationand copulation occur in spring while summer marks the depositionand spawning of egg capsules. B. digitalis and B. pura migrateoffshore to lay their eggs, the juveniles of both species restrictingthemselves to beyond the breaker zone. Newly hatched snailsof B. rhodostoma appear in the intertidal towards late summer.The discovery of a penis-like structure on females of B. rhodostomais investigated. The pseudopenis was also found on B. pura butnever on B. digitalis. The possibility of a sex-change was eliminatedon the histological investigation of the gonads of a range ofsnails and the examination of sex-ratios over a period of fivemonths. General trends in reproductive behaviour are also discussed. (Received 18 March 1984;     

Controlled trial of methylprednisolone pulses and low dose oral prednisone for the minimal change nephrotic syndrome.

In a multicentre, randomised, prospective trial 89 patients (67 children and 22 adults) with the minimal change nephrotic syndrome were treated with three intravenous pulses of methylprednisolone followed by low dose oral prednisone for six months (group given methylprednisolone) or with high dose oral prednisone for four weeks followed by low dose oral prednisone for five months (control group). Five patients in the group given methylprednisolone and one in the control group did not respond initially. The time to response was shorter in children treated with methylprednisolone. No significant differences between the two groups were observed in the number of patients who relapsed or number of relapses per patient per year. Patients given methylprednisolone tended to relapse earlier than patients in the control group. Side effects related to treatment were significantly fewer in the group given methylprednisolone than in the control group. These data suggest that a short course of methylprednisolone pulses followed by low dose oral prednisone is only marginally less effective than a regimen of high dose oral steroids but can improve the ratio of risk to benefit associated with treatment of the minimal change nephrotic syndrome.     

Nucleic acid relationships among Acholeplasma species.

3H-labeled Acholeplasma DNA probes were generated in vitro by the nick-translation method and used to determine the nucleotide sequence homology among the type strains of the eight currently recognized species of Acholeplasma. Very little nucleotide sequence homology (less than or equal to 18%) was found among the eight species, with heteroduplexes showing at least 12% or more mismatching as determined by thermal elution midpoints. The small amount of nucleotide sequence homology among the eight species indicates that these species are quite distinct and are not closely related to each other genomically.     

Monosomy 21: a new case confirmed by in situ hybridization

Summary A new case of total monosomy 21 in a newborn is described. The diagnosis was first made using the cytogenetic data; it was then confirmed by the dosage of copper-superoxide dismutase (SOD-1) which showed a 50% decrease. In situ hybridization with a probe previously assigned to chromosome 21 was used to rule out the possibility of a partial monosomy with an unbalanced reciprocal translocation.