Conserved cystatin segments as models for designing specific substrates and inhibitors of cysteine proteinases

Peptide segments derived from consensus sequences of the inhibitory site of cystatins, the natural inhibitors of cysteine proteinases, were used to develop new substrates and inhibitors of papain and rat liver cathepsins B, H, and L. Papain hydrolyzedAbz-QVVAGA-EDDnp andAbz-LVGGA-EDDnp at about the same rate, with specificity constants in the 10⁷M^–1 sec^–1 range; cathepsin L also hydrolyzes both substrates with specificity constants in the 10⁵ M^–1 sec^–1 range due to lowerk _cat values, with theK _m 's being identical to those with papain. OnlyAbz-LVGGA-EDDnp was rapidly hydrolyzed by cathepsin B, and to a lesser extent by cathepsin H. Peptide substrates that alternate these two building blocks (LVGGQVVAGAPWK and QVVAGALVGGAPWK) discriminate the activities of cathepsins B and L and papain. Cathepsin L was highly selective for cleavage at the G-G bond of the LVGG fragment in both peptides. Papain and cathepsin B cleaved either the LVGG fragment or the QVVAG fragment, depending on their position within the peptide. While papain was more specific for the segment located C-terminally, cathepsin B was specific for that in N-terminal position. Peptidyl diazomethylketone inhibitors based on these two sequences also reacted differently with papain and cathepsins. GlcA-QVVA-CHN₂ was a potent inhibitor of papain and reacted with papain 60 times more rapidly (k ₊₀= 1,100,000 M^–1 sec^–1) than with cathepsin L, and 220 times more rapidly than with cathepsin B. Cathepsins B and L were preferentially inhibited by Z-RLVG-CHN₂. Thus cystatin-derived peptides provide a valuable framework for designing sensitive, selective substrates and inhibitors of cysteine proteinases.     

A role of cathepsin B1 in polymorphonuclear leukocytes chemotaxis.

Cathepsin B_I¹ was purified from rat liver lysosomal fraction by ammonium sulfate fractionation, followed by chromatography on Sephadex G-200 and DEAE-Sephadex. Formation of chemotactic factor for guinea pig polymorphonuclear (PMN) leukocytes was demonstrated $\text{in vitro}$ when guinea pig serum was incubated with cathepsin B_I. This factor formation was dependent on SH-reagent dithiothreitol (DTT) and was maximal at pH 6.0. ZnSO₄, an inhibitor of cathepsin B_I, inhibited the chemotactic factor formation likewise.     

Effects of Vitamin B6 Deficiency on the Conversion Ratio of Tryptophan to Niacin

To investigate how vitamin B₆ (B₆) deficiency affects the whole metabolism of tryptophan-niacin, rats were fed for 19 days with each of the following four kinds of diets; a complete 20% casein diet (control diet), the control diet without B₆, the control diet without nicotinic acid, and the control diet without nicotinic acid and B₆, and the urinary excretion of such tryptophan metabolites as kynurenic acid, xanthurenic acid, nicotinamide, N¹-methylnicotinamide, N¹-methyl-2-pyridone-5-carboxamide, and N¹-methyl-4-pyridone3- carboxamide each and the enzyme activities involved in tryptophan-niacin pathway were measured. The urinary excretion of kynurenic acid decreased while that of xanthurenic acid increased drastically in the two B₆-deficient groups, when compared with the B₆-containing groups. These results indicate that the rats fed with the B₆-free diets were in the vitamin-deficient state. The conversion ratio was calculated from the ratio of the urinary excretion of sum of nicotinamide, N¹-methylnicotinamide, N¹-methyl-2-pyridone-5carboxamide, and N¹-methyl-4-pyridone-3-carboxamide, to the Trp intake. The ratio was statistically lower in the B₆-free diet than in the B₆-containing diet under the niacin-free conditions.     

Effects of Nucleotides on [3H]Bradykinin Binding in Guinea Pig: Further Evidence for Multiple B2 Receptor Subtypes

Abstract: We have suggested recently the existence of three subtypes of B₂ bradykinin receptors in tissues of guinea pigs. We have classified these B₂ bradykinin receptors into B_2a, B_2b, and B_2c subtypes depending on their affinity for various bradykinin antagonists. Because the actions of bradykinin in different cell systems appear to be both dependent on and independent of G proteins, we sought to determine whether the binding of [³H]bradykinin to the B₂ subtypes is sensitive to guanine nucleotides and, therefore, possibly coupled to G proteins. In the ileum, where we have demonstrated B_2a and B_2b subtypes, specific [³H]bradykinin binding was reduced with GDP (100 μM) and the nonmetabolized analogue of GTP, guanyl-5′-yl-imidodiphosphate (GppNHp; 100 μM). Competition studies with bradykinin and with [Hyp³]-bradykinin, which shows approximately 20-fold greater selectivity for the B_2a subtype than bradykinin, were performed in the presence or absence of GppNHp (100 μM). The competition experiments demonstrated that binding to the B_2a subtype, which has higher affinity for [Hyp³]-bradykinin and bradykinin than the B_2b subtype, was lost in the presence of GppNHp, whereas binding to the B_2b subtype was unaffected. In contrast, GppNHp (100 μM) and GDP (100 μM) failed to alter specific [³H]bradykinin binding to B_2b and B_2c subtypes in lung. [³H]Bradykinin binding was unaffected by AMP, ADP, ATP, and GMP (100 μM each). Based on this evidence, we suggest that the B_2a bradykinin subtype is coupled to G proteins. The B_2b and B_2c subtypes are either not coupled to G proteins, or may be coupled to the G^o-type GTP binding proteins, which have been suggested to be less sensitive to guanine nucleotides. These data provide further evidence for three subtypes of B₂-type bradykinin receptors in guinea pig.     

MHC and Non-MHC genetic influences on rous sarcoma metastasis in chickens

The B ⁵/B ⁵ genotype, in Leghorns, was associated with a high degree of metastasis of Rous sarcoma virus-induced tumors, but in combination with a Leghorn-New Hampshire background markedly less metastasis occurred. Initially, four mating types were used: B ⁵/B ⁵ × B ⁵/B ⁵ chickens from the F₅ generation of the cross of Leghorn lines 6₁ and 15₁, B ²⁴/B ²⁴ × B ²⁴/B ²⁴ chickens from line UNH 105 (New Hampshires), and reciprocal crosses of B ⁵/B ⁵ × B²⁴/B²⁴ chickens. Subsequently, F₂ generation progeny of the cross of B⁵/B⁵ and B ²⁴/B ²⁴ breeders, as well as B ²⁴/B ²⁴ line UNH 105 and B ⁵/B ⁵ (6₁ × 15₁)F₂ chickens, were used. Six-week-old chickens were inoculated in the wingweb with Rous sarcoma virus. Chickens dying during a 10-week period after inoculation were necropsied and suspect metastatic lesions examined histologically. Among 234 terminal chickens from the initial four mating types the incidence of metastasis associated with B ⁵/B ⁵ Leghorns (66%) was substantially higher than for B ²⁴/B ²⁴ New Hampshires (12%) and B ⁵/B ²⁴ progeny of reciprocal Leghorn-New Hampshire crosses (19 and 24%). Subsequently, among 524 terminal hosts in the Leghorn-New Hampshire F₂ population, B genotype significantly influenced tumor dissemination. However, among 52 concurrently challenged B ⁵/B ⁵ hosts from the (6₁ × 15₁)F₂ population the incidence of metastasis (60%) was significantly higher than among 122 B ⁵/B ⁵ hosts from the Leghorn-New Hampshire F₂ population (31%), indicating a non-major histocompatibility complex genetic effect on metastasis.     

Major histocompatibility (B) complex effects on acquired immunity to cecal coccidiosis

The influence of the major histocompatibility (B) complex on acquired immunity to the avian coccidium Eimeria tenella was studied in 217 F₄ segregants (B ² B ², B ² B ⁵, B ⁵ B ⁵) of a cross between inbred lines 6₁ (B ² B ²) and 15₁ (B ⁵ B ⁵) and segregating haplotype combinations of UNH105 (B ²³ B ²³ B ²³ B ²⁴, B ²⁴ B ²⁴), a noninbred line of New Hampshire chickens. Chickens were immunized at 6 weeks of age with 500 oocysts daily for 5 days, then challenged 14 days later with 10000 oocysts. Responses to infection were evaluated by cecal lesion scores, body weight gain, delayed wattle reaction (DWR), and spleen weight. The F₄ segregants of genotypes B ² B ⁵ and B ⁵ B ⁵ exhibited greater immunity to challenge than B ² B ² chickens. B ⁵ B ⁵ chickens showed a significantly greater DWR following immunization and larger spleens 6 days after the challenge than either of the other genotypes. However, both BIBS and B ⁵ B ⁵ chickens demonstrated significantly lower lesion scores than B ² B ² chickens. There were no significant differences in weight gain among these genotypes. Among 139 line UNH105 segregants, B ²³ B ²³ hosts had significantly lower lesion scores than B ²⁴ B ²⁴ chickens. No other differences in immune response among line UNH105 genotypes were detected.     

Estrogen modulation of osteoclast lysosomal enzyme secretion

Osteoclast-mediated bone resorption is accomplished by secretion of lysosomal proteases into an acidic extracellular compartment. We have previously demonstrated that avian osteoclasts and human osteoclast-like giant cell tumor cells respond in vitro to treatment with 17β-estradiol (17β-E₂) by decreased bone resorption activity. To better understand the mechanism by which this is accomplished, we have investigated the effects of 17β-E₂ treatment on lysosomal enzyme production and secretion by isolated avian osteoclasts and multinucleated cells from human giant cell tumors in vitro. Isolated cells were cultured with bone particles in the presence of either vehicle or steroid. The conditioned media and cells were harvested, and the levels of cathepsin B, cathepsin L, β-glucuronidase, lysozyme, and tartrate-resistant acid phosphatase (TRAP) activities were determined. There was a steroid dose-dependent decrease in secreted levels of these enzymes. Cell-associated levels of cathepsin L, β-glucuronidase, and lysozyme decreased, whereas cell-associated levels of cathepsin B and TRAP increased. These changes were measurable at 10^?10 M and maximal at 10^?8 M 17β-E₂. The changes were detectable at 4–18 h of treatment and increased through 24 h of treatment. The response was steroid specific, since the inactive estrogen isomer, 17β-E₂, failed to alter the activity levels. Moreover, the effects of 17β-E₂ were blocked when the cells were treated simultaneously with the estrogen antagonist ICI182–780 in conjunction with 17β-E₂. Human osteoclast-like cells obtained from giant cell tumors of bone responded similarly to estrogen with respect to cathepsin B, cathepsin L, and TRAP activities. However, secretion of β-glucuronidase and lysozyme were not altered by treatment with 10^?8 M 17β-E₂. These data indicate that estrogen effects on osteoclast resorption activity may be mediated by decreasing the secretion of cathepsin B, cathepsin L, and TRAP.     

Production of aflatoxin byAspergillus parasiticus and its control

The aim of the present work was to investigate the production of aflatoxin byAspergillus parasiticus and to find out the possible ways to control it. Of 40 food samples collected from Abha region, Saudi Arabia, only 25% were contaminated with aflatoxins. Oil-rich commodities had the highly contaminated commodities by fungi and aflatoxins while spices were free from aflatoxins.Bacillus megatertum andB cereus were suitable for microbiological assay of aflatoxins. Czapek’s-Dox medium was found a suitable medium for isolation of fungi from food samples. The optimal pH for the growth ofA. parasiticus and its productivity of aflatoxin B₁ was found at 6.0, while the best incubation conditions were found at 30°C for 10 days. D-glucose was the best carbon source for fungal growth, as well as aflatoxin production. Corn steep liquor, yeast extract and peptone were the best nitrogen sources for both fungal growth and toxin production (NH₄)₂HPO₄ (1.55 gL^-1) and NaNO₂ (1.6 gL^-1) reduced fungal growth and toxin production with 37.7% and 85%, respectively. Of ten amino acids tested, asparagine was the best for aflatoxin B₁ production. Zn²⁺ and Co²⁺ supported significantly both fungal growth, as well as, aflatoxin B₁ production at the different tested concentrations. Zn²⁺ was effective when added toA. parasiticus growth medium at the first two days of the culture age. The other tested metal ions expressed variable effects depending on the type of ion and its concentration. Water activity (a_w) was an important factor controlling the growth ofA. parasiticus and toxin production. The minimum a_w for the fungal growth was 0.8 on both coffee beans and rice grains, while a_w of 0.70 caused complete inhibition for the growth and aflatoxin B₁ production. H₂O₂ is a potent inhibitor for growth ofA. parasiticus and its productivity of toxins. NaHCO₃ and C₆H₅COONa converted aflatoxin B₁ to water-soluble form which returned to aflatoxin B₁ by acidity. Black pepper, ciliated heath, cuminum and curcuma were the most inhibitory spices on toxin production. Glutathione, quinine, EDTA, sodium azide, indole acetic acid, 2,4-dichlorophenoxy acetic acid, phenol and catechol were inhibitory for both growth, as well as, aflatoxin B₁ production. Stearic acid supported the fungal growth and decreased the productivity of AFB₁ gradually. Lauric acid is the most suppressive fatty acid for both fungal growth and aflatoxin production, but oleic acid was the most potent supporter. Vitamin A supported the growth but inhibited aflatoxin B₁ production. Vitamins C and D₂ were also repressive particularly for aflatoxin production The present study included studying the activities of some enzymes in relation to aflatoxin production during 20-days ofA. parasiticus age in 2-days intervals. Glycolytic enzymes and pyruvate-generating enzymes seems to be linked with aflatoxin B₁ production. Also, pentose-phosphate pathway enzymes may provide NADPH for aflatoxin B₁ synthesis. The decreased activities of TCA cycle enzymes particularly from 4th day of growth up to 10th day were associated with the increase of aflatoxin B₁ production. All the tested enzymes as well as aflatoxin B₁ production were inhibited by either catechol or phenol.     

TheB locus (MHC) in the chicken: Association with the fate of RSV-induced tumors

The fate of tumors induced by Rous sarcoma virus (RSV) was determined in anF ₂ population segregating at three alloantigen loci. TheF ₁ resulted from crossing tumor-resistant RPRL line 6₁ (B ² B ² D ³ D ³ I ² I ²) with tumor-susceptible RPRL line 15₁ (B ⁵ B ⁵ D ⁴ D ⁴ I ⁸ I ⁸). Among theF ₂ segregantsB ² B ²,B ² B ⁵, andB ⁵ B ⁵, the percentage of chicks dying of terminal tumors (by 70 days post-inoculation) was 5, 26, and 93, respectively (P0.01). NeitherD orI genotypes nor sex significantly affected tumor growth. In chickens with terminal tumors, the incidence of metastatic lesions was also significantly associated withB genotypes. Thus, the MHC chromosomal region in the chicken appears to exert a crucial role in determining the outcome of RSV-induced tumors.     

Increased growth of RSV-induced tumors in chickens partially tolerant to MHC alloantigens

Chickens withB ² B ² MHC genotypes were made partially tolerant to B₅ MHC cell-surface antigens and the fate of their Rous-sarcoma-virus (RSV)-induced tumors was determined.B ² B ² chickens partially tolerant to viable or lysed white blood cells (WBC) or viable red blood cells (RBC) fromB ⁵ B ⁵ chickens had a significantly higher incidence of tumor progression than untreated, PBS-treated, orB ² B ² chickens inoculated with WBC from otherB ² B ² chickens. The criteria for tolerance were absence of antibody titer to the cell type inoculated and acceptance of allografts fromB ⁵ B ⁵ donors byB ² B ² chickens. Graft-vs-host reactions occurred only inB ² B ² chickens inoculated with viable WBC fromB ⁵ B ⁵ chickens. It appears thatB ² B ² chickens partially tolerant to B₅ antigens failed to mount a successful immune response to RSV-induced tumors partly because a B₅ MHC antigen(s) cross-reacted with a tumor associated antigen(s) thereby severely limitingB ² B ² host recognition of the tumor as foreign. Since WBC and RBC cell-surface antigens appear to contribute similarly to the effect, theB-F- region of the MHC may be involved.     

Effects of various metabolites on two phosphoglucomutase allozyme activities from Drosophila melanogaster

The effects of various metabolites on the two most common phosphoglucomutase allozymes (PGM^A and PGM^B) in Drosophila melanogaster have been investigated in vitro. 2,3-Diphosphoglycerate (2,3DPG) inhibited PGM^A and PGM^B to the same degree in the presence of 25 µM glucose-1,6-diphosphate (G1, 6P₂). However a higher concentration of G1,6P₂ partially reversed the inhibition of PGM^A exerted by 2,3DPG, so that in the presence of 150 µM G1,6P₂ the inhibition of PGM^A was half that of PGM^B at pH 6.0. Glycerol-3-phosphate (G3P) had no significant effect at pH 7.4 but exerted an activating effect at pH 6.0 which was more pronounced in the case of PGM^B. ATP, citrate, and fructose-1, 6-diphosphate (F1,6P₂) inhibited both PGM^A and PGM^B. The differences found in vitro between these two allozymes can have a significant impact on in vivo function and, therefore, on the maintenance of PGM polymorphism in experimental populations of D. melanogaster studied in the laboratory.     

Mechanism and Putative Structure of B0-like Neutral Amino Acid Transporters

The Na⁺-dependent transport of neutral amino acids in epithelial cells and neurons is mediated by B⁰-type neutral amino acid transporters. Two B⁰-type amino acid transporters have been identified in the neurotransmitter transporter family SLC6, namely B⁰AT1 (SLC6A19) and B⁰AT2 (SLC6A15). In contrast to other members of this family, B⁰-like transporters are chloride-independent. B⁰AT1 and B⁰AT2 preferentially bind the substrate prior to the Na⁺-ion. The Na⁺-concentration affects the K _m of the substrate and vice versa. A kinetic scheme is proposed that is consistent with the experimental data. An overlapping binding site of substrate and cosubstrate has been demonstrated in the bacterial orthologue LeuT _Aa from Aquifex aeolicus, which elegantly explains the mutual effect of substrate and cosubstrate on each other’s K _m -value. LeuT _Aa is sequence-related to transporters of the SLC6 family, allowing homology modeling of B⁰-like transporters along its structure.     

Cross-reactivity between RSV-induced tumor antigen and B5 MHC alloantigen in the chicken

Lymphocytes from chickens homozygous (B ² B ²) at the major histocompatibility complex (MHC) were tested for cytotoxic activity against five types of target chicken embryo fibroblasts (CEF). Lymphocytes from B²B² chickens bearing RSV-induced tumors lysed in vitro targets of B ² B ² and B ⁵ B ⁵ RSV-infected CEF and B ⁵ B ⁵ normal CEF, but did not lyse B ² B ² and B ²⁴ B ²⁴ normal CEF. Lymphocytes from normal B ² B ² chickens did not lyse any of the five types of CEF targets. Alloantisera absorption studies showed that both RSV-infected and uninfected CEF shared alloantigens, in particular B-F alloantigens, with syngeneic erythrocytes. Absorption with B ² B ² RSV-infected CEF significantly lowered the titer of B ² B ² anti-B ⁵ B ⁵ alloantisera. Cross-reactivity between B ₅ antigen(s) and tumor-associated antigen was suggested and the nature of the cross-reactivity was discussed. It is hypothesized that this cross-reactivity prevents B ⁵ B ⁵ chickens from recognizing RSV-induced tumors as foreign, enhances tumor growth and leads to death of the host.     

Role of calpains in diabetes mellitus-induced cataractogenesis: A mini review

Premature visual impairment due to lens opacification is a debilitating characteristic of untreated diabetes. Lens opacification is primarily due to the insolubilization of crystallins, proteins essential for lens optical properties, and recent studies have suggested that a major cause of this insolubilization may be the unregulated proteolysis of crystallins by calpains. These are intracellular cysteine proteases whose activation requires the presence of calcium (Ca²⁺) and elevated levels of lens Ca²⁺ is a condition associated with both diabetic cataractogenesis and other forms of the disorder. A number of calpains have been identified in the lens, including calpain 2, calpain 10 and two isozymes of calpain 3:Lp82 and Lp85. The use of animal hereditary cataract models have suggested that calpain 2 and/or Lp82 may be the major calpains involved in murine cataractogenesis with contributions from calpain 10 and Lp85. However, calpain 2 appears to be the major calpain involved in murine diabetic cataractogenesis and the strongest candidate of the calpains for a role in human types of cataractogenesis. Here, we present an overview of recent evidence on which these observations are based with an emphasis on the ability of calpains to proteolyse lens crystallins and calpain structural features, which appear to be involved in the Ca²⁺-mediated activation of these enzymes. (Mol Cell Biochem 261: 151–159, 2004)     

Insights into ET<Subscript>A</Subscript> subtype selectivity of benzodiazepine endothelin receptor antagonists by 3D-QSAR approaches

ET_A subtype selective antagonists constitute a novel and potentially important class of agents for the treatment of pulmonary hypertension, heart failure, and other pathological conditions. In this paper, 60 benzodiazepine derivatives displaying potent activities against ET_A and ET_B subtypes of endothelin receptor were selected to establish the 3D-QSAR models using CoMFA and CoMSIA approaches. These models show excellent internal predictability and consistency, external validation using test-set 19 compounds yields a good predictive power for antagonistic potency. Statistical parameters of models were obtained with CoMFA-ET_A (q ² = 0.787, r ² = 0.935, r ² _pred  = 0.901), CoMFA-ET_B (q ² = 0.842, r ² = 0.984, r ² _pred  = 0.941), CoMSIA-ET_A (q ² = 0.762, r ² = 0.971, r ² _pred  = 0.958) and CoMSIA-ET_B (q ² = 0.771, r ² = 0.974, r ² _pred  = 0.953) respectively. Field contour maps (CoMFA and CoMSIA) corresponding to the ET_A and ET_B subtypes reflects the characteristic similarities and differences between these types. The results of this paper provide valuable information to facilitate structural modifications of the title compounds to increase the inhibitory potency and subtype selectivity of endothelin receptor.     

Changes in lysosomal cathepsins during liver regeneration

The effect if $\text{2}\text{3}$ hepatectomy in the adult rat on the specific and total activities of two lysosomal endopeptidases, cathepsins B₁ and D, has been examined. The specific activity of both enzymes fell rapidly following hepatectomy when compared to paired, sham-operated controls. When changes in total protein were compared to changes in cathepsins B₁ and D, all three decreased in a parallel fashion for the first 18 h. At 24 h, total liver protein increased rapidly while cathepsins B₁ and D continued to decrease. Cathepsin B₁ fell to a level of 12% of non-operated control levels at 36 h, while total protein was already back to 40% of control levels. In contrast to the decreases in the activities of the cathepsins, there was an increase in the activities of lactate dehydrogenase and malate dehydrogenase during the first two days of regeneration. The clear lag in replacement of the cathepsins relative to other liver proteins following partial hepatectomy suggests that cathepsin activity is selectively controlled and that lowering the levels of cathepsins B₁ and D may play an important role in the decreased degradation of protein seen during the early phases of liver regeneration.     

Mycobiota in poultry feeds and natural occurrence of aflatoxins, fumonisins and zearalenone in the Rio de Janeiro State, Brazil

The intake of mycotoxin-contaminated feeds can lead to nutrient losses and may have adverse effects on animal health and on productivity. The aims of this study were (1) to determine the mycobiota present in poultry feed samples, and (2) to evaluate the natural occurrence of aflatoxin B₁, fumonisin B₁ and zearalenone. Fungal counts were similar between all culture media tested (10³ CFU g⁻¹). The most frequent genus isolated was Penicillium spp. (41.26%) followed by Aspergillus spp. (33.33%) and Fusarium spp. (20.63%). High precision liquid chromatography was applied to quantify aflatoxin B₁ and fumonisin B₁. Thin layer chromatography was used to determine zearalenone levels. Aflatoxin B₁ values ranged between 1.2 and 17.5 μg kg⁻¹. Fumonisin B₁ levels ranged between 1.5 and 5.5 μg g⁻¹. Zearalenone levels ranged between 0.1 and 7 μg g⁻¹. The present study shows the simultaneous occurrence of two carcinogenic mycotoxins, aflatoxin B₁ and fumonisin B₁, together with another Fusarium mycotoxin (zearalenone) in␣feed intended for poultry consumption. Many samples contained AFB₁ levels near the permissible maximum and it could affect young animals. A synergistic toxic response is possible in animals under simultaneous exposure.     

PHOTOSYNTHETIC RESPONSE OF NATURAL ASSEMBLAGES OF MARINE BENTHIC MICROALGAE TO SHORT-AND LONG-TERM VARIATIONS OF INCIDENT IRRADIANCE IN BAFFIN BAY,TEXAS1

This study was designed to understand the high variability characterizing primary production rates of microphytobenthos. The photosynthetic efficiency (α^B) and photosynthetic capacity (P^B_max) of the microphytobenthos were measured at different times of the day on two different dates (8 May and 7 July 1990). In July, unusually low light conditions were caused by the development of a brown tide (chrysophytes). Both light-limited and light-saturated photosynthesis changed at hourly and monthly scales. There was a linear relationship between α^B and P^B_max, suggesting a common response to environmental factors [α^B= 0.0075(±0.00063)·P^B_max+ 0.00097(±0.0071), R²= 0.94]. Incident irradiance at the sediment-water interface was the primary physical factor that explained variability of both α^B (84%) and P^B_max (92%). Temperature had a negative but minor effect that explained an extra 8% and 2% of the variance, respectively. There was no diel rhythm of α^B and P^B_max and incident irradiance was regulated by wind-induced currents. Therefore, microphytobenthos photosynthesis seemed to be primarily controlled by wind events in Baffin Bay.     

亚热带区4种林地土壤微生物生物量碳氮磷及酶活性特征

在位于亚热带丘陵区的长沙县大山冲林场选取地域毗邻、环境条件(立地、土壤、气候)基本一致的杉木人工林(CL)和3种次生林:马尾松-柯(又名石栎)针阔混交林(PM-LG)、南酸枣落叶阔叶林(CA)、柯-青冈常绿阔叶林(LG-CG),每种林地随机设置5个20 m×20 m的样地,分别采集表层(0—15 cm)和亚表层(15—30 cm)土壤样品,测定土壤微生物生物量碳(B_C)、氮(B_N)、磷(B_P)和蔗糖酶(INV)、脲酶(URE)、酸性磷酸酶(ACP)、过氧化氢酶(CAT)活性,分析4种林地土壤微生物生物量和酶活性及其与土壤化学性质的关系。结果表明:表层和亚表层土壤B_C、B_N、B_P和ACP活性依次为:CA LG-CG PM-LG CL,INV和URE活性依次为:LG-CG CA PM-LG CL,CAT活性依次为:CA PM-LG LG-CG CL,说明森林植被恢复对土壤微生物生物量和酶活性有明显的促进作用。通径分析表明,土壤B_C、B_N、B_P的直接影响因素和主要影响因素分别为SOC和TN/TP,TN和TN/TP,TP和SOC/TP,而TN/TP与B_C之间,TN与B_N之间具有较强的负相关;INV、ACP活性的直接影响因素主要是TN、TN/TP,其中TN/TP与INV、ACP活性具有较强的负相关;URE、CAT活性分别为B_P/TP和B_P,B_C/SOC和SOC,其中B_P与URE活性具有较强的负相关,B_C/SOC、SOC两者与CAT活性具有较强的正相关。此外,土壤B_C、B_N、B_P以及INV、URE、ACP、CAT活性的剩余余项通径系数较低,说明土壤化学性质对土壤微生物生物量,以及土壤化学性质和微生物生物量对土壤酶活性具有较大的影响。土壤B_C、B_N、B_P之间及其与土壤酶活性呈显著正相关。     

Chemically reacting systems of the type A + B ⇄ AB. II. Osmometry

The osmotic pressure equation for nonideal, associating systems of the type nA +mB ? A_nB_m, has been derived, by using the assumption yA _nB _m/yA ⁿyB ^m = 1. This treatment can also be applied to related associations such as nA + mB ? AB + AB₂ + A₂B + …. From osmotic pressure experiments on the pure reactants it is possible to obtain the molecular weights (M_A and M_B) of the reactants and also the virial coefficients (B_AA and B_BB) of the reactants. The osmotic pressure of a nonreacting mixture of A and B can be calculated from these measurements. It can be used along with osmotic pressure measurements on equilibrium mixtures of A and B to obtain expressions containing the equilibrium constant (or constants) and the cross-virial coefficients (B_AB and B_BA). Several procedures are described for the evaluation of the equilibrium constant (or constants) and the B_AB or B_BA terms. It appears that this procedure is a general one which is applicable to associations of the type nA + mB ? AB + A₂B + AB₂ + …. By correcting for nonideal behavior, one should then be able to apply it to any method available for analyzing ideal associations of the types considered here. In addition it is possible, subject to certain restrictions, to analyze associations of the type 3A + B ? A₂ + AB.