马齿苋叶片PEP羧化酶的分子特性

马齿苋叶片PEPCase由四个相同的亚基组成,亚基分子量为83kD。远紫外CD光谱分析表明,此酶含有36.6％α—螺旋结构。马齿苋叶片PEPCase可被G6P激活,但不能被Gly、Ser激活。G6P可防止酶的尿素变性和枯草杆菌蛋白酶的作用。这种保护效应与G6P诱导的酶构象变化有关。 从酶对低温、高温及尿素的反应来看,马齿苋叶片PEPCase的稳定性高于高粱叶片PEP—Case,两者的免疫特性和电泳特性亦不同。     

Synthesis and secretion of lipids by long-term cultures of female rat hepatocytes.

The objective of this work was to characterize lipid metabolism in long-term cultures of adult rat hepatocytes from female rats and explore the potential use of this culture system to study the effect of hormones, drugs and toxic chemicals on it. Hepatocytes, seeded on a feeder layer of 3T3 cells, maintained for 2 weeks their typical morphology. The cultures were able to take up [14C]acetic and [14C]oleic acid from the culture medium and incorporate them into lipids. The synthesis and secretion of lipids by [14C]acetic acid-labeled cultures had a maximum value after 11 and 13 days in culture. Triacylglycerols were the main lipidic species synthesized and secreted by hepatocytes (up to 67% of the total lipids); they also synthesized and secreted phospholipids, cholesterol and cholesterol esters from [14C]acetic acid. Similarly, [14C]oleic acid-labeled cultures synthesized and secreted mostly triacylglycerols (up to 60-70% of the total lipids), but they were also able to incorporate the labeled precursor into both cellular and secreted phospholipids and cholesterol esters. The activity of glycerol-phosphate-dehydrogenase, marker enzyme of glycerolipid synthesis, decreased slightly during the culture time whereas the activity of malic enzyme, marker of fatty acid synthesis, increased. Our results show that long-term cultures of female rat hepatocytes are able to synthesize and secrete several lipids, specially triacylglycerols, from both [14C]acetic and [14C]oleic acid for at least 2 weeks and that they maintain enzyme activities related with the synthetic pathways of glycerolipids and fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)     

Amino acid metabolism in several tissues of the obese Zucker rat as indicated by the tissue accumulation of α-amino[1-14C]isobutyrate

Measurements of the tissue accumulation of α-amino[1-¹⁴C]isobutyrate [1-¹⁴C]AIB) in lean (+/?) and obese (fa/fa) Zucker rats showed an augmented tissue/plasma ratio in the liver of the obese animals. In contrast, brown adipose tissue AIB accumulation was lower in the fa/fa animals. In response to a 24h starvation period AIB accumulation was significantly elevated in the liver and plasma of the lean animals and was unchanged in the liver of the fa/fa animals. The circulating concentration of alanine and branched-chain amino acids was elevated in the fa/fa animals as compared to their lean counterparts. These observations suggest that amino acid uptake is not involved in the impaired muscle development observed in the obese Zucker rat and that the ability of brown adipose tissue for amino acid utilization is decreased in the obese animals suggesting that this may partially explain the impaired thermoregulatory capacity observed in brown adipose tissue of obese Zucker rats.     

Phosphorus-31 magnetic resonance spectroscopy of forearm flexor muscles in student rowers using an exercise protocol adjusted for differences in cross-sectional muscle area

To assess exercise energy metabolism of forearm flexor muscles in rowers, six male student rowers and six control subjects matched for age and sex were studied using phosphorus-31 magnetic resonance spectroscopy (31P-MRS). Firstly, to adjust for the effect of differences in cross-sectional muscle area, the maximal cross-sectional area (CSAmax) of the forearm flexor muscles was estimated in each individual using magnetic resonance imaging. Multistage exercise was then carried out with an initial energy production of 1 J.cm-2 CSAmax for 1 min and an increment of 1 J.cm-2 CSAmax every minute to the point of muscle exhaustion. A series of measurements of 31P-MRS were performed every minute. The CSAmax was significantly greater in the student rowers than in the control subjects [19.8 (SD 2.2) vs 17.1 (SD 1.2) cm2, P less than 0.05]. The absolute maximal exercise intensity (J.min-1) was greater in the rowers than in the control subjects. However, the maximal exercise intensity per unit of muscle cross sectional area (J.min-1.cm-2) was not significantly different between the two groups. During mild to moderate exercise intensities, a decrease in phosphocreatine and an increase in inorganic phosphate before the onset of acidosis were significantly less in the rowers, indicating a requirement of less adenosine 5'-diphosphate to drive adenosine 5'-triphosphate production. The onset of acidosis was also significantly delayed in the rowers. No difference was observed in forearm blood flow between the two groups at the same exercise intensity (J.min-1.cm-2).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of formaldehyde on growth of obligate methylotroph Methylobacillus flagellatum in a substrate non-limited continuous culture

The ribulose monophosphate cycle methylotroph Methylobacillus flagellatum was grown under oxyturbidostat conditions on mixtures of methanol and formaldehyde. Formaldehyde when added at low concentration (50 mg/l) increased the methanol consumption and the yield of biomass. The presence of 150–300 mg/l of formaldehyde resulted in an increase of the growth rate from 0.74 to about 0.79–0.82 h^-1. The presence of 500 mg/l of formaldehyde in the inflow decreased culture growth characteristics. Activities of methanol dehydrogenase and enzymes participating in formaldehyde oxidation and assimilation were measured. The enzymological profiles obtained are discussed.Abbreviations MDH methanol dehydrogenase - NAD-linked FDDH NAD-linked formaldehyde dehydrogenase - DLFDDH dye-linked formaldehyde dehydrogenase - DLFDH dye-linked formate dehydrogenase - GPDH glucose-6-phosphate dehydrogenase - PGDH 6-phosphogluconate dehydrogenase - RuMP cycle ribulose monophosphate cycle     

Anaerobic metabolism of Nitrosomonas europaea

Nitrosomonas europaea is capable of maintaining an anaerobic metabolism, using pyruvate as an electron donor and nitrite as an electron acceptor; utilization of nitrite depends upon supply of both pyruvate and ammonia. The role of ammonia in this reaction was not determined. N europaea also assimilates CO₂ anaerobically into cell material in the presence of nitrite (0.5–1.0 mM), pyruvate and ammonia. This reaction was partially inhibited by nitrite which apparently competed with CO₂ for reducing power. Anaerobic nitrite respiration is sensitive to ionophores, FCCP being the most effective.Non-standard-abbreviations TCA trichloroacetic acid - FCCP carbonylcyanide-p-trifluoromethoxyphenylhydrazon     

Genetic,biochemical and immunological evidence for the involvement of two alcohol dehydrogenases in the metabolism of propane by Rhodococcus rhodochrous PNKb1

NAD⁺-dependent propan-1-ol and propan-2-ol dehydrogenase activities were detected in cell-free extracts of Rhodococcus rhodochrous PNKb1 grown on propane and potential intermediates of propane oxidation. However, it was unclear whether this activity was mediated by one or more enzymes. The isolation of mutants unable to utilize propan-1-ol (alcA^-) or propan-2-ol (alcB^-) as sole carbon and energy sources demonstrated that these substrates are metabolized by different alcohol dehydrogenases. These mutants were also unable to utilize propane as a growth substrate indicating that both alcohols are intermediates of propane metabolism. Therefore, propane is metabolized by terminal and sub-terminal oxidation pathways. Westernblot analysis demonstrated that a previously purified NAD⁺-dependent propan-2-ol dehydrogenase (Ashraf and Murrell 1990) was only synthesized after growth on propane and sub-terminal oxidation intermediates (but not acetone), and not propan-1-ol or terminal oxidation intermediates. Therefore, our evidence suggest that another dehydrogenase is involved in the metabolism of propan-1-ol and this agrees with the isolation of the alcA^- and alcB^- phenotypes. The previously characterized NAD⁺-dependent propan-2-ol dehydrogenase from R. rhodochrous PNKb1 is highly conserved amongst members of the propane-utilizing Rhodococcus-Nocardia complex.     

Guanine nucleotide- and muscarinic agonist-dependent phosphoinositide metabolism in synaptoneurosomes from cerebral cortex of immature rats

Guanine nucleotide-, neurotransmitter-, and fluoride-stimulated accumulation of [³H]inositol phosphates ([³H]InsPs) was measured in [³H]inositol-labeled synaptoneurosomes from cerebral cortex of immature (7-day-old) and adult rats, in order to clarify the role of GTP-binding proteins (G-proteins) in modulating phosphoinositide (PtdIns) metabolism during brain development. GTP(S) [Guanosine 5-O-(3-thio)triphosphate] time- and concentration-dependently stimulated PtdIns hydrolysis. Its effect was potentiated by full (carbachol, metacholine) and partial (oxotremorine) cholinergic agonists through activation of muscarinic receptors. The presence of deoxycholate was required to demonstrate agonist protentiation of the guanine nucleotide effect. The response to GTP(S) was higher in adult than in immature rats, while the effect of cholinergic agonists was similar at the two ages examined. At both ages, histamine potentiated the effect of GTP(S), while norepinephrine was ineffective. At both ages, guanosine 5-O-(2-thio)diphosphate [GDP(S)] and pertussis toxin significantly decreased GTP(S)-induced [³H]InsPs formation. The phorbol ester phorbol 12-myristate 13-acetate (PMA), on the other hand, did not inhibit the guanine nucleotide response in synaptoneurosomes from immature rats. NaF mimicked the action of GTP(S) in stimulating PtdIns hydrolysis. Its effect was not affected by carbachol and was highly synergistic with that of AlCl₃, according to the concept that fluoroaluminate (AlF₄ ^–) is the active stimulatory species. No quantitative differences were found in the response to these salts between immature and adult animals. These results provide evidence that, in both the immature and adult rat brain, neuroreceptor activation is coupled to PtdIns hydrolysis through modulatory G-proteins.     

Energy metabolism and selective neuronal vulnerability following global cerebral ischemia

A short period of global ischemia results in the death of selected subpopulations of neurons. Some advances have been made in understanding events which might contribute to the selectivity of this damage but the cellular changes which culminate in neuronal death remain poorly defined. This overview examines the metabolic state of tissue in the post-ischemic period and the relationship of changes to the development of damage in areas containing ischemia-susceptible neurons. During early recirculation there is substantial recovery of ATP, phosphocreatine and related metabolites in all brain regions. However, this recovery does not signal restitution of normal energy metabolism as reductions of the oxidative metabolism of glucose are seen in many areas and may persist for several days. Furthermore, decreases in pyruvate-supported respiration develop in mitochondria from at least one ischemia-susceptible region at times coincident with the earliest histological evidence of ischemia-induced degeneration. These mitochondrial changes could simply be an early marker of irreversible damage but the available evidence is equally consistent with these contributing to the degenerative process and offering a potential site for therapeutic intervention.Submitted as an Overview article for the volume of Neurochemical Research in honor of Alan N. Davison.     

一个拖延了半个多世纪的研究课题的总结汇报

本文是汤佩松教授为北京植物生理学会1992年年会准备的发言稿,并以庆祝中国植物生理学会成立30周年。后“北京年会”因故未开,本刊应汤老要求按原样发表于此(并附英文摘要),以飨读者。