5''-Nucleotidases in rat heart. Evidence for the occurrence of two soluble enzymes with different substrate specificities.

Chromatography of soluble proteins from rat heart on phosphocellulose columns separates two 5'-nucleotidases. The first to emerge from the column shows a preference for AMP over IMP as substrate, whereas the second shows a preference for IMP over AMP. The properties of the IMP-preferring enzyme, including the conditions under which it is eluted from phosphocellulose columns, show it to be the enzyme studied by Itoh, Oka & Ozasa [Biochem. J. (1986) 235, 847-851]. The kinetic properties of the AMP-preferring enzyme indicate that it is likely to be the enzyme responsible for the production of adenosine under conditions of hypoxia and increased work load, and with metabolic stresses such as a high load of acetate.     

Nitric oxide control of steroidogenesis: endocrine effects of NG-nitro-L-arginine and comparisons to alcohol.

Recent studies suggest that nitric oxide (NO) may regulate hormone biosynthesis and secretion. This was tested by treating male rats with NG-nitro-L-arginine methyl ester (NAME), a NO synthase inhibitor, and measuring serum and testicular interstitial fluid testosterone and serum corticosterone, luteinizing hormone (LH), and prolactin (PRL). The effect of NG-nitro-L-arginine (NA), a less-soluble form of the same NO synthase inhibitor, on the reproductive suppressant actions of alcohol was also examined. NAME increased testosterone and corticosterone secretion dose-dependently without affecting LH and PRL secretion. The alcohol-induced suppression of testosterone or LH secretion was not altered by treatment with NA. Although effects of NAME and NA on other systems may be involved, these results indicate that testicular and adrenal steroidogenesis are negatively regulated by endogenous NO and that NO does not regulate LH and PRL secretion or inhibit the testicular steroidogenic pathway in the same way as alcohol.     

l-Phenylalanyl-l-Glutamate-Stimulated, Chloride-Dependent Glutamate Binding Represents Glutamate Sequestration Mediated by an Exchange System

Stimulation of glutamate binding by the dipeptide L-phenylalanyl-L-glutamate (Phe-Glu) was inhibited by the peptidase inhibitor bestatin, suggesting that the stimulation was caused by glutamate liberated from the dipeptide and not by the dipeptide itself. It further suggests that this form of glutamate binding should be reinterpreted as glutamate sequestration and that stimulation of binding both by dipeptides and after preincubation with high concentrations of glutamate is likely to be due to counterflow accumulation. Several other criteria indicate that most of glutamate binding stimulated by chloride represents glutamate sequestration: Binding is reduced when the osmolarity of the incubation medium is increased, when membranes incubated with [3H]glutamate are lysed before filtration, and when membranes are made permeable by transient exposure to saponin. Moreover, dissociation of bound glutamate after a 100-fold dilution of the incubation medium is accelerated about 50 times by the addition of glutamate to the dilution medium. This result would be anomalous if glutamate were bound to a receptor site; it suggests instead that glutamate is transported in and out of membrane vesicles by a transport system that preferentially mediates exchange between internal and external glutamate. Glutamate binding contains a component of glutamate sequestration even when measured in the absence of chloride. Sequestration is adequately abolished only after treating membranes with detergents; even extensive lysis, sonication, and freezing/thawing may be insufficient.     

Monoamine oxidase inhibitory properties of milacemide in rats

Milacemide is a glycine prodrug with reported antiepileptic antimyoclonic properties. In this study, milacemide increased "wet dog shakes" in rats pretreated with 5-Hydroxytryptophan (5-HTP) and carbidopa. Moreover, it worsened the serotonin behavior syndrome precipitated by 5-HTP and the monoamine oxidase inhibitor tranylcypromine. The serotonin syndrome was also elicited by the combination of milacemide and 5-HTP without tranylcypromine. In vitro, milacemide inhibited both monoamine oxidase A and B from the frontal cortex of rats, to a greater extent for MAO B. This drug is currently under investigation in humans as an antiepileptic agent and precautions for the consequences of monoamine oxidase inhibition should be considered when the drug is used in high doses.     

小麦叶绿体中CTK结合蛋白的结合活性研究

叶绿体中存在着与细胞分裂素(CTK)专一结合的蛋白质。这一蛋白与6-苄氨基嘌呤(6 BA)的亲和力很强,解离常数达3.7×10~(-8)mol/L。最大结合量为10.7 pmol 6 BA/mg蛋白,Seatchard分析表明只有一类结合位点。不同的叶绿体纯化步骤对CTK结合蛋白的活性有不同的影响,分离步骤少而快速的差速离心法可以得到具有较高结合活性的叶绿体。叶绿体经分离纯化后,低温保存时的结合活性较稳定,-20℃以下可以较长期保存。用EDTA预处理叶绿体,不降低CTK结合蛋白对6 BA的结合活性,而用高浓度的NaCl处理,可以使叶绿体结合6BA的能力明显下降。这说明EDTA不能使CTK结合蛋白从叶绿体膜系统表面解离,而高浓度NaCl则有这种可能性。     

Sex determination and differentiation in organisms other than higher plants

The understanding of sex determination in general, but in particular in mammals, has been a subject of scientific speculation for a long time. It has been shown that in many vertebrate and invertebrate species, the sex of an individual is determined by the individual's chromosomal constitution. Initial studies of classical genetic searching for sex-transforming mutations and the scrupulous analyses of modified phenotypes have shed light on the mechanism(s) of sex-determination. They paved the road to successful studies at molecular level. After a brief review on sex determination in chosen model species, the “Drosophila system” is presented to exemplify a possible general principle for sex determinism.     

Specific binding of three neurotoxins with phospholipase A2 activity to synaptosomal membrane preparations from the guinea pig brain

Snake presynaptic toxins such as crotoxin, β-bungarotoxin and taipoxin block neuromuscular transmission through inhibiting the release of acetylcholine by their phospholipase A₂ activities. On the other hand, many other phospholipase A₂s show little neurotoxicity. It is likely that the difference lies in whether high affinity binding to nerve cell membranes exists or not. To test this idea, crotoxin, β-bungarotoxin and taipoxin were first radioactively labeled with Na(¹²⁵I) without loss of their neurotoxicity. Using the radioactive toxins we have found that each of the three showed specific binding to synaptosomal membranes from guinea pig brain. In contrast, we could not detect specific binding of a non-neurotoxic pancreatic phospholipase A₂. Crotoxin and taipoxin, but not β-bungarotoxin, also bound specifically to membrane preparation from other tissues. The binding of each toxin was not greatly affected by the other two toxins. The photoaffinity labeling technique has been used to obtain further information about the components which bind crotoxin. For this purpose, (¹²⁵I) crotoxin was derivatized with N-hydroxysuccinimidyl-4-azidobenzoate. Autoradiographic analysis of the membranes following photoirradiation in the presence of the modified crotoxin revealed that an 85K dalton component was preferentially covalently conjugated with the crotoxin analogue in a specific manner.     

Comparative antioxidant enzyme study in freshwater fishes. II. Distribution of antioxidant enzymes and lipid peroxidation in omnivorous fish organs

Comparative studies were performed on the activities of superoxide dismutases, catalase and glutathione peroxidase in organ homogenates from three omnivorous fishes, the barbel, crucian carp and common carp. The lipid peroxidation and protein contents of organ homogenates were also compared. These comparative measurements primarily provide control values for subsequent toxicological examinations. The highest total superoxide dismutase activities were found in the liver or roe, kidney, heart and spleen in every cases. The antioxidant enzyme activities and other studied parameters of the organ homogenates partly appear to depend on the feeding mode, but are rather characteristic of the fish variety.