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排序方式: 共有1987条查询结果,搜索用时 15 毫秒
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The role of the polypeptide matrix in electron transfer processes in proteins has been studied in two distinct systems: first
in a protein where the induced ET is artificial, and second as part of the catalytic cycle of an enzyme. Azurins are structurally
well-characterized blue single-copper proteins consisting of a rigid β-sheet polypeptide matrix. We have determined rate constants
and activation parameters for intramolecular long-range electron transfer between the disulfide radical anions (generated
by pulse radiolysis) and the copper(II) centre as a function of driving force and nature of the intervening medium in a large
number of wild-type and single-site-mutated proteins. In ascorbate oxidase, for which the three-dimensional structure is equally
well characterized, the internal ET from the type-I Cu(I) to the trinuclear Cu(II) centre has been studied. We find that the
results correlate well with distance through well-defined pathways using a through-bond electron tunnelling mechanism.
Received: 2 January 1997 / Accepted: 6 February 1997 相似文献
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A "fatigue" of acetylcholine (ACh) release is described in cholinergic synaptosomes stimulated with the calcium ionophore A23187 or gramicidin. A small conditioning calcium entry, which did not trigger a large ACh release, led to a decrease of transmitter release elicited by a second large calcium influx. This fatigue was half-maximal at approximately 30 microM external calcium and developed in a few minutes. In contrast, activation of release by calcium was very rapid and was half-maximal at approximately 0.5 mM external calcium. Activation and desensitization of release could be attributed to the recently identified presynaptic membrane protein, the "mediatophore." Proteoliposomes equipped with purified mediatophore showed a calcium-dependent activation and "fatigue" of ACh release similar to that of synaptosomes. It was found that the ionophore A23187 rapidly equilibrated internal and external calcium concentrations in proteoliposomes. Thus, the external calcium concentration gave the internal concentration required for activation or desensitization of proteoliposomal ACh release. The mediatophore showed remarkable calcium binding properties (20 sites/molecule) with a KD of 25 microM. The physiological implications of desensitization on the organization of release sites are discussed. 相似文献
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M Denechaud J M Israel F Belloc J D Vincent 《Comptes rendus de l'Académie des sciences. Série III, Sciences de la vie》1987,304(16):427-431
The different antehypophysical cell types which synthetize and release somatotroph (GH), corticothroph (ACTH), gonadotroph (LH-FSH) and lactotroph (PRL) hormones were analysed. The experiments were performed on hypophyses from five groups of animals: adult males, 14 days-old female, adult females, gestating adult females and lactating adult females. The cells were analysed by immunofluorescence using flow cytometry. For each of the hormones studied, there was a characteristic spectral distribution of cells. The evolution of cell size and granular content with respect to sex and physiological state of each group was studied by the analysis of diffused light. Small, slightly granular cells represented 50% of the cell population in males and 14 day-old females but only 8% in gestating or lactating females. The study of the cell cycle showed the presence of dividing cells in the population of large, granular cells from gestating and from lactating females. No features of cell division were observed in the population of small, slightly granular cells. This study indicates the potential value of multiparametric analysis in the separation of pure sub-populations of antehypophysial cells. 相似文献
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M B Honnebier J P Figueroa J Rivier W Vale P W Nathanielsz 《Journal of developmental physiology》1989,12(4):225-232
Previous observations have demonstrated that under several different circumstances the pregnant rhesus monkey myometrium shows a spontaneous shift in activity from contractures to contractions around the beginning of the hours of darkness. Preliminary studies were conducted to demonstrate that the competitive oxytocin antagonist ([1-beta-Mpa(beta-(CH2)5)1,) Me)Tyr2, Orn8] oxytocin was effective in vivo in inhibiting oxytocin induced contraction type myometrial activity in the pregnant rhesus monkey in the last third of gestation. Four pregnant and one fetectomized rhesus monkey (98-141 days gestation) received one intra-arterial dose of oxytocin antagonist to study its ability to inhibit myometrial contractions occurring spontaneously around the onset of prevailing nighttime. In three pregnant monkeys (105-121 days gestation) maternal arterial plasma oxytocin levels were measured at 4-h intervals for a period of 48 h. Maternal plasma oxytocin concentration was maximal during the early hours of darkness and demonstrated a significant 24-h rhythm. From the combined results of both experiments it may be concluded that circulating oxytocin and/or a change in one of the many potential regulatory sites for oxytocin function plays a role in the switch from contractures to contractions that occurs around the beginning of the hours of darkness. 相似文献