Membrane receptors and hormone action: Recent developments

Vasopressin binds to microvessels from rat hippocampus

Recent evidence suggests that vasopressin may influence the permeability of the endothelium of brain capillaries. We measured the binding of [125I]arginine-8-vasopressin ([125I]AVP) to microvessels isolated from different regions of the rat brain. The study revealed saturable and specific binding of [125I]AVP to microvessels isolated from hippocampus. Scatchard analysis confirmed a single class of high affinity sites with an equilibrium dissociation constant, Kd, of 3.2 nM and an apparent maximal binding capacity of 205 fmol/mg protein. No binding was observed to microvessels from neocortex and striatum.

Dihydrotetrabenazine binding and monoamine uptake in mouse brain regions

Binding of [2-3H]dihydrotetrabenazine and uptake of 5-hydroxytryptamine (serotonin) were studied in mouse brain cerebellum, pons-medulla, frontal cortex, hypothalamus, hippocampus, and striatum. Binding of [2-3H]dihydrotetrabenazine to homogenates of these brain areas is stable for several hours and occurs at a homogeneous class of binding sites (KD = 2.4 nM). Subcellular fractionation and regional distribution of [2-3H]dihydrotetrabenazine binding and serotonin uptake showed that the ligand binds to synaptic vesicles. Dihydrotetrabenazine inhibited serotonin uptake with the same inhibitory constant (IC50 = 2.6 nM) for synaptic vesicles from brain regions containing 3,4-dihydroxyphenylethylamine (dopamine) or serotonin and noradrenaline in different proportions. This constant is similar to the KD of [2-3H]dihydrotetrabenazine, which suggests that the latter ligand labels specifically and with the same affinity the monoamine transporter from various monoaminergic synaptic vesicles. Therefore the regional differences in central monoamine depletion induced in vivo by tetrabenazine are not due to regional differences in inhibition of vesicular monoamine uptake. Moreover, vesicular monoamine transporters from the central and peripheral nervous systems of various mammals and from bovine adrenal glands have comparable affinity for substrate and inhibitor (Km values for serotonin and IC50 for dihydrotetrabenazine are about 0.8 microM and 3 nM, respectively) and comparable turnover number (10-35 molecules transported per transporter per minute), which suggests the involvement of a common transporter molecule in the process of monoamine uptake by the various monoaminergic storage vesicles.

Mass spectral analysis of murine epidermal growth factor

Fast atom bombardment mass spectrometry has been used to characterize epidermal growth factor isolated from mouse submaxillary glands. The preparation is found to consist of two peptides, one of which has the average molecular weight predicted for the familiar gene product. The molecular weight of the second component is found to be reduced by the mass of one asparagine residue. These observations are discussed in light of previous reports of heterogeneity.

Hyaluronate binding proteins also bind to fibronectin, laminin and collagen

Small molecular weight proteins, isolated from the culture medium of embryonic chick heart fibroblasts and 3T3 cell lines by hyaluronate affinity chromatography, bind in order of apparent affinity, to hyaluronate, fibronectin, collagen and laminin. Such proteins isolated from the MSV-transformed 3T3 cell line bind in greater amounts to the nectins and hyaluronate than do similar proteins isolated from heart fibroblasts or 3T3 cells. These small hyaluronate binding proteins are immunologically distinct from other well characterized proteins such as laminin, fibronectin, bovine serum albumin and actin. Their relationship to other small, extracellular proteins and their possible role in structuring of extracellular matrix are discussed.

Total internal reflection fluorescence study of energy transfer in surface-adsorbed and dissolved bovine serum albumin

A simple adaptation of a commercial spectrofluorometer allows selective excitation of fluorescent biomolecules adsorbed to a solid surface while they are in equilibrium with a bulk solution. As a demonstration of this technique, we have detected a change in the effective singlet-singlet energy transfer in fluorescence-labeled bovine serum albumin (BSA) upon adsorption to a fused silica surface. The technique combines total internal reflection fluorescence excitation of surface-adsorbed BSA with a fluorescence spectroscopic examination of energy transfer between two different fluorophores that are covalently bound to amino groups in each BSA molecule. Two donor--acceptor pairs were used, 4-chloro-7-nitro-2,1,3-benzoxadiazole-rhodamine and dansyl-eosin. For studies of surface-adsorbed BSA, we constructed a device in which the excitation light of a standard fluorescence spectrometer totally internally reflects from a surface at which adsorbed BSA is in equilibrium with the bulk solution. A shallow evanescent wave is created, which excites fluorescence from only those BSA molecules in close proximity to the surface. Spectral examination shows significantly less effective singlet-singlet energy transfer from the donor to the acceptor in surface-adsorbed BSA relative to that in native bulk-dissolved BSA. Under appropriate and reasonable assumptions, the energy transfer change between native and adsorbed states of fluorescent BSA can be interpreted as a conformational change of BSA upon adsorption.

Alterations of synaptic high and low affinity opiate binding sites after acute and chronic morphine administration in mice

1. The opiate receptor binding to whole brain synaptic membranes obtained from mice subjected to acute and chronic morphine administration and precipitated withdrawal was investigated. 2. The number of high and low affinity binding sites was significantly higher in morphine tolerant group than in control; acute administration of morphine also induced an increase in the number of binding sites, though this increase was significantly lower as compared to tolerant group. 3. The affinity of both high and low affinity sites, however remained unchanged after acute or chronic treatment. In contrast, both the affinity and the number of binding sites were significantly reduced after precipitated withdrawal, compared to the tolerant group. 4. Sodium chloride enhanced the antagonist binding and inhibited the agonist binding in both tolerant and non-tolerant groups. 5. It is concluded that (a) the increase in the number of receptors during tolerance development is an extension of the acute effect of morphine, (b) the character of tolerant membrane is qualitatively same as that of non-tolerant membrane, (c) upon withdrawal, the receptor population is brought back to normal; the altered higher, affinity after withdrawal may be a compensatory effect as a corollary to the withdrawal-induced decrease in receptor population.