Membrane-bound histamine N-methyltransferase in mouse brain:: possible role in the synaptic inactivation of neuronal histamine
Membrane-bound histamine N-methyltransferase in mouse brain:: possible role in the synaptic inactivation of neuronal histamine
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DOI：
10.1046/j.1471-4159.2002.01063.x
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发表时间：
2002-09-01
影响因子：
4.7
通讯作者：
Hough, LB
中科院分区：
文献类型：
--
作者：
Barnes, WG;Hough, LB
在中枢神经系统中，组胺是一种被组胺N -甲基转移酶（HNMT）灭活的神经递质，HNMT是一种定位于神经元和内皮细胞细胞质的可溶性酶。然而，细胞外组胺（一种生理pH值的带电分子）如何到达细胞内HNMT尚不清楚。目前的研究探讨了小鼠脑神经末梢部分（突触体）组胺失活的两种可能途径：(i)组胺摄取和（ii） HNMT组胺代谢。完整的突触体表现出较弱的温度依赖性组胺摄取（0.098 pmol/min-mg蛋白），但通过HNMT代谢组胺的能力要大得多（1.4 pmol/min-mg蛋白）。对突触体内HNMT分布的测定表明，缺乏可溶性HNMT的突触体膜提供的HNMT活性与完整的突触体相当（分别为14.3 +/- 2.2和18.2 +/- 4.3 pmol/min-tube），并表明组胺甲基化活性与膜组分有关。支持这一假设的其他实验发现包括：(i)组胺代谢物远端甲基组胺（tMH）仅在具有完整突触体的HNMT检测后的上清部分中被发现；（ii）经0.1% Triton X-100增溶后，膜结合的HNMT活性增加了6.5倍；（iii） S2部分、破裂的突触体和突触体膜的HNMT活性在-20℃下储存23天后显示出不同的稳定性。综上所述，这些研究证明了膜结合HNMT存在的功能证据。尽管分子研究尚未确定这种活性的性质，但目前的工作表明，生物活性组胺的水平可能受到细胞外过程的控制。
In the CNS, histamine is a neurotransmitter that is inactivated by histamine N -methyltransferase (HNMT), a soluble enzyme localized to the cytosol of neurons and endothelial cells. However, it has not been established how extracellular histamine, a charged molecule at physiological pH, reaches intracellular HNMT. Present studies investigated two potential routes of histamine inactivation in mouse brain nerve terminal fractions (synaptosomes): (i) histamine uptake and (ii) histamine metabolism by HNMT. Intact synaptosomes demonstrated a weak temperature-dependent histamine uptake (0.098 pmol/min-mg protein), but contained a much greater capacity to metabolize histamine by HNMT (1.4 pmol/min-mg protein). Determination of the distribution of HNMT within synaptosomes revealed that synaptosomal membranes (devoid of soluble HNMT) contribute HNMT activity equivalent to intact synaptosomes (14.3 +/- 2.2 and 18.2 +/- 4.3 pmol/min-tube, respectively) and suggested that histamine-methylating activity is associated with the membrane fraction. Additional experimental findings that support this hypothesis include: (i) the histamine metabolite tele-methylhistamine (tMH) was found exclusively in the supernatant fraction following an HNMT assay with intact synaptosomes; (ii) the membrane-bound HNMT activity was shown to increase 6.5-fold upon the solubilization of the membranes with 0.1% Triton X-100; and (iii) HNMT activity from the S2 fraction, ruptured synaptosomes, and synaptosomal membranes displayed different stability profiles when stored over 23 days at -20degreesC. Taken together, these studies demonstrate functional evidence for the existence of membrane-bound HNMT. Although molecular studies have not yet identified the nature of this activity, the present work suggests that levels of biologically active histamine may be controlled by an extracellular process.