Choline transport is not coupled to acetylcholine synthesis
Choline transport is not coupled to acetylcholine synthesis
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胆碱转运不与乙酰胆碱合成耦合
DOI：
10.1038/279542a0
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发表时间：
1979
期刊：
影响因子：
64.8
通讯作者：
R. Marchbanks
中科院分区：
文献类型：
--
作者：
P. Kessler;R. Marchbanks
有人提出1 -3，胆碱转运，特别是由所谓的高亲和力系统进行的转运，必然与乙酰胆碱合成偶联，因此只有刚刚通过转运机制进入的胆碱才会被乙酰化。如果是这样的话，这将意味着胆碱乙酰转移酶被结合在膜上的转运位点附近。这个问题很重要，因为它对突触的分子组织的影响，也因为亚细胞分级分离研究4，5相当清楚地表明，在细胞内的离子强度，胆碱乙酰转移酶是可溶的，而不是绑定到血浆或突触囊泡膜（如果强制性耦合的假设是正确的，这将是预期的）。这方面的证据是间接的，并依赖于这样的发现，即胆碱及其类似物在原位乙酰化的半最大浓度更接近于其转运的半最大浓度，而不是其被分离的酶乙酰化的浓度。然而，这只需要意味着运输和合成之间的耦合，如果后者是速率控制，从乙酰胆碱（ACh）的释放和合成速率的调查6，似乎胆碱乙酰转移酶的活性大大超过正常的需求。已经注意到运输和合成的强制性偶联的某些例外情况，但这些例外情况涉及胆碱的合成类似物7或不寻常的代谢物8，9。乙酰辅酶A水平的降低会降低乙酰胆碱浓度，而不会伴随胆碱转运的减少10，但这一点和以前的研究都不排除残留乙酰胆碱合成保持偶联的可能性;也就是说，强制性依赖于刚刚穿过膜转运的胆碱。我们在这里描述了一个实验，我们相信，表明胆碱，最近已被运输没有特权进入胆碱乙酰转移酶在突触末端。相反，它与预先存在的胆碱池混合，因此在运输和合成之间没有耦合。
IT has been proposed1–3 that choline transport, particularly that performed by the so-called high-affinity system, is obligatorily coupled to acetylcholine synthesis and therefore only choline that has just entered by the transport mechanism is acetylated. If this were so it would imply that the enzyme choline acetyltransferase is bound close to the transport sites on the membrane. The problem is important because of its implications for the molecular organisation of the synapse and also because subcellular fractionation studies4,5 show fairly clearly that, at the ionic strengths prevailing inside the cell, choline acetyltransferase is soluble and not bound to the plasma or synaptic vesicle membrane (as would be expected if the hypothesis of obligatory coupling is true). The evidence for this is indirect and relies on the finding that the half-maximal concentration for acetylation of choline and its analogues in situ approximates more nearly to the half-maximal concentration of their transport than of their acetylation by the isolated enzyme1. However, this need only imply a coupling between transport and synthesis if the latter is rate controlling, and from a survey6 of the rates of release and synthesis of acetylcholine (ACh) it seems that the activity of choline acetyltransferase is in considerable excess of normal requirements. Certain exceptions to the obligatory coupling of transport and synthesis have been noted but these involve synthetic analogues of choline7 or unusual preparations8,9. Reduction of the acetyl-CoA levels reduces ACh concentrations without a concomitant reduction in choline transport10 but neither this nor the previous studies exclude the possibility that the residual ACh synthesis remains coupled; that is, obligatorily dependent on choline that has just been transported across the membrane. We describe here an experiment which we believe demonstrates that choline that has been recently transported has no privileged access to choline acetyltransferase in the synaptic terminal. Instead it mixes with the pre-existing pool of choline and therefore there is no coupling between transport and synthesis.