Eisenhofer G, Goldstein D S, Ropchak T G, Kopin I J
Clinical Neuroscience Branch, National Institute of Neurological and Communicative Disorders and Stroke; Bethesda, Maryland.
J Neurochem. 1988 Oct;51(4):1204-13. doi: 10.1111/j.1471-4159.1988.tb03088.x.
To elucidate the source and physiological significance of plasma 3,4-dihydroxyphenylalanine, the immediate product of the rate-limiting step in catecholamine biosynthesis, plasma 3,4-dihydroxyphenylalanine was quantified in conscious rats after administration of reserpine, desipramine, clorgyline, or forskolin, treatments that affect tyrosine hydroxylase activity. Plasma 3,4-dihydroxyphenylalanine was also examined during infusions of norepinephrine with or without clorgyline, reserpine, or desipramine pretreatment. After reserpine, the plasma 3,4-dihydroxyphenylalanine level decreased by 22% and then increased by 40%, a result consistent with modulation of tyrosine hydroxylase activity first by an increased axoplasmic norepinephrine content and then by depletion of norepinephrine stores. After desipramine, the plasma 3,4-dihydroxyphenylalanine level decreased by 20%, reflecting the depressant effect of neuronal uptake blockade on norepinephrine turnover. Forskolin increased the plasma 3,4-dihydroxyphenylalanine level by 30%, consistent with activation of tyrosine hydroxylase by cyclic AMP-dependent phosphorylation. Acute administration of clorgyline was without effect on the plasma 3,4-dihydroxyphenylalanine level. Norepinephrine infusions decreased the plasma 3,4-dihydroxyphenylalanine concentration, as expected from end-product inhibition of tyrosine hydroxylase. Pretreatment with desipramine prevented the norepinephrine-induced decrease in plasma dihydroxyphenylalanine content, indicating that inhibition of tyrosine hydroxylase required neuronal uptake of norepinephrine. Both reserpine and clorgyline augmented the norepinephrine-induced decrease in plasma 3,4-dihydroxyphenylalanine level, suggesting that retention of norepinephrine in the axoplasm--due to inhibition of norepinephrine sequestration into storage vesicles or catabolism--caused further inhibition of tyrosine hydroxylase. Changes in plasma 3,4-dihydroxyphenylalanine concentration during norepinephrine infusions were negatively correlated with those in plasma 3,4-dihydroxyphenylglycol level, a finding consistent with modulation of tyrosine hydroxylase activity by axoplasmic norepinephrine. In reserpinized animals, clorgyline and norepinephrine infusion together decreased the plasma 3,4-dihydroxyphenylalanine content by 50%, a result demonstrating that hydroxylation of tyrosine was depressed by at least half. The results indicate that quantification of plasma 3,4-dihydroxyphenylalanine can provide a simple and direct approach for examination of the rate-limiting step in catecholamine biosynthesis.
为阐明血浆3,4 - 二羟基苯丙氨酸（儿茶酚胺生物合成限速步骤的直接产物）的来源及生理意义，我们对清醒大鼠在给予利血平、地昔帕明、氯吉兰或福斯高林（这些处理会影响酪氨酸羟化酶活性）后血浆3,4 - 二羟基苯丙氨酸进行了定量分析。在输注去甲肾上腺素期间，无论有无氯吉兰、利血平或地昔帕明预处理，也对血浆3,4 - 二羟基苯丙氨酸进行了检测。给予利血平后，血浆3,4 - 二羟基苯丙氨酸水平先下降22%，随后上升40%，这一结果与酪氨酸羟化酶活性先因轴浆中去甲肾上腺素含量增加而后因去甲肾上腺素储存耗竭受到调节相一致。给予地昔帕明后，血浆3,4 - 二羟基苯丙氨酸水平下降20%，反映出神经元摄取阻断对去甲肾上腺素周转的抑制作用。福斯高林使血浆3,4 - 二羟基苯丙氨酸水平升高30%，与环磷酸腺苷依赖性磷酸化激活酪氨酸羟化酶相一致。急性给予氯吉兰对血浆3,4 - 二羟基苯丙氨酸水平无影响。正如酪氨酸羟化酶终产物抑制所预期的那样，输注去甲肾上腺素使血浆3,4 - 二羟基苯丙氨酸浓度降低。用地昔帕明预处理可防止去甲肾上腺素引起的血浆二羟基苯丙氨酸含量下降，表明酪氨酸羟化酶的抑制需要去甲肾上腺素的神经元摄取。利血平和氯吉兰均增强了去甲肾上腺素引起的血浆3,4 - 二羟基苯丙氨酸水平下降，提示由于去甲肾上腺素向储存囊泡的隔离或分解代谢受到抑制，导致轴浆中去甲肾上腺素潴留，从而进一步抑制酪氨酸羟化酶。输注去甲肾上腺素期间血浆3,4 - 二羟基苯丙氨酸浓度的变化与血浆3,4 - 二羟基苯乙二醇水平的变化呈负相关，这一发现与轴浆中去甲肾上腺素对酪氨酸羟化酶活性的调节相一致。在利血平化的动物中，氯吉兰和去甲肾上腺素输注共同使血浆3,4 - 二羟基苯丙氨酸含量下降50%，这一结果表明酪氨酸的羟化至少被抑制了一半。结果表明，血浆3,4 - 二羟基苯丙氨酸的定量分析可为检测儿茶酚胺生物合成中的限速步骤提供一种简单直接的方法。
