运动后乳酸的转化

What Happens to Lactic Acid After Exercise?

　

 

当你的肌肉在短时间的运动中无法获得足够的氧气时，他们就开始使用乳酸发酵的途径。乳酸发酵产生一种叫做乳酸（Lactic Acid）或乳酸盐（Lactate）的三碳化合物，作为葡萄糖分解的副产品。运动中的肌肉细胞不能利用乳酸，但你的肝脏在运动后会将其转化为葡萄糖。

　

　

　

 

血液的乳酸

当乳酸在你的肌肉细胞内聚集时，它进入你的血液。你的肝脏吸收了血循环的乳酸。当你休息的时候，你的肝脏正忙着将乳酸氧化（需要氧气）成丙酮酸（pyruvic acid），通过一种叫做乳酸脱氢酶(lactic dehydrogenase)的酶催化反应。该酶利用从乳酸中去除的电子，将NAD的分子还原为NADH。丙酮酸通过转运体进入被称为线粒体的小的胶囊状结构，在那里它可能会遇到一对不同的命运。

　

　

　

 

三羧酸循环 (Citric Acid Cycle)

在线粒体内，丙酮酸可以通过一种叫做丙酮酸脱氢酶复合物的酶转化为乙酰辅酶A和CO2。在这种情况下，乙酰辅酶A会进入一种叫做柠檬酸循环的生化途径，你的肝细胞通过氧化这些碳来提取能量,以三磷酸腺苷或ATP的形式储存。然而，以这种方式，肝脏仅满足了它自己的需求，而不是其他细胞的需求。肝脏还需要将乳酸转化为葡萄糖。它是通过一种叫做糖异生（gluconeogenesis））的过程来实现的。

　

 

糖异生（Gluconeogenesis）

当运动后乳酸在你的肝细胞大量存在时，肝脏葡萄糖生成途径与在其他时间所使用的有一点不同。它从线粒体开始，在线粒体中，一种叫做丙酮酸羧化酶的酶在丙酮酸中加入了一种碳酸氢盐分子，并将其转化为草酰乙酸。这种反应需要能量消耗以ATP分子的形式。接下来，另一种叫做线粒体PEP羧激酶的酶将草酰乙酸转化为磷酸烯醇丙酮酸或PEP和游离二氧化碳（CO2）。这一步还需要消耗GTP分子的形式的能量。PEP羧激酶所产生的PEP是从线粒体中输出的，通过在细胞内的一系列有9种酶催化的反应转化为葡萄糖。

 

效果

葡萄糖（Glucose) 转化为乳酸(Lactate)，乳酸转化为葡萄糖的一系列事件被称为Cori循环（Cori Cycle）。你的肌肉最终从葡萄糖分解和乳酸发酵获得的能量要比你的肝脏消耗乳酸，使乳酸重新转化为葡萄糖。因此，Cori循环需要净能量消耗。

　

　

　

　

你的身体在剧烈运动时使用它，当你的血液不能为你的肌肉提供他们所需的氧气时。在这种情况下，乳酸发酵成为你的肌肉能保持葡萄糖代谢作为能量的唯一途径。

 

　

Cori循环的局限性

使用Cori循环，人体能够将代谢产物转化为肌肉的能量来源。然而，它不能无限地继续这样做。

 

与许多其他自然周期类似，Cori循环并不是完全封闭的循环。在肌肉中，糖酵解产生两个ATP的单位。然而，肝脏消耗6个ATP来进行糖原生成过程。Cori循环也需要氧气的初始引入，没有它，它就不能开始。因此，最终，肌肉必然需要新的葡萄糖和氧气供应。

　

如果体力活动过于剧烈，肌肉的能量需求就会超过Cori循环从乳酸中再生出葡萄糖的能力。这将导致一种称为乳酸酸中毒的情况，它是系统中过量乳酸的累积。乳酸酸中毒会降低血液的pH值，从而导致组织损伤。它还会引起与恐慌有关的症状，如过度换气、腹部绞痛、呕吐等，这些都是人体的自然防御机制，旨在减缓剧烈的活动，防止永久性损伤的发生。

　

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What Happens to Lactic Acid After Exercise?

BY  JOHN BRENNAN  SEPT. 11, 2017

　

When your muscles can't get enough oxygen during a short burst of exercise, they start to make use of a pathway called lactic acid fermentation, which generates a small three-carbon compound called lactic acid or lactate as a byproduct of glucose breakdown. Lactic acid is not useful to your muscle cells, but your liver turns it back into glucose later on after exercise.

 

 

Bloodstream

 

As lactic acid accumulates inside your muscle cells, it enters your bloodstream. Your liver soaks up the circulating lactate. Later on while you are resting, your liver is busy oxidizing the lactic acid to pyruvate through a reaction catalyzed by an enzyme called lactate dehydrogenase. The enzyme uses the electrons removed from lactate to reduce a molecule of NAD to NADH. Pyruvate enters small capsule-shaped structures called mitochondria via a transporter, where it may meet with one of a couple different fates.

 

Citric Acid Cycle

 

Inside the mitochondria, pyruvate may be converted to acetyl-CoA and CO2 by an enzyme called the pyruvate dehydrogenase complex. In this case, the acetyl-CoA will feed into a biochemical pathway called the citric acid cycle, and your liver cell will use the energy it extracts by oxidizing these carbons to store energy in the form of adenosine triphosphate or ATP. By so doing, however, the liver merely satisfies its own requirements and not those of other cells. The liver also needs to turn the lactic acid into glucose. It does so through a process called gluconeogenesis.

 

Gluconeogenesis

 

When lactic acid is abundant in your liver cells after exercise, the gluconeogenesis pathway is a little bit different from the one your liver employs at other times. It begins in the mitochondria, where an enzyme called pyruvate carboxylase adds a molecule of bicarbonate to pyruvate and converts it to oxaloacetate. This reaction requires energy expenditure in the form of a molecule of ATP. Next, another enzyme called mitochondrial PEP carboxykinase converts oxaloacetate into phosphoenolpyruvate or PEP and free carbon dioxide. This step also requires energy investment in the form of a molecule of GTP. The PEP produced by PEP carboxykinase is exported from the mitochondria and converted back to glucose through a series of nine enzyme-catalyzed reactions inside the cell.

 

Effects

 

The series of events by which glucose is converted into lactate and back again is called the Cori cycle. Your muscles ultimately gain less energy from glucose breakdown and lactic acid fermentation than your liver must expend to make the lactate back into glucose. Consequently, the Cori cycle entails a net energy loss. Your body makes use of it during intense workouts, when your bloodstream can't furnish your muscles with all the oxygen they need. At times like these, lactic acid fermentation becomes the only way your muscles can keep metabolizing glucose for fuel.

　

Limitations of the Cori Cycle

Using the Cori cycle, the human body is able to convert metabolic by products into a source of energy for the muscles. However, it cannot continue to do so infinitely.

 

Similar to many other natural cycles, the Cori cycle isn't a completely closed loop. In the muscles, glycolysis results in the production of two units of ATP. However, the liver uses up six units of ATP to carry out the process of gluconeogenesis. The Cori cycle also requires the initial introduction of oxygen, without which it cannot begin. As such, eventually, the muscles are bound to require a new supply of glucose as well as oxygen.

 

If a physical activity is too strenuous, the energy requirements of the muscles will exceed the capacity of the Cori cycle to regenerate glucose from lactate. This will result in a condition known as lactic acidosis, which is an accumulation of excess lactic acid in the system. Lactic acidosis brings down the pH level of the blood, which can lead to tissue damage. It also induces symptoms associated with panic, such as hyperventilation, abdominal cramps, vomiting, etc., all of which are the body's natural defense mechanisms designed to slow down the rigorous activity, and prevent permanent damage from occurring. A Brief Explanation of the Importance of Cori Cycle in Metabolism  https://biologywise.com/brief-explanation-of-cori-cycle

　

 

What Happens to Lactic Acid After Exercise? | LIVESTRONG.COM  https://www.livestrong.com/article/544580-what-happens-to-lactic-acid-after-exercise/