Regulation of branched-chain amino acid metabolism and pharmacological effects of branched-chain amino acids

https://doi.org/10.1016/j.hepres.2004.09.001Get rights and content

Abstract

Significant evidence of the pharmacological and physiological effects of branched-chain amino acids (BCAA) has accumulated, attracting the interest of not only clinicians but also basic medical researchers. We summarize here the characteristic features of BCAA catabolism, focusing on the initial two enzymes in the pathway, branched-chain aminotransferase and branched-chain α-keto acid dehydrogenase complex. In addition, we describe a unique characteristic of the valine catabolic pathway. Finally, we present evidence obtained in animal studies that indicates that BCAA treatment may be appropriate for liver cirrhosis, but not acute liver failure.

Introduction

Leucine, isoleucine, and valine, branched-chain amino acids (BCAA), are indispensable amino acids and work not only as building blocks of proteins but also as physiological stimulants (especially for leucine) for protein synthesis [1], [2]. Due to the unique action for the protein synthesis, clinical use of BCAA has been considered. It has been demonstrated that BCAA administration improves the production of albumin in the liver of cirrhotic patients [3] and clinical conditions of patients with advanced cirrhosis [4].On the other hand, BCAA obtained in excess are immediately catabolyzed in cells, because the amino acids are little stored in free form in animal bodies. The enzymes catalyzing initial two steps in the BCAA catabolic pathway are critically important for saving as well as disposing of BCAA, branched-chain aminotransferase (BCAT) and branched-chain α-keto acid dehydrogenase (BCKDH) complex (Fig. 1) [5]. It has been postulated that the regulation of BCAA catabolism by these two enzymes may be related with the stimulatory effect of leucine on protein synthesis [6]. We describe here the unique features of BCAA catabolism and our recent findings in relation to the clinical application of BCAA.

The entire pathway of BCAA catabolism is located in mitochondria. The initial two steps of the pathway, which are common to all three of the BCAA, have characteristic features of the catabolism (Fig. 1) [5]. The first step reaction is reversible transamination of BCAA catalyzed by BCAT to form branched-chain α-keto acids (BCKA) corresponding to each BCAA. The second step reaction is oxidative decarboxylation of BCKA catalyzed by BCKDH complex to form acyl-CoA derivatives. This reaction is irreversible and therefore considered the rate-limiting step in the BCAA catabolism [7]. Each acyl-CoA derivative formed has individual catabolic pathways, except that the dehydrogenation (the third step reaction) of both acyl-CoA derivatives derived from valine and isoleucine appears to be catalyzed by the identical enzyme, 2-methyl-branched-chain acyl-CoA dehydrogenase [8].

It has been reported that there are two isoforms of BCAT, mitochondrial (BCATm) and cytosolic (BCATc) [9]. The former is the predominant isoenzyme in mammals, and the latter is restricted to the brain, ovary, and placenta [9]. It is believed that BCATc plays an important role in nitrogen shuttling and glutamate metabolism in the brain [10].

In contrast to BCATc, BCATm is a ubiquitous enzyme in mammalian tissues. BCAT activity is relatively high in the heart and kidney, whereas the activity is low in the liver due to minor expression of the enzyme [9]. Therefore, the initial step of BCAA catabolism is localized primarily in extrahepatic tissues. This fact indicates that liver uses BCAA for protein synthesis but cannot directly degrade the amino acids. This may be the systemic strategy for saving BCAA absorbed from gut and BCAA delivery to other tissues such as skeletal muscles. These findings suggest that the high concentrations of BCAA absorbed from the gut reach the liver and stimulate protein synthesis in the tissue when BCAA are orally administered. Furthermore, a large part of the BCAA may be delivered to other tissues such as skeletal muscles and are also effective in the tissues.

Since BCKDH complex catalyses the committed step of the BCAA catabolic pathway, this enzyme regulates the BCAA catabolism in mammalian tissues, apart from the liver as described above. The BCKDH complex is a mitochondrial multi-enzyme complex consisting of E1, E2, and E3 components [5]. This enzyme activity is regulated by covalent modification of the enzyme: BCKDH complex is inactivated by phosphorylation of the E1 component by the specific protein kinase, BCKDH kinase [11], and is re-activated by dephosphorylation by the specific phosphoprotein phosphatase, BCKDH phosphatase [12]. Many studies provide evidence that the BCKDH kinase plays an important role in regulating the activity state of the BCKDH complex under a number of physiological conditions [13], suggesting that the kinase regulates BCAA catabolism. Although purification of BCKDH phosphatase has been reported, little information is available on the physiological regulation of the complex by the phosphatase.

The BCKDH complex has been intensively studied using rats, because rat liver has an extremely high activity of the enzyme compared to the human liver [7], [9], [14]. It is believed that hepatic BCKDH complex works to dispose the BCKA formed by extrahepatic tissues (mainly skeletal muscle) in rats, because rat skeletal muscle has the minor BCKDH activity due to the small amount of BCKDH complex and the majority of the complex in an inactive (phosphorylated) form under normal and resting conditions [15], suggesting that BCKA catabolism is negligible under physiological conditions. The low activity of muscle BCKDH complex is suggested to be important for saving BCAA for muscle protein synthesis [16], [17].

On the other hand, several physiological conditions such as exercise, starvation, and diabetes promote BCAA oxidation by activation of muscle and liver BCKDH complex through reducing the BCKDH kinase activity [18], [19], [20]. Hormonal regulations of the BCKDH kinase activity are also reported; insulin, thyroid hormone, and female hormone increase the kinase activity, resulting in a decrease in the BCKDH complex activity, and glucocorticoid acts in the opposite direction [21], [22], [23], [24]. These findings suggest that BCAA are actively involved in protein synthesis as well as in energy metabolism.

In humans, it is believed that the major tissue for BCAA catabolism is skeletal muscles, whereas other indispensable amino acids are catabolyzed in the liver. This may be reasonable, because the human liver has extremely low activity of BCKDH complex compared to rat liver, and the total activity of the complex in all muscle tissues is the greatest in the body because skeletal muscle consists of about 40% of body weight [9], [14].

The catabolic pathways of individual BCAA are shown in Fig. 2, Fig. 3, Fig. 4. In the valine catabolic pathway (Fig. 2), the CoA ester (HIB-CoA) that occurs in the middle of the pathway is hydrolyzed to free carboxylic acid and CoA-SH by HIB-CoA hydrolase. Two steps downstream after hydrolyzing a CoA ester, the pathway reforms a CoA ester to give propionyl-CoA. It seems paradoxical that an acyl-CoA hydrolase should exist at the middle of a pathway that otherwise involves CoA esters from beginning to end. However, hydrolysis of HIB-CoA is an important strategy for disposal of methacrylyl-CoA in the pathway occurring upstream of HIB-CoA, because methacrylyl-CoA is a thiol-reactive molecule and therefore a toxic compound through nonenzymatic Michael addition reactions. The hydrolysis of HIB-CoA allows the free carboxylic acid (β-hydroxyisobutyric acid) to readily diffuse out of the cells in which it is formed. Further, the reaction catalyzed by crotonase is equilibrated toward formation of metharylyl-CoA [25]. This may be the system to protect cells against the toxic effects of methacrylyl-CoA [25]. We purified the HIB-CoA hydrolase from rat livers and established the method for measurement of this enzyme activity in a coupled reaction with crotonase [25], [26]. The activities of both crotonase and HIB-CoA hydrolase are extremely high compared to the activity of BCKDH complex in mammalian tissues examined (rat [25], dog [26], and man [14]). Therefore, methacrylyl-CoA is rapidly degraded to the free acid and CoA-SH by the high activities of two enzymes. As a consequence, methacrylyl-CoA and HIB-CoA are not detectable in liver cells even when incubated under conditions that should maximize the concentrations of intermediates of the valine pathway [27]. These findings suggest that a supplement of valine as the BCAA mixture is not toxic for humans under normal conditions.

We measured the activities of crotonase and HIB-CoA hydrolase in human liver tissues obtained from 30 patients with hepatocellular carcinoma or metastatic liver cancer [28]. The activities of both crotonase and HIB-CoA hydrolase were significantly lower by 36–46% in livers with cirrhosis or hepatocellular carcinoma compared with normal, suggesting a decrease in the capability of detoxifying methacrylyl-CoA with these diseases. Further studies are required to elucidate the relationship between alterations in enzyme activities and the progression of liver failure.

A number of pharmacological effects of BCAA would be expected and these effects and responsible mechanisms are described in detail in other sections of this issue. Therefore, we describe here our recent findings in relation to administration of BCAA to rats with acute or chronic liver failure [29].

The BCKDH complex is responsible for the regulation of BCKA catabolism, and the actual activity (active form) of the hepatic enzyme complex is responsive to changes in various physiological conditions as described above. It is interesting to compare responses of the BCKDH complex to acute and chronic liver failure [29]. We prepared acute liver failure rats by injection of a single dose (2.0 ml/kg body weight) of carbon tetrachloride and liver cirrhotic rats by repeated injection of the drug (twice a week at 0.5 mg/kg body weight) for 21 weeks. Acute liver failure markedly decreased hepatic BCKDH complex activity and significantly elevated the concentrations of plasma BCAA and BCKA, suggesting that hepatic BCKA catabolism was suppressed in the rats. On the other hand, liver cirrhosis did not suppress the hepatic enzyme activity, rather the actual activity of BCKDH complex was increased, and decreased the concentrations of both plasma BCKA and BCAA, suggesting BCKA catabolism may be enhanced in the cirrhotic liver in comparison to that of normal liver.

It has been reported that BCAA administration improves protein turnover in rats and humans with liver cirrhosis [30], [31], [32], suggesting that BCAA administered to patients with cirrhosis is appropriate. Furthermore, BCAA may be a preferable substrate to fulfill the energy requirements of patients with cirrhosis [33], because the energy efficacy of BCAA is higher than that of other substrates such as glucose and fatty acids and glucose tolerance is deteriorated in the majority of liver cirrhotic patients [34]. However, BCAA administration in patients with acute liver failure, especially those with high serum BCAA concentrations, may not be appropriate because of the potential for BCAA overload. These findings support that BCAA administration therapy is suitable for patients with liver cirrhosis but not those with acute liver failure.

Section snippets

Acknowledgements

This work was supported in part by a grant-in-aid for scientific research (14370022 to YS) from the Ministry of Education, Science, Sports and Culture, Japan.

References (34)

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