Inhibition of rabbit muscle isozymes by vitamin C

Uremia Impedes Skeletal Myocyte Myomixer Expression and Fusogenic Activity: Implication for Uremic Sarcopenia

In patients with chronic kidney disease (CKD), skeletal muscle mass and function are known to occasionally decline. However, the muscle regeneration and differentiation process in uremia has not been extensively studied. In mice with CKD induced by adenine-containing diet, the tibialis anterior muscle injured using a barium chloride injection method recovered poorly as compared to control mice. In the cultured murine skeletal myocytes, stimulation with indoxyl sulfate (IS), a representative uremic toxin, morphologically jeopardized the differentiation, which was counteracted by L-ascorbic acid (L-AsA) treatment. Transcriptome analysis of cultured myocytes identified a set of genes whose expression was down-regulated by IS stimulation but up-regulated by L-AsA treatment. Gene silencing of myomixer, one of the genes in the set, impaired myocyte fusion during differentiation. By contrast, lentiviral overexpression of myomixer compensated for a hypomorphic phenotype caused by IS treatment. The split-luciferase technique demonstrated that IS stimulation negatively affected early myofusion activity that was rescued by L-AsA treatment. Lastly, in mice with CKD compared with control mice, myomixer expression in the muscle tissue in addition to the muscle weight after the injury was reduced, both of which were restored with L-AsA treatment. Collectively, data showed that the uremic milieu impairs the expression of myomixer and impedes the myofusion process. Considering frequent musculoskeletal injuries in uremic patients, defective myocyte fusion followed by delayed muscle damage recovery could underlie their muscle loss and weakness.

Ascorbic acid stimulates the in vitro myoblast proliferation and migration of pacu (Piaractus mesopotamicus)

The postembryonic growth of skeletal muscle in teleost fish involves myoblast proliferation, migration and differentiation, encompassing the main events of embryonic myogenesis. Ascorbic acid plays important cellular and biochemical roles as an antioxidant and contributes to the proper collagen biosynthesis necessary for the structure of connective and bone tissues. However, whether ascorbic acid can directly influence the mechanisms of fish myogenesis and skeletal muscle growth remains unclear. The aim of our work was to evaluate the effects of ascorbic acid supplementation on the in vitro myoblast proliferation and migration of pacu (Piaractus mesopotamicus). To provide insight into the potential antioxidant role of ascorbic acid, we also treated myoblasts in vitro with menadione, which is a powerful oxidant. Our results show that ascorbic acid-supplemented myoblasts exhibit increased proliferation and migration and are protected against the oxidative stress caused by menadione. In addition, ascorbic acid increased the activity of the antioxidant enzyme superoxide dismutase and the expression of myog and mtor, which are molecular markers related to skeletal muscle myogenesis and protein synthesis, respectively. This work reveals a direct influence of ascorbic acid on the mechanisms of pacu myogenesis and highlights the potential use of ascorbic acid for stimulating fish skeletal muscle growth.

Effect of lithium salts on lactate dehydrogenase, adenylate kinase, and 1-phosphofructokinase activities

Inhibitions of 30 nM rabbit muscle 1-phosphofructokinase (PFK-1) by lithium, potassium, and sodium salts showed inhibition or not depending upon the anion present. Generally, potassium salts were more potent inhibitors than sodium salts; the extent of inhibition by lithium salts also varied with the anion. Li(2)CO(3) was a relatively potent inhibitor of PFK-1 but LiCl and lithium acetate were not. Our results suggest that extents of inhibition by monovalent salts were due to both cations and anions, and the latter needs to be considered before inhibition can be credited to the cation. An explanation for monovalent salt inhibitions is proffered involving interactions of both cations and anions at negative and positive sites of PFK-1 that affect enzyme activity. Our studies suggest that lithium cations per se are not inhibitors: the inhibitors are the lithium salts, and we suggest that in vitro studies involving the effects of monovalent salts on enzymes should involve more than one anion.

Effect of ammonium, sodium, and potassium ions on rabbit muscle phosphofructokinase-1 and adenylate kinase activities

This report shows that 30 nM PFK-1 and 30 nM AK were both affected by the presence of NH(4)(+), Na(+), and K(+) salts but with opposite consequences. Low concentrations of PFK-1 lose about half of its activity as a result of dilution and become susceptible to further activity losses owing to the presence of monovalent salts. On the other hand low concentrations of AK lose about 75 percent of its activity but regains activity losses owing to the presence of monovalent salts. It was determined that regain of AK activity did not appear to be a reflection of a major effect on the K(m) value of either AMP or ATP. Dilution to 30 nM AK resulted in no increase K(m) values compared to K(m) values at 140 nM AK. Dilution caused major decreases in the maximum velocities, V(max), when ATP or fructose 6-phosphate was the variable substrate. It was shown in earlier reports that these same low concentrations of PFK-1 and AK were susceptible inhibitions by ascorbate. These attributes are discussed as they may relate to the role of ascorbate facilitation glycogen synthesis in resting muscle and the role that the cytoskeleton infrastructure scaffold may play is also discussed.

Some characteristics of rabbit muscle phosphofructokinase-1 inhibition by ascorbate

These studies relate to a working hypothesis that glycogen storage is facilitated in resting muscle by inhibiting glycolysis via inhibition of LDH, AK, and PFK-1 by ascorbate; when muscle is active, these isozymes combine with muscle proteins and are released and protected from inhibition by ascorbate and glycolysis proceeds. Focus in these studies is on the ability of G-actin and aldolase to prevent PFK-1 inhibition by ascorbate. We found that inhibition by ascorbate was PFK-1 concentration dependent; ascorbate does not inhibit above 200 nM PFK-1. We conclude that ascorbate inhibits PFK-1 dimers (and perhaps monomers) but not PFK-1 tetramers. Separation of PFK-1 dimers from tetramers was achieved with centrifugal filter devices and differences in their sensitivity to ascorbate inhibition were demonstrated. Some comparisons are made with attributes of AK inhibitions by ascorbate that, like PFK-1, are also enzyme concentration dependent. Discussions relate findings to cellular infrastructure and the role of ascorbate in glycogen synthesis.

Effects of vitamin C on muscle glycogen and oxidative events in experimental diabetes

Streptozotocin (STZ) is an agent used in creating experimental diabetes. Varying findings have been reported about the striated muscle glycogen levels in diabetes. In this study, it was planned to observe interaction of vitamin C (AA), of which deficiency has been shown in diabetics, with soleus muscle glycogen levels and oxidative events on STZ-diabetic subjects.

MATERIAL AND METHOD

In the study, 38 male adult Wistar Albino rats with weights 200 +/- 20 g were used by separating them into four groups: Control, Vitamin C, Diabetes, Diabetes + Vitamin C. Body weights and fasting blood glucose were measured at the beginning and end of the experiment. AA, TBARS, GSH, NOx and glycogen levels of soleus muscles, and AA level of blood were measured. The results were compared using Anova variance and Mann-Whitney U tests. Results showed that AA levels in blood increased with vitamin C administration; AA, GSH and NOx levels in the muscle were low and MDA and glycogen levels were high in diabetics; and that vitamin C in the given dosage partially corrected these values. These results indicate that higher dosage than daily 20 mg/kg Vitamin C is required for being effective on metabolic and oxidizing events in diabetic rats.

Characteristics of rabbit muscle adenylate kinase inhibition by ascorbate

Earlier studies [1-3] showed that of the glycolytic enzymes, the muscle isozymes PFK-1, LDH, and AK were inhibited by ascorbic acid. These studies on the characteristics of the inhibition of RMAK by ascorbate are part of a hypothesis [3] that ascorbate facilitates the storage of skeletal muscle glycogen by inhibiting glycolysis when the muscle is at rest. These studies examine conditions for RMAK inhibition, prevention of inhibition, and reversal of ascorbate inhibition. We found that the concentration of RMAK was an important condition for inhibition. Above 200 nM RMAK, inhibition by ascorbate could not be demonstrated and below that concentration RMAK became increasingly sensitive to ascorbate inhibition. Associated with increased sensitivity to inhibition by ascorbate is a deviation from a linear to a concave relationship between low RMAK concentrations and enzyme activity. At low RMAK concentrations, the concave relationship becomes convex in the presence of muscle aldolase. In addition, aldolase reverses inhibitions by ascorbate. A comparison of inhibition of RMAK byascorbate and inhibition of LDH-m4 [3] is discussed. Other proteins prevent RMAK inhibition but do not reverse inhibition by ascorbate. The role of RMAK as a factor in the control of the rate of glycolysis is presented as is the role of compartmentalization with respect to the proposed role for ascorbate inhibition.

Vitamin C homeostasis in skeletal muscle cells

In skeletal muscle, vitamin C not only enhances carnitine biosynthesis but also protects cells against ROS generation induced by physical exercise. The ability to take up both ascorbic and dehydroascorbic acid from the extracellular environment, together with the ability to recycle the intracellular vitamin, maintains high cellular stores of ascorbate. In this study, we examined vitamin C transport and recycling, by using the mouse C2C12 and rat L6C5 muscle cell lines, which exhibit different sensitivity to oxidative stress and GSH metabolism. We found that: (1) both cell lines express SVCT2, whereas SVCT1 is expressed at very low levels only in proliferating L6C5 cells; furthermore L6C5 myoblasts are more efficient in ascorbic acid transport than C2C12 myoblasts; (2) C2C12 cells are more efficient in dehydroascorbic acid transport and ascorbyl free radical/dehydroascorbic acid reduction; (3) differentiation is paralleled by decreased ascorbic acid and dehydroascorbic acid transport and reduction and increased ascorbyl free radical reduction; (4) differentiated cells are more responsive to oxidative stress induced by glutathione depletion; indeed, myotubes showed increased SVCT2 expression and thioredoxin reductase-mediated dehydroascorbic acid reduction. From our data, SVCT2 and NADPH-thioredoxin-dependent DHA reduction appears to belong to an inducible system activated in response to oxidative stress.

Aldolase and actin protect rabbit muscle lactate dehydrogenase from ascorbate inhibition

Muscle-type LDH (LDH-m4) activity is critical for efficient anaerobic glycolysis. The results here show that rabbit LDH-M4 is inhibited by concentrations of ascorbate normally found in tissues. Aldolase and muscle G-actin were found to protect and to reverse inhibitions of LDH-m4 by ascorbate. G-actins showed some species specificity. Myosin, tropomyosin and troponin from rabbit muscle and muscle proteins from other animal sources had no affect on the inhibitions by ascorbate. The substrate inhibition of LDH-m4 by pyruvate is partially relieved by the presence of aldolase and lowers the Km without affecting the Vm. G-actin under similar conditions has no affect. It is believed that these studies reflect some of the resting properties of glycolytic enzymes that bind and unbind to contractile elements. It is proposed that ascorbate facilitates the storage of glycogen in muscle at rest by inhibiting glycolysis.

Vitamin C is a kinase inhibitor: dehydroascorbic acid inhibits IkappaBalpha kinase beta

Reactive oxygen species (ROS) are key intermediates in cellular signal transduction pathways whose function may be counterbalanced by antioxidants. Acting as an antioxidant, ascorbic acid (AA) donates two electrons and becomes oxidized to dehydroascorbic acid (DHA). We discovered that DHA directly inhibits IkappaBalpha kinase beta (IKKbeta) and IKKalpha enzymatic activity in vitro, whereas AA did not have this effect. When cells were loaded with AA and induced to generate DHA by oxidative stress in cells expressing a constitutive active IKKbeta, NF-kappaB activation was inhibited. Our results identify a dual molecular action of vitamin C in signal transduction and provide a direct linkage between the redox state of vitamin C and NF-kappaB signaling events. AA quenches ROS intermediates involved in the activation of NF-kappaB and is oxidized to DHA, which directly inhibits IKKbeta and IKKalpha enzymatic activity. These findings define a function for vitamin C in signal transduction other than as an antioxidant and mechanistically illuminate how vitamin C down-modulates NF-kappaB signaling.