Sarcoplasm

The sacroplasmic proteins myoglobin and hemoglobin are responsible for much of the color in meat. Species vary tremendously in the amount of sacroplasmic proteins within skeletal muscle with catde, sheep, pigs, and poultry Hsted in declining order of sarcoplasmic protein content. Fat is also an important component of meat products. The amount of fat in a portion of meat varies depending on the species, anatomy, and state of nutrition of the animal. The properties of processed meat products are greatiy dependent on the properties of the fat included. Certain species, such as sheep, have a relatively higher proportion of saturated fat, whereas other species, such as poultry, have a relatively lower proportion of saturated fat. It is well known that the characteristic davors of meat from different species are in part determined by their fat composition. 

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

Another mechanism in initiating the contraction is agonist-induced contraction. It results from the hydrolysis of membrane phosphatidylinositol and the formation of inositol triphosphate (IP3)- IP3 in turn triggers the release of intracellular calcium from the sarcoplasmic reticulum and the influx of more extracellular calcium. The third mechanism in triggering the smooth muscle contraction is the increase of calcium influx through the receptor-operated channels. The increased cytosolic calcium enhances the binding to the protein, calmodulin [73298-54-1]. 

M FIGURE 10.14 The arrangement of Ca -ATPase in the sarcoplasmic reticulum membrane. Ten transmembrane segments are postulated on the basis of hydropathy analysis. 

FIGURE 10.15 A mechanism for Ca -ATPase from sarcoplasmic reticulum. Note the similarity to the mechanism of Na, K -ATPase (see also Figure 10.11). ( Out here represents the cytosol In represents the lumen of the SR.)... 

The trigger for all musele eontraetion is an increase in Ca eoneentration in the vicinity of the muscle fibers of skeletal muscle or the myocytes of cardiac and smooth muscle. In all these cases, this increase in Ca is due to the flow of Ca through calcium channels (Figure 17.24). A muscle contraction ends when the Ca concentration is reduced by specific calcium pumps (such as the SR Ca -ATPase, Chapter 10). The sarcoplasmic reticulum, t-tubule, and sarcolemmal membranes all contain Ca channels. As we shall see, the Ca channels of the SR function together with the t-tubules in a remarkable coupled process. 

Muscle cells contain two different isozymes of creatine kinase, one in the mitochondria and one in the sarcoplasm. Explain. 

Saito, A., et al., 1988. Ultrastrnctnre of die calcium release channel of sarcoplasmic redcnlnm. of Cell Biology 107 211-219. 

Wagenknecht, T, et al., 1989. Three-dimensional architecture of die calcium channel/foot structure of sarcoplasmic reticnlnm. Nature 338 167-170. 

Gitelzon, G. I., Tugai, V. A., and Zakharchenko, A. N. (1990). Production of obelin, a calcium-activated photoprotein, from Obelia longissima and its application for registration of the calcium efflux from the fragmented sarcoplasmic reticulum of skeletal muscles. Ukr. Biokhim. Zh. 62 69-76. 

Calsequestrin is the major calcium storage protein of the sarcoplasmic reticulum in skeletal and cardiac muscles. It is highly acidic and has a large capacity for Ca2+. Calsequestrin functions to localize calcium near the junctional face of the terminal cistemae from which calcium can be released into the cytosol via the ryanodine receptor. 

Tlie Na+/K+-ATPase belongs to the P-type ATPases, a family of more than 50 enzymes that also includes the Ca2+-ATPase of the sarcoplasmic reticulum or the gastric H+/K+-ATPase. P-Type ATPases have in common that during ion transport an aspartyl phos-phointermediate is formed by transfer of the y-phosphate group of ATP to the highly conserved sequence DKTGS/T [1]. 

Protein kinase A (PKA) is a cyclic AMP-dependent protein kinase, a member of a family of protein kinases that are activated by binding of cAMP to their two regulatory subunits, which results in the release of two active catalytic subunits. Targets of PKA include L-type calcium channels (the relevant subunit and site of phosphorylation is still uncertain), phospholam-ban (the regulator of the sarcoplasmic calcium ATPase, SERCA) and key enzymes of glucose and lipid metabolism. 

Ryanodine receptor (RyR) is an intracellular Ca2+ release channel in the sarcoplasmic reticulum (SR) or the endoplasmic reticulum (ER). RyR binds ryanodine (a plant alkaloid, see Drugs) with a high affinity, after which it is named. 

Sarcoplasmic reticulum (SR) is a form of the smoothfaced endoplasmic reticulum (ER) in muscles. It functions as an intracellular Ca2+ store for muscle contraction. Ca2+ is energetically sequestered into the SR by Ca2+-pump/sarcoplasmic endoplasmic reticulum Ca2+-ATPase (SERCA) and released via Ca2+ release channels on stimuli (ryanodine receptor in striated muscles and inositol 1,4,5-trisphosphate receptor in most smooth muscles). Endoplasmic reticulum in non-muscle tissues also functions as an intracellular Ca2+ store. 

Sarcoplasmic calcium ATPase this enzyme utilizes the energy gained from hydrolysis of ATP to pump calcium from the cytosol into the stores of the sarcoplasmic reticulum. Its activity is negatively regulated by the closely associated protein phospholamban, and this inhibition is relieved upon phosphorylation of phospholamban by protein kinase A (PKA). 

A skinned fiber is a muscle fiber, the sarcolemma of which has been mechanically removed or which is made freely permeable to small molecules, such as Ca2+, Mg2+, EGTA, ATP, soluble enzymes and others by a chemical agent (saponin, (3-escin or Staphylococcus a-toxin). The organization of the sarcoplasmic reticulum (SR) and myofibrils is kqrt as they are in the living muscle. 

The structure and mechanism of the sarcoplasmic reticulum Ca2+-ATPase a bioinorganic perspective. E. M. Stephens andC. M. Grisham, Adv. Inorg. Biochem., 1982, 4, 263-288 (151). 

Sandwich complexes nickel. 5, 35 Sapphyrins, 2, 888 demetallation, 2, 891 metallation, 2, 891 reactions, 2, 891 synthesis, 2, 889 Sarcoplasmic reticulum calcium/magnesium ATPase, 6, 566 skeletal muscle... 

Site symmetry symbols, I, 128 Six-coordinate compounds stereochemistry, 1, 49-69 Six-membered rings metal complexes, 2, 79 Skeletal muscle sarcoplasmic reticulum calcium pump, 6, 565 Slags... 

Figure 4. The citrate cycle. There is complete oxidation of one molecule of acetyl-CoA for each turn of the cycle CH3COSC0A + 2O2 - 2CO2 + H2O + CoASH. The rate of the citrate cycle is determined by many factors including the ADP/ATP ratio, NAD7NADH ratio, and substrate concentrations. During muscle contraction, Ca is released from cellular stores (mainly the sarcoplasmic reticulum) and then taken up in part by the mitochondria (see Table 2). Ca " activates 2-oxoglutarate and isocitrate dehydrogenases (Brown, 1992). Succinate dehydrogenase may be effectively irreversible. Enzymes ...

In the sarcoplasm of smooth muscle cells there is a membrane bound compartment usually referred to as the SR by analogy with skeletal muscle. However, it is not at all clear that the interior of these membrane-bound regions are continuous as they are in skeletal muscle. The primary properties of this system seem to be quite similar to those of the endoplasmic reticulum of many other cell types. In general, calcium is concentrated into the membrane-bound reticulum and then released to initiate the characteristic action of the cell. 

Sarcoplasmic reticulum Ca -channels. In many smooth muscle cells the rise of intracellular calcium which triggers contraction comes from the flow of calcium from the SR through Ca channels. In others, the SR contributes some unknown fraction of the triggering calcium relative to the amount which comes from the extracellular space through the plasma membrane Ca -channels. There are at least two kinds of Ca -channels in the SR. 

Figure 2. Muscle stimulation, a) a single nerve impulse (stimulus) causes a single contraction (a twitch). There is a small delay following the stimulus before force rises called the latent period, b) A train of stimuli at a low frequency causes an unfused tetanus. Force increases after each progressive stimulus towards a maximum, as calcium levels in the myofibrillar space increase. But there is enough time between each stimulus for calcium to be partially taken back up into the sarcoplasmic reticulum allowing partial relaxation before the next stimulus occurs, c) A train of stimuli at a higher frequency causes a fused tetanus, and force is maximum. There is not enough time for force to relax between stimuli. In the contractions shown here, the ends of the muscle are held fixed the contractions are isometric.

Calcium levels in the myofibrillar space are usually low, to prevent contraction. The calcium ions are stored in an internal membrane system called the sarcoplasmic... 

T-Tubular Depolarization and Ca Release From the Sarcoplasmic Reticulum 246... 

Voluntary muscle contraction is initiated in the brain-eliciting action potentials which are transmitted via motor nerves to the neuromuscular junction where acetylcholine is released causing a depolarization of the muscle cell membrane. An action potential is formed which is spread over the surface membrane and into the transverse (T) tubular system. The action potential in the T-tubular system triggers Ca " release from the sarcoplasmic reticulum (SR) into the myoplasm where Ca " binds to troponin C and activates actin. This results in crossbridge formation between actin and myosin and muscle contraction. 

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