Muscle enzymes creatine kinase

Liver A survey of abnormal liver function (raised alanine aminotransferase) or raised muscle enzyme creatine kinase (CK) activity in a large ambulatory population showed that values over three times the upper limit of the reference range were deemed to be rare and differed little from the incidence of enzyme rises that have been reported for placebo [8 ]. 

In our laboratory, we have focused our attention on the syntheses of various analogs of creatine, 17, a substrate for the enzyme creatine kinase from rabbit muscle (2). The reaction catalyzed by this enzyme is... 

The enzyme creatine kinase (CK) facilitates the transfer of phosphate and energy to a molecule of ADP to form ATP. Stores of creatine phosphate are sufficient to sustain approximately 15 more seconds of muscle contraction. Because this is a single-step process, it provides ATP very rapidly and is the first pathway for formation of ATP to be accessed. 

In addition to actin and myosin, other proteins are found in the two sets of filaments. Tropomyosin and a complex of three subunits collectively called troponin are present in the thin filaments and play an important role in the regulation of muscle contraction. Although the proteins constituting the M and the Z bands have not been fully characterized, they include a-actinin and desmin as well as the enzyme creatine kinase, together with other proteins. A continuous elastic network of proteins, such as connectin, surround the actin and myosin filaments, providing muscle with a parallel passive elastic element. Actin forms the backbone of the thin filaments [4]. The thin... 

The enzyme creatine kinase is present in two positions in the muscle fibre within the intermembrane space of the mitochondria and close to the myofibrils. 

The enzyme creatine kinase (CK) is formed of two subunits that can either be of the brain (B) type or the muscle (M) type, and different combinations of these types lead to isozymes that predominate in the brain (BB), skeletal muscle (MM), and heart muscle (MB). 

Phosphocreatine (Fig. 13-5), also called creatine phosphate, serves as a ready source of phosphoryl groups for the quick synthesis of ATP from ADP. The phosphocreatine (PCr) concentration in skeletal muscle is approximately 30 nra, nearly ten times the concentration of ATP, and in other tissues such as smooth muscle, brain, and kidney [PCr] is 5 to 10 mM. The enzyme creatine kinase catalyzes the reversible reaction... 

Enzymes have been classified by an international Enzyme Commission (EC) and assigned EC numbers. Thus the enzyme creatine kinase (the muscle enzyme that catalyses the energy storage reaction ATP + creatine —> ADP + phosphocreatine) has the EC number 2.7.3.2, these numbers successively referring to a transferase function (2), a phosphotransferase function (7), phosphotransfer with a nitrogen (N) acceptor (3) and creatine kinase per se (2). 

Proteins that differ somewhat in primary structure and properties from tissue to tissue, but retain essentially the same function, are called tissue-specific isoforms or isozymes. The enzyme creatine kinase is an example of a protein that exists as tissue-specific isozymes, each composed of two subunits with 60 to 72% sequence homology. Of the two creatine kinases that bind to the muscle sarcomere, the M form is produced in skeletal muscle, and the B polypeptide chains are produced in the brain. The protein comprises two subunits, and skeletal muscle therefore produces an MM creatine kinase, and the brain produces a BB form. The heart produces both types of chains and therefore forms a heterodimer, MB, as well as an MM dimer. Two more creatine kinase isozymes are found in mitochondria, a heart mitochondrial creatine kinase, and the universal isoform found in other tissues. (In general, most proteins present in both the mitochondria and cytosol will be present as different isoforms.) The advantage conferred on different tissues by having their own isoform of creatine kinase is unknown. However, tissue-specific isozymes such as MB creatine kinase are useful in diagnosing sites of tissue injury and cell death. 

A somewhat similar picture has been obtained by electron-spin resonance studies of two manganese-containing enzymes creatine kinase and muscle enolase. In the former, manganese was shown to form a bond to the coenzyme (adenosine diphosphate) and to the substrate, but not to the protein in the latter, manganese seemed to act as a bridge between substrate and protein (Cohn and Leigh, 1962). 

The effect of a statin is usually determined by measuring fasting plasma lipids and lipoproteins after 4-6 weeks of treatment. Liver enzymes and eventually creatine kinase (in case of myositis liver enzymes are usually also elevated) are measured simultaneously to exclude side effects related to liver and muscles. After the treatment goal has been reached, blood sampling is usually performed 1-2 times a year. 

Creatine phosphate is formed from ATP and creatine (Figure 49-16) at times when the muscle is relaxed and demands for ATP are not so great. The enzyme catalyzing the phosphorylation of creatine is creatine kinase (CK), a muscle-specific enzyme with clinical utility in the detection of acute or chronic diseases of muscle. 

Creatine kinase, creatine kinase myocardial band Creatine kinase (CK) enzymes are found in many isoforms, with varying concentrations depending on the type of tissue. Creatine kinase is a general term used to describe the nonspecific total release of all types of CK, including that found in skeletal muscle (MM), brain (BB) and heart (MB). CK MB is released into the blood from necrotic myocytes in response to infarction and is a useful laboratory test for diagnosing myocardial infarction. If the total CK is elevated, then the relative index (RI), or fraction of the total that is composed of CK MB, is calculated as follows RI = (CK MB/CK total) x 100. An RI greater than 2 is typically diagnostic of infarction. 

The same reaction was recently proposed to detect creatine kinase (CK), an enzyme of high clinical significance in relation to the investigation of skeletal muscle disease and the diagnosis of myocardial infarct or cerebrovascular accidents. As ATP is a reaction product obtained from the reaction of ADP with creatine phosphate catalyzed by CK, this enzyme can be indirectly measured by the CL intensity read from the subsequent reaction of ATP with luciferin. Using the technique of electrophoretically mediated microanalysis (EMMA), it is possible to detect the enzyme using nanoliter volumes of biological sample with an improved speed and simplicity with respect to a conventional colorimetric method [100],... 

The enzyme responsible for this topping-up ATP in active muscle is CK. CK is found in high concentration in muscle cells, both free within the sarcoplasm and also associated with membranes of mitochondria and the sarcoplasmic reticulum. Structurally, creatine kinase is a dimeric enzyme of B and/or M subunits, each of about 40 kDa. Three quaternary structure isoenzyme forms arise CK-MM, CK-BB and CK-MB. The predominant form in all muscles is CK-MM, but cardiac muscle also contains a significant amount of CK-MB and this isoenzyme can be used as a specific marker of myocardial damage (see Case Notes at the end of this chapter). 

This muscle phosphotransferase (EC 2.132) catalyzes the reversible rephosphorylation of ADP to form ATP (/.c., T eq = [ATP][Creatine]/([Creatine phosphate] [ADP]) = 30). In resting muscle, creatine phosphate is synthesized at the expense of abundant stores of ATP intracellular creatine phosphate stores often reach 50-60 mM. If ATP is suddenly depleted by muscle contraction, its product ADP is immediately converted back into ATP by the reverse of the creatine kinase reaction. Depending on the pH at which the enzyme is studied, the kinetic reaction can be either rapid equilibrium random or rapid equilibrium ordered. A-Ethylglycocyamine can also act as a substrate. 

Creatine (Cr) plays an important role in energy transmission and storage in cells and tissues with high energy demands. Tissues like the brain, retina, spermatozoa and cardiac and skeletal muscle contain the enzyme Cr kinase, which catalyses the interconversion of Cr and its phosphorylated analogue, phosphocreatine. The dephosphorylation of phosphocreatine yields energy, as ADP is simultaneously converted into ATP. 

Nakagawa, T. Nagayama, E Enzymic properties of fish muscle creatine kinase. Comp. Biochem. Physiol. B, 98, 349-354 (1991)... 

Reasons for the presence of enzymes in the plasma Enzymes can normally be found in the plasma either because they were specifically secreted to fulfill a function in the blood, or because they were released by dead or damaged cells. Many diseases that cause tissue damage result in an increased release of intracellular enzymes into the plasma. The activities of many of these enzymes (for example, creatine kinase, lactate dehydrogenase, and alanine aminotransferase) are routinely determined for diagnostic purposes in diseases of the heart, liver, skeletal muscle, and other tissues. 

Creatine kinase sequences are known for many different species and iso-forms, so species-specificity of MAbs can often be used for refining the details of epitope mapping. Natural variants that prevent MAb binding are likely to involve contact residues, because the overall protein structures (and enzyme activity) are likely to be retained. The CK-2A7 MAb in Fig. IB binds between Met-29 and Cys-73. It recognizes rabbit and Torpedo CKs, as well as chick CK, but it fails to bind to either rat muscle CK or rabbit brain CK. This suggests that Lys-39 is required for CK-2A7 binding, since it is replaced by Asn m rat muscle CK and by Ala in rabbit brain CK (7), and is the only amino acid change consistent with the observed CK-2A7 specificity. 

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