In vitro translation of canine mitochondrial creatine kinase messenger RNA

Molecular biology of heart disease

Dr. Robert Roberts is currently Professor of Medicine and Director of the Ruddy Canadian Cardiovascular Genetics Centre along with being President and CEO of the University of Ottawa Heart Institute. Prior to this appointment, he was Chief of Cardiology for 23 years at Baylor College of Medicine, Houston, Texas. His original research was in cardiac enzymology which led to the development of the MBCK test which was the standard diagnostic assay for myocardial infarction for more than 3 decades. In the late 1970s, his research interests switched to molecular biology and the genetics of cardiomyopathies. He is regarded as one of the founders of molecular cardiology and has identified and sequenced more than 20 genes responsible for cardiovascular disorders. In the past 6 years, he has pursued genome-wide association studies to identify genes predisposing to coronary artery disease (CAD) and myocardial infarction. The first genetic variant for CAD, 9p21, was identified by Dr. Robert's laboratory and, in collaboration with the international consortium, CARDIoGRAM, has identified 13 novel genes for CAD.

Early diagnosis of myocardial infarction with MB CK isoforms

Over 5 million patients with chest pain present annually to the ER in the US but only 10% will have infarction with < 50% identified by ECG. Early diagnosis of infarction in the ER to triage provides more appropriate and early therapy, less unnecessary use of CCU, and is more cost effective. A rapid automated MB CK isoform assay (25 min) was evaluated in a prospective study of 1110 patients presenting to the ER with chest pain. This assay was shown to have a sensitivity of 96% and specificity of 93% to diagnose infarction within 6 h of onset of chest pain. In 92% the diagnosis was confirmed within 1 h of arrival at the ER. Similar sensitivity and specificity for total MB CK, troponin T, and troponin I required 16 h from onset of pain. Thus, MB CK isoforms provide a rapid diagnosis early after onset of infarction and have the requisite sensitivity and specificity for triaging patients in the ER.

Reversible myocardial ischemic injury is not associated with increased creatine kinase activity in plasma

Creatine kinase (CK) isoenzymes MM, MB, and BB are located primarily in the cell cytosol, and increased CKMB in plasma is the hallmark of myocardial infarction. However, whether CK is released with reversible ischemic injury remains controversial. Here, we assessed plasma CK activity—cytosolic and mitochondrial CK—in serial samples (every 10 min for 60 min, then hourly or every 4 h for 48 h) from 46 conscious dogs after transient or sustained coronary occlusion. Four dogs were sham-operated (controls); four underwent sustained coronary occlusion (96 h); and 38 underwent transient coronary occlusion (10–40 min) followed by 48 h of reperfusion. In postmortem histological examination of the dogs’ hearts by light and electron microscopy, we looked for ischemia or necrosis. The presence of cell swelling and glycogen depletion was indicative of ischemia, whereas the added presence of cell disruption indicated necrosis. Coronary occlusion for ≥20 min consistently increased plasma mitochondrial and total CK activity and produced histologically evident myocardial necrosis. In contrast, after 10 to 15 min of coronary occlusion, 12 of 14 animals, despite extensive severe reversible ischemia, showed no increase in plasma CK; the remaining 2, which had increased plasma CK, had subendocardial necrosis. Thus, cytosolic or mitochondrial CK is released from the heart only when there has been irreversible myocardial injury—a finding with significant diagnostic and therapeutic implications.

Intracellular compartmentation, structure and function of creatine kinase isoenzymes in tissues with high and fluctuating energy demands: the 'phosphocreatine circuit' for cellular energy homeostasis

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Distinct tissue specific mitochondrial creatine kinases from chicken brain and striated muscle with a conserved CK framework

cDNA clones for chicken mitochondrial creatine kinase (Mi-CK) were isolated from a lambda gt11 leg muscle cDNA library and sequenced. The deduced amino acid sequence showed 6 blocks of extensive homologies with the cytosolic creatine kinases and contained twenty N-terminal amino acids, with characteristic features of part of a mitochondrial presequence. The mature enzyme contained 380 amino acids with a calculated Mr of 43'195. RNA hybridization analysis showed corresponding Mi-CK transcripts in cardiac and skeletal muscle, but not in brain RNA. Within the 30 N-terminal amino acids purified brain Mi-CK contained 10 changes with respect to cardiac Mi-CK. Thus multiple isoproteins of mitochondrial creatine kinases of brain and striated muscle are encoded by multiple mRNA's.

Complete nucleotide sequence of dog heart creatine kinase mRNA: conservation of amino acid sequence within and among species

Creatine kinase (CK; EC 2.7.3.2) plays an important role in energy metabolism in brain and muscle. Expression of CK isoenzymes is regulated during development and is tissue specific. To define the structures of canine CK isoenzymes and to elucidate the mechanism of regulation in their expression, CK cDNA clones from dog myocardium were isolated. Myocardial CK mRNA is predicted to encode a protein of 381 amino acids. The nontranslated regions of the mRNA comprise at least 38 bases at the 5' end and exactly 345 bases before the poly(A) tail. Partial protein sequences of dog muscle (M) CK and brain (B) CK subunits were determined and compared with the derived amino acid sequence of the myocardial enzyme and of M CK subunits of other species. The M CK subunits from different species share a very high degree (83-96%) of sequence identity. Dog M and B subunits share extensive sequence identity (74%), a degree of similarity not previously suspected. Southern blot analysis suggests that a CK gene family exists. These observations imply that evolutionary changes in the M CK subunit structure are constrained by the need for preservation of functional properties other than the kinase activity. This conservation is consistent with the possibility that the M subunit plays a structural role in cardiac and skeletal muscle.

Enzyme kinetics of a highly purified mitochondrial creatine kinase in comparison with cytosolic forms

Mitochondrial creatine kinase (CK) purified from canine myocardium showed a single protein band on SDS-PAGE and was free of MMCK. Its amino acid composition was different than MMCK or BBCK and did not react to antiserum to MMCK or BBCK. Using purified mitochondrial, MM and BBCK, the velocity of reaction (V) was estimated for creatine phosphate (CP), creatine (C), adenosine triphosphate (ATP) and adenosine diphosphate (ADP) over a wide range of concentrations including those at Vmax. The values for Km (mM/L) derived from Lineweaver-Burke plots are shown: (Table: see text). The affinity of mitochondrial CK for C is much greater than MMCK which is compatible with the energy shuttle hypothesis, namely ATP is converted by mitochondrial CK to CP, and then diffuses to the myofibril for conversion to ATP for utilization.

Two tissue-specific isozymes of creatine kinase have closely matched amino acid sequences

Creatine kinase activity is associated with different isozyme species. We have examined two of these: the cytoplasmic brain (B) isozyme that is expressed in many tissues and is reported to be induced by estrogen and the developmentally regulated cytoplasmic muscle (M) isozyme that is found predominantly in differentiated muscle tissue. Recently, we cloned and sequenced the cDNA for the M isoenzyme of rabbit creatine kinase. We now report the isolation of B-isozyme cDNAs and the deduced primary structure of the polypeptide. The translated cDNA nucleotide sequence was cross-checked by fast-atom bombardment/mass spectrometry of tryptic fragments from the protein. The sequence is exactly colinear with the rabbit M isozyme and the two isozymes have 80% nucleotide and amino acid sequence identity. There are blocks of 36 and 41 amino acids where the amino acid sequence is conserved exactly. The colinearity of the two sequences and the extent of their identity makes it unlikely that either isozyme has unique polypeptide domains that account for specialized functions. The rationale for the existence of these creatine kinase isozymes, with distinct biological features, evidently is at the level of regulation of individual isozyme expression.

Transport of proteins into mitochondria

There is still much that is obscure concerning the transport of proteins into or through the mitochondrial membrane systems. In addition, as pointed out previously, it is unlikely that the details of the process are the same for proteins destined for different compartments of the organelle. A brief summary of the process for matrix proteins might be as follows: The proteins are synthesized on free polysomes as precursors of higher molecular weight than the native forms. These precursors are liberated into the cell cytosol and subsequently translocated into the mitochondria. This timing might be different in yeast under some circumstances, synthesis being completed in association with the mitochondria. The precursors interact with a receptor in the outer mitochondrial membrane interaction being mediated by the presequences of the precursors. The presequences therefore act as addressing signals as well as possibly playing a role in one or all of (a) solubilization of precursors, (b) prevention of premature assembly into multimeric structures, or (c) maintenance of nonnative configurations required for transport. Interaction occurs with a second receptor, this time in the inner membrane of the mitochondria, interaction being with multiple sites in the polypeptide chain. Transport across the inner membrane then occurs, this transport depending on a transmembrane electrochemical gradient of which the proton component is the essential part. Transport is accompanied or followed by proteolysis of the prepiece, and formation of the native structure. While steps 1 and 2 of this sequence can be considered well established, the remaining steps are still poorly understood or purely hypothetical. Nevertheless, this sequence of events is consistent with known facts about the process and provides a framework for future investigations.