Myoglobin (Mb) is a small molecular pigment proteinGlobinAndHeme(Heme), which can be reversibly combined with oxygen to form MbOtwo，MbOtwobe calledOxymyoglobinMb is called deoxymyoglobin.It has the function of transporting and storing oxygen in muscle cells.When myocardial damage occursCardiac myocyteIt diffuses out into the blood circulation.Myoglobin only exists inmyocardiumAnd skeletal muscle, other tissues includesmooth muscleIt does not contain this protein.[1-2][7-8]
The increase of serum Mb level is due toskeletal muscleAnd (or)Cardiac myocyteInjury (dissolution/necrosis) is released into the blood circulation.Serum Mb<70ng/ml, and its level varies with age, sex, and race.miocardial infarctionMb can be detected in the serum at the early stage, and the peak time (1-2 hours) ratiocreatine kinase(create kinase, CK) early.Mb has a small molecular weight and is easy to be excreted from urine.After reperfusion treatment, the serum Mb level increases rapidly, so it has been used as a useful indicator to evaluate the success of reperfusion treatment or the size of myocardial infarction. However, because the half-life of Mb is short (15 minutes), the serum Mb level does not increase 6~12 hours after the onset of chest pain, which is helpful to excludeacute myocardial infarction。At the same time, because the duration of Mb level increase is very short (<24 hours), Mb measurement is helpful to observe whether there is reinfarction or infarction expansion in the course of acute myocardial infarction.The frequent increase of Mb level indicates that the original myocardial infarction is still ongoing.It should be pointed out that Mb is also a component of skeletal muscle and lacks specificity, so the clinical value of a series of Mb measurements after myocardial infarction is relatively limited.For patients with chest discomfort accompanied by non diagnostic ECG manifestations, the diagnosis of acute myocardial infarction can not rely on Mb measurement alone within the first 4-8 hours of onset, but needs more specific examinations to supplement, such as CK-MB orcardiac troponin 。[4]
Tertiary structure
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Myoglobin (Mb) isMammalian cellIt is mainly the protein that stores and distributes oxygen in muscle cells.The muscle of diving mammals, such as whales, seals and dolphins, is rich in myoglobin, which makes their muscles brownish red.Because myoglobin stores oxygen, these animals can potentially go underwater for a long time.Myoglobin is composed of a polypeptide chain and a cofactor heme,relative molecular mass16700, including 153Amino acid residue。The apo myoglobin that removes heme is called globinhemoglobinOfSubunit(α - globin chain and β - globin chain) have obvious homology in amino acid sequence, and their conformation and function are also very similar.[3]
Globular proteinOfX-ray diffractionThe figure is extremely complex, often with hundreds, thousands or even tens of thousands of diffraction points.For analyzing globular proteinscrystal structureAppropriate heavy metal atoms must be introduced into the protein crystals to be analyzed in order to obtainIsomorphic replacement crystal。The preparation of suitable isomorphic replacement crystals is one of the key tasks for the determination of spherical protein crystal structure by X-ray structure analysis technology.[3]
The spatial structure of myoglobin was determined by Kendrew j and his colleagues in 1963.The diffraction pattern of sperm whale myoglobin crystal measured by Kendrew et al. has about 25000 diffraction points, which requires a lot of complex computer operations.The X ray crystallographic analysis of myoglobin was completed in three stages: the first stage, with a resolution of 0.6 nm, could identify the folding and trend of the main chain of myoglobin molecular polypeptide;In the second stage of analysis, the resolution reaches the level of 0.2mm, and the side chain groups of the molecule can be identified;In the third stage, the resolution is 0.14nm, and all amino acid residues can be recognized.The observed residue sequence is completely consistent with the results obtained by chemical analysis.[3]
myoglobin[9]
Myoglobin molecules are flat prisms with a molecular size of about 4.5nm × 3.5nm × 2.5nm.The main chain of the polypeptide in the molecule consists of eight straight α - helices with different lengths. The longest helix contains 23 residues and the shortest 7 residues. Almost 80% of the amino acid residues in the molecule are in the α - helix region.The 8 helices are named A, B, C,... H. The corresponding non helical peptide segments are called NA (terminal segment), AB, BC,... FG, GH, HC (C-terminal segment).Therefore, each residue has a set of serial numbers calculated from the N end and additionally numbered according to the position in each spiral segment. For example, the 93rd His is also numbered as F8, indicating that the His is at the 8th position of the F spiral.[3]
It is used to diagnose myocardial and skeletal muscle injury and determine the extent of injury.[5]
Because Mb is a globulin of small molecule, Mb can leak from muscle tissue to circulating blood when myocardial or skeletal muscle is injured, and it can be filtered through glomeruli and appear in urine.Therefore, the determination of Mb in serum and urine can be used for the diagnosis of certain myopathy and heart disease, such as acute muscle injury, acute and chronic renal failure, severe congestive heart failure, long-term shock, neuromuscular diseases such asMuscular dystrophy, muscular atrophy, dermatomyositis, and myopathy caused by various reasons.Serum Mb is significantly higher in the early stage of myocardial infarction than serum creatine kinaseisozymeThe increased sensitivity is even higher.However, due to the same immunological properties of myocardial and skeletal muscle Mb, it is still not possible to distinguish between serum Mb from central muscle and Mb from skeletal muscle.If skeletal muscle disease is excluded, serum and urine Mb determination can be used as early diagnostic indicators of myocardial infarction.The reference range of human serum Mb is 16~87ng/m, and its content varies with gender, age and race.Generally, men are higher than women, black men are significantly higher than white men, and women do not have this racial difference.Mb was higher in older people of other races except blacks.[6]
Myoglobin is widely distributed in myocardium and skeletal muscle. The blood content of normal people is very low. When myocardium and skeletal muscle are injured, the blood Mb is significantly increased. Therefore, Mb determination is conducive to the diagnosis of acute myocardial infarction.At the same time, it is necessary to grasp the analysis of abnormal results. The content of myoglobin in serum reaches the peak within 4~12 hours after the onset of myocardial infarction, and returns to normal within 48 hours, which is an early indicator for the diagnosis of myocardial infarction.But there are skeletal muscle diseases, shock, surgical traumaRenal failurepatientSerum myoglobinIt can also be raised, pay attention to identification;Pseudohypertrophic myopathy, acute dermatomyositis,PolymyositisMyoglobin andCreatine phosphokinaseIt rises in parallel.[6]
In heart disease, the rise of myoglobin in serum or urine is a reliable indicator of myocardial damage.Angina pectoris, acute coronary insufficiency andOld myocardial infarctionMyoglobin in patients without acute myocardial infarction is normalAcute pericarditisNon myocardial infarctionheart failureandArrhythmiaThe patient's serum myoglobin was also in the normal range.[6]
matters needing attention:Fasting blood sampling;The examinees should live and eat normally, avoid drinking alcohol and strenuous exercise, and suspend the use of drugs that affect liver function.[5]
Clinical significance
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1. The elevation of blood and urine myoglobin levels is seen in acute myocardial infarction, which starts to rise at 4 hours of onset, and usually returns to normal at 24 hours.It starts to be discharged from the urine at 5~40h and lasts for 3~4d.Also visible inischemic heart disease、angina pectoris、myocardial damage 、Cardiogenic shock、Hemorrhagic shockEtc.[1]
2. Rise is seen inmuscular dystrophy, such as Duchenne muscular dystrophy and its genetic gene carriers (60%~80% Mb value increases), congenital muscular dystrophy, myotonic dystrophy, polymyositis and dermatomyositisProgressive muscular atrophy、Hypothyroidism、 Drug induced myopathy[1]。It is used for early diagnosis of acute myocardial infarction (AMI). Serum Mb starts to rise within 2-4h after chest pain occurs in AMI, reaches the peak within 5-12h, and returns to normal within 18-30h;It can also be used for skeletal muscle diseases, such as rhabdomyolysis, myopathyMalignant hyperthermiaEvaluation of, etc;And monitoring of excessive training in sports medicine[5]。
