Myosin plays a fundamental role in the contraction of muscle cells. Some structural differences of this protein are related to the different physical properties of muscle--that is, speed of shortening. The typical myosin protein has a molecular weight of 460 000 daltons and is made up of six subunits: two heavy chains (200 000 daltons each) and two pairs of light chains (LC) weighting 15 000-25 000 daltons each (LC1 and LC2). Myosin was extracted from six specimens of human oesophagus in order to find out whether any biochemical differences exist between the oesophageal body and the lower oesophageal sphincter, to account for their different properties. Myosin was examined by two dimensional gel electrophoresis. Peptide mapping of myosin heavy chains was obtained by carrying out enzymatic digestion during the electrophoretic run. A quantitative difference between LC1 of the oesophageal body and the lower oesophageal sphincter was found in the circular fibres, thereby suggesting that two populations of muscle fibres exist in the inner coat of the oesophagus. The presence of more than one type of myosin strongly implies functional differentiation of the two specialised zones.
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