Contents of Myofibrillar Proteins in Cardiac, Skeletal, and Smooth Muscles

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Abstract
The in situ contents of myosin, actin, α-actinin, tropomyosin, troponin, and desmin were estimated in dog cardiac, rabbit skeletal, and chicken smooth muscles. Whole muscle tissues were dissolved with 8 M guanidine hydrochloride and subjected to two-dimensional gel electrophoresis, which is a nonequilibrium pH gradient electrophoresis (Murakami, U. & Uchida, K. (1984) J. Biochem. 95, 1577–1584) with some modification. The amount of protein in a spot on a slab gel was determined by quantification of the extracted dye. Dye binding capacity of individual myofibrillar proteins was determined by using the purified protein. Myosin contents were 82±7 pmol/mg wet weight in cardiac muscle, 105±10 pmol±mg wet weight in skeletal muscle, and 45±4 pmol±mg wet weight in smooth muscle. Actin contents were 339±15 pmol/mg wet weight in cardiac muscle, 625±27 pmol/mg wet weight in skeletal muscle, and 742±13 pmol±mg wet weight in smooth muscle. The subunit stoichiometry of myosin in the three types of muscles was two heavy chains and four light chains, and there was one light chain 2 for every heavy chain. The molar ratio of actin to tropomyosin was 7/1 in the three types of muscles. Striking differences were seen in the molar ratio of myosin to actin: 1.0/4.1 in cardiac muscle, 1.0/6.0 in skeletal muscle, and 1.0/16.5 in smooth muscle.