A study and comparison was made of α-actinin purified from cardiac and both light and dark portions of semitendinosus muscle of the pig. Distinct differences in elution patterns were observed between cardiac (or dark skeletal) and light skeletal α-actinins during purification by DEAE-cellulose chromatography. Examination by polyacrylamide gel electrophoresis demonstrated a single band present in cardiac α-actinin that migrated faster than the major band present in α-actinin from light skeletal muscle, but coincided with the major band of α-actinin from dark skeletal muscle. α-Actinin prepared from either of the separated portions of semitendinosus muscle also exhibited a lightly stained band that migrated to a point equidistant between the faster major band of dark skeletal (or single band of cardiac) and the slower major band of α-actinin of the light skeletal preparations. The intermediate band may represent an intermediate or hybrid type of α-actinin molecule. Results of amino acid composition substantiated the chromatographic and electrophoretic differences found between the cardiac (or majority of protein in dark skeletal) and light skeletal preparations. All three α-actinin preparations exhibited a subunit molecular weight of approx. 95 000 as determined in polyacrylamide gel electrophoresis runs made in the presence of sodium dodecyl sulfate, indicating the α-actinin molecule is composed of two subunits of equal size. No significant differences were observed among cardiac and skeletal α-actinins in their relative abilities to activate the enzymic and turbidity responses of reconstituted actomyosin. Cardiac α-actinin and the major proportion of α-actinin prepared from dark skeletal muscle appear similar, if not identical, in this study, but both differ significantly in certain molecular properties from the major share of α-actinin isolated from light skeletal muscle. The biological significance of these differences is not presently clear.
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