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RECENT investigations have shown that actomyosin can be isolated from human muscle biopsy samples; it has a magnesium (Mg ++ ) moderated adenosine triphosphatase (ATPase) activity and at low ionic strength it undergoes superprecipitation, an in vitro analogue of contraction, in the presence of adenosine triphosphate (ATP), magnesium chloride and calcium chloride. 1 Both ATPase activity and superprecipitation are inhibited by removal of calcium with either a chelating agent or human fragmented sarcoplasmic reticulum. Fragmented sarcoplasmic reticulum can also be obtained from human biopsy samples 2 and, as in the case of rabbit muscle, 3-6 it manifests an ATP dependent calcium uptake by which it can lower the concentration of ionized calcium below the 10- 6 M threshold required for actomyosin superprecipitation. 4 In extension of the above studies, the present work on purified myosin from normal and myotonic dystrophic human muscle was undertaken. The procedure described is geared to the
Samaha et al. (1969) studied this question.