The Nucleoside Triphosphate (NTP)-Induced Superprecipitation and NTPase Reaction of Chicken Gizzard Actomyosin as a Function of the NTP Concentration1

The ATPase or ITPase reaction and ATP- or ITP-induced superprecipitation were studied as a function of the ATP or ITP concentration with suspensions of chicken gizzard "native " myosin B or "reconstituted " myosin B (a combination of actin, myosin, and native tropomyosin). The specific aim of the study was to answer the following questions: i) Is the superprecipitation or the ATPase reaction sensitive to calcium ions even at very low concentrations of ATP? ii) Is tropomyosin required for calcium sensitivity? iii) Does "native " myosin B from gizzard muscle behave differently from "recon-stituted " myosin B? iv) Does the troponin-tropomyosin complex of rabbit skeletal muscle act as a regulatory protein for the contractile activity of acto-phosphorylated myosin? Considering the overall time course of reaction rather than single values of ac-tivity, we found that the answers to the first three questions were negative, while that to the last question was positive. These results favor the kinase-phosphatase mechanism of calcium regulation rather than the leiotonin mechanism. According to Szent-Gyorgyi (7), muscle contraction study both the ATPase reaction and superprecipi-is simply the coupling of the actomyosin-ATPase tation simultaneously. In the present study, this reaction (chemical change) with actomyosin super- approach was adopted. precipitation (physical change). We, therefore, There has been a general tendency to regard feel that if one aims at elucidation of the molecular results on the ATPase activity as being more mechanism of muscle contraction, one should reliable than those on superprecipitation activity