Interaction between actomyosin and 8-substituted ATP analogs.

Various 8-substituted ATP analogs were synthesized, and their reactions with myosin and actomyosin were studied. The nucleoside triphosphates (NTPs) with an amino group at the 6 position and hydrogen at the 8 position, and formycin 5'-triphosphate (FTP) were hydrolyzed by myosin very slowly in the presence of Mg2+ and rapidly in the presence of EDTA and K+. In contrast, NTPs with substitution of the 8 position, other than FTP, were readily hydrolyzed by myosin in the presence of Mg2+ but were hardly hydolyzed in the presence of EDTA and K+. The Michaelis constant (Km) for hydrolysis of 8-substituted NTP by heavy meromyosin was much larger than the dissociation constant (Kfl) for binding of heavy meromyosin with NTP estimated from the change in tryptophan fluorescence. All the NTPs with no substitution at the 8 position, and FTP, caused an initial Pi burst, actin activation of myosin NTPase, superprecipitation of actomyosin, and myofibrillar contraction. On the other hand, all the 8-substituted NTPs in three possible conformations did not cause these phenomena, regardless of the conformation. These results were discussed in relation to the hindrance of rotation about the glycosidic bond accompanying an 8 substitution.