the adenosine triphosphatase activity of the ; natural ' actomyosin system ( containing tropomyosin and troponin ) stimulated by the smallest cation , mg ( 2+ ) ( ionic radius 0.65a ) , is low when the system is deprived of ca ( 2+ ) but high in the presence of small amounts of ca ( 2+ ) .
C0007447
cation
C0022023
ionic