Calcium sensitivity of actomyosin ATPase: its modification by substitution of myosin sulfhydryl groups.

Abstract

SH group substitution by DTNB enabled natural actomyosin to split ATP (in the prescence of Mg2+) also in the absence of Ca2+, when assayed at low ionic strength. At higher KCl concentrations the ATPase activity of SH group substituted actomyosin was still Ca-dependent. Addition of unsubstituted myosin to natural actomyosin whose SH groups had been substituted increased the ATPase activity. This increase was Ca-insensitive indicating that SH group substitution of myosin in actomyosin can make the interaction of additional myosin molecules Ca-independent. In natural actomyosin Ca-insensitivity of ATPase activity was attained at a lower degree of SH group substitution when substitution was performed in the presence of EDTA. The part of ATPase activity which still remained Ca-sensitive after DTNB treatment could be activated by lower concentrations of free Ca2+ than the Ca-sensitive ATPase of untreated actomyosin. In reconstituted actomyosin the Ca-sensitivity of ATPase activity could more easily be reduced when the myosin-actin ratio was high. For demonstrating remaining Ca-sensitivity in SH group substituted reconstituted actomyosin more tropomyosin-troponin was needed than for sensitizing unsubstituted actomyosin to Ca2+. The similarities between the ATPase acitivity of SH group substituted actomyosin on the one hand and that of actomyosin at low concentrations of ATP on the other hand suggest that SH group substitution modifies actin-myosin interaction in a similar way as does nucleotide-free myosin (rigor myosin).