Reaction of myosin with 1-fluoro-2, 4-dinitrobenzene at low ionic strength.

Adenosine triphosphate has a dual effect on the incorporation of I-fluoro-2,4dinitrobenzene into myosin at low ionic strength: free ATP increases the incorporation, whereas magnesium-chelated ATP decreases it. Most of the reagent was incorporated into the smallest active fragment isolated from the stepwise digestion of the modified myosin with trypsin. Free ITP, GTP, CTP, UTP, ADP, PPPi, and PPi were equally as effective as ATP in increasing the incorporation, whereas Pi was less effective and AMP had no effect. The protecting effect of MgATP*on the incorporation could only be duplicated by MgCTP2or MgUTP2-. The increased incorporation of 1-fluoro-2,4-dinitrobenzene into myosin in the presence of free ATP and PPPi was the result of a greater reactivity of cysteine and tyrosine (and some lysine) residues in myosin. In the presence of MgATP2-, the reactivity of cysteine and tyrosine (and some lysine) residues of myosin was decreased. Actin also protected myosin from the incorporation of 1 -fluoro-2,4-dinitrobenzene at low ionic strength. This protection similarly was localized in the smallest active fragment of myosin. Combination with actin promoted a change in the conformation of myosin in which no enhancement of incorporation was produced by substances that otherwise increased the incorporation into myosin alone. Tyrosine groups of myosin were less accessible to l-fluoro2,4-dinitrobenzene in the presence of actin than in its absence. During the interaction of myosin with F-actin and MgATP2at low ionic strength, certain groups of myosin became available for reaction with I-fluoro-2,4-dinitrobenzene.