Studies on the amino groups of myosin ATPase III. Effect of nucleotides on the fluorescence of β-naphtho-quinone-4-sulfonate bound to amino groups of myosin

β-Naphthoquinone-4-sulfonate was used for chemical modification of amino groups of myosin. The reagent was found to affect also the sulfhydryl groups if the reaction was not prevented by previous disulfide exchange with cystamine. When cystamine protection was employed the ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity was enhanced in the presence of Mg²⁺ and decreased in the presence of K⁺ or Ca²⁺, a pattern typical of myosin with blocked essential amino groups. On addition of ATP or ADP a blueshift was observed in the fluorescent emission spectrum of β-naphthoquinone-4-sulfonate bound by myosin, presumably owing to conformational changes in the environment of essential amino groups induced by the binding of nucleotides.