Does muscle-type myosin have ADPase activity?

Adenosine diphosphate (ADP) is a nucleotide that is structurally very similar to ATP but lacks one of the two high-energy bonds due to hydrolysis. In muscle studies, ADP is usually considered exclusively as a product formed during myosin cross-bridge cycling and is not otherwise involved in this process. In our study, we question the widely held view of ADP as a final product formed during muscle contraction. Using biophysical and biochemical methods, we managed to show that ADP can act as a substrate for myosins in at least three types of muscles: smooth and striated adductor muscles of bivalves (Mytilidae and Pectinidae), and also vertebrate skeletal muscles. According to our data, the differences in the effect of ATP and ADP on the optical, biochemical, and structural properties of actomyosins are exclusively quantitative. We explain the previous ideas about ADP as a compound capable of inhibiting the ATPase activity of actomyosin by the ability of ATP and ADP to depolymerize the polymeric myosin when the concentration in the medium reaches more than 0.3 mM.