Porcine cardiac myosin monomers in equilibrium with filaments under physiological conditions were observed to have two conformations, extended and folded forms, upon electron microscopy and gel filtration HPLC. The conformational state was independent of ATP and the phosphorylation of regulatory light chain. The folded monomers of cardiac myosin were mainly in an open conformation with only one bend in the tail, and may not trap the hydrolysis products of ATP, as assessed by single turnover experiments. These properties are similar to those of the folded monomers of rabbit skeletal myosin [Katoh, T., Konishi, K., and Yazawa, M. (1998) J. Biol. Chem. 273, 11436-11439]. The conformational states of skeletal and cardiac myosin monomers were not affected by pH between 7.0 and 8.5. Although significant disassembly of filaments and thus an increase in the monomer concentration were observed with an increase in pH. The results indicate that the pH-dependent change in filament assembly is due to a shift of equilibrium between the filaments and extended monomers toward filament disassembly. The Mg2+-ATPase activity of these myosin monomers decreased with a decrease in the salt concentration below approximately 0.1 M, suggestive of the formation of a closed conformation similar to the conformation of 10S smooth myosin. The results suggest that the conformational change from the extended to the folded form is a common property of various myosin IIs.

Acanthopanax senticosus HARMS (AS) is used as a Chinese herbal medicine and as a health supplement in Japan. However, little is known about the interaction between AS and other drugs. In this study, we investigated the effect of AS extract on intestinal drug transporter (P-glycoprotein, or P-gp) and peptide transporter activities in Caco-2 cells. Caco-2 cells were cultured on a culture dish and a permeable membrane for 1-3 weeks. The apical-to-basolateral (A-to-B) transport of digoxin, a P-gp substrate, was significantly increased by the addition of AS extract in a concentration-dependent manner. In contrast, the A-to-B transport of cephalexin, a peptide transporter substrate, was significantly decreased by the addition of AS extract in the same manner. The effects of AS extract addition on the kinetics of the uptake of rhodamine 123, a P-gp substrate, and Gly-Sar, a peptide transporter substrate, were investigated. V (max) for rhodamine 123 uptake was significantly increased by AS extract addition compared with the control, whereas that for Gly-Sar uptake was significantly decreased. On the other hand, K (m) and K (d) for rhodamine 123 and Gly-Sar uptake were not affected. We conducted further investigations to clarify the effect of AS extract addition on P-gp activity. When AS extract was added to the apical side, B-to-A transport of rhodamine 123 was significantly decreased compared with the control. Furthermore, the amount of intracellular rhodamine 123 was increased by AS extract addition compared with the control. These results suggest that P-gp and peptide transporter activities are suppressed by AS extract addition in a non-competitive manner.