The inhibition of muscle contraction by adenosine 5' (beta, gamma-imido) triphosphate and by pyrophosphate.

We have studied the inhibition of the contraction of glycerinated rabbit psoas muscle caused by ligands that bind to the ATPase site of myosin. Two ligands, adenosine 5' (beta, gamma-imido) triphosphate (AMPPNP) and pyrophosphate (PPi), decreased the force and stiffness developed in isometric contractions and the velocity of shortening of isotonic contractions. The force exerted by isometric fibers was measured as a function of MgATP in the presence and absence of a constant concentration of the ligands. As the MgATP concentration decreased, the inhibition of tension caused by the ligand increased, reaching approximately 50% at 25 microM MgATP and either 2 mM MgPPi or 2 mM MgAMPPNP. The maximum velocity of shortening was also measured as a function of MgATP concentration in the presence of 1 and 2 mM MgPPi and 2.5 and 5 mM MgAMPPNP. Both ligands acted as pure competitive inhibitors with Ki = 3.0 mM for PPi and 5.1 mM for MgAMPPNP. These data show that both ligands are weak inhibitors of the contraction of fibers. The results provided information on the energetics of actin-myosin-ligand states that occur in the portion of the cross-bridge cycle where MgATP binds to myosin. A simple analysis of the inhibition of velocity suggests that MgAMPPNP binds to the actomyosin complex at this step of the cycle with an effective affinity constant of approximately 2 X 10(2) M-1.     

Effects of alkali ions on myosin conformation

We have used two probes to study the effects of alkali ions on the conformation of myosin. One was paramagnetic, the “spin label” N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)-maleimide, which binds primarily to SH groups; and the other was fluorescent, l-anilino-8-naphthalenesulfonate, which binds to an apolar niche. The bonding of the spin label to myosin was carried out in 0.6M LiCl, 0.6M NaCl, or 0.6M KCl, and the resulting labeled myosin was studied in the same medium in which the myosin was labeled as well as in other alkali chlorides. The electron paramagnetic resonance spectra of the spin label showed that the structure of myosin in the vicinity of the labeled groups differed in the various salts. The protein surface in the region of the labeled groups restricted the rotational freedom of the spin label more in KCl than in any of the other salts. Although ions are known to influence the properties of myosin, our results show that these ions also effect the molecular structure. The fluorescence of l-anilino-8-naphthalenesulfonate, noncovalently attached to myosin in the presence of alkali chlorides, decreased progressively with increasing size of the cations, again showing the protein structure near the probe attachment to be a function of the cation, in the solvent. Ca²⁺ quenched the fluorescence of the bound probe, indicating an interaction between Ca²⁺ and the myosin molecule. The effect of Ca²⁺ on the fluorescence was greatest in KCl.     

Structural investigations of the extracellular polysaccharides elaborated by Beijerinckia mobilis

The extracellular mucilage from Beijerinckia mobilis, a member of the Azotobacteriaceae, after removal of contaminating protein, was separated into a neutral polysaccharide (N-2, 10%); a neutral, dialysable fraction (N-1, 5%), consisting of glucose and oligosaccharides containing glucose, arabinose, and rhamnose; and an acidic polysaccharide (85%). N-2 (mol. wt, 1900) was highly branched and comprised glucopyranose, mannopyranose, and arabinofuranose residues (1:1:1). The various linkages were determined. The acid fraction was a polymer of high molecular weight composed of L-guluronic acid (65%), D-glucose (15%), and D-glycero-D-mannoheptose (20%), together with acetic and pyruvic acids. From the results of methylation, periodate oxidation, and partial hydrolysis, a branched molecule with a backbone of guluronic acid and heptose, and side chains of glucose and guluronic acid is proposed. Pyruvic acid was found to be acetal-linked to 2?5% of the heptose residues. The similarities between this polysaccharide and that from the related species Azotobacter indicum are discussed.