Parvalbumin concentration and diffusion coefficient in frog myoplasm

The concentrations and diffusivity of two isoforms of parvalbumin, IVa and IVb, were measured using quantitative SDS PAGE in single fibers from semite ndinosus muscles of the frog Rana temporaria. The concentrations of IVa and IVb were 2.9 +/- 0.3 (SEM) and 4.5 +/- 0.5 gl(-1) total fiber volume, resp ectively. The total concentration of parvalbumin (7.4 +/- 0.8 gl(-1) total fiber) corresponds to a cytosolic concentration of 0.9 +/- 0.1 mmol l(-1) m yoplasmic water. Estimates for the transverse and longitudinal diffusion co efficients for parvalbumin at 4 degrees C were obtained in two ways: (1) by diffusion of parvalbumin out of skinned fibers into droplets of relaxing s olution, and (2) by diffusion of parvalbumin between two juxtaposed skinned fibers under oil. The transverse diffusion coefficient obtained using the droplet method was significantly lower than that obtained using juxtaposed fibers, but the longitudinal diffusion coefficients obtained from both meth ods were similar. The juxtaposed fiber method more accurately approximates parvalbumin diffusion in undisturbed myoplasm because no artificial solutio ns were used and, upon fiber-to-fiber contact, a potentially confounding oi l barrier at the interface rapidly disperses. The juxtaposed fiber method y ielded values for transverse (4.27 +/- 0.87 x 10(-7) cm(2) s(-1)) and longi tudinal (3.20 +/- 0.74 x 10(-7) cm(2) s(-1)) diffusion coefficients that we re not significantly different, suggesting that diffusion of parvalbumin in myoplasm is essentially isotropic. The average diffusion coefficient of fr og parvalbumin in myoplasm (3.74 +/- 0.81 x 10(-7) cm(2) s(-1); 4 degrees C ) is approximately a third of that estimated for frog parvalbumin diffusing in bulk water into and out of 3% agarose cylinders (10.6 x 10(-7) cm(2) s( -1); 4 degrees C). The reduced translational mobility of parvalbumin in myo plasm reflects an elevated effective viscosity due to tortuosity and viscou s drag imposed by the fixed proteins of the cytomatrix and the numerous dif fusible particles of the cytosol.