Colloid osmotic pressure of steer alpha- and beta-crystallins: possible functional roles for lens crystallin distribution and structural diversity.
This study addresses the general mechanisms whereby the major cytoplasmic proteins from the adult bovine lens contribute both to transparency and maintenance of the refractive index gradient across the lens. Colloid osmotic properties and quaternary structure were measured for alpha- and beta-crystallins isolated from the steer lens, including low-molecular-weight crystallins from the cortex (alpha Le and beta L) and nucleus (alpha Ln) and high-molecular-weight crystallins from the nucleus (alpha H and beta H). In electron microscopic images of rotary-shadowed preparations alpha Le appears as spherical particles 16 nm in diameter, alpha Ln appeared as individual spheres or small aggregates of spherical subunits, alpha H contained large irregular aggregates as large as 180 nm, and both beta L and beta H appeared as elliptical particles of 7-9 nm diameter. Secondary osmometry showed that for all these crystallins colloid osmotic pressure increased monotonically in a non-linear fashion with protein concentration. For the alpha-crystallins, osmotic pressure rose more steeply with concentration for alpha Le than for either alpha Ln or alpha H, so that at 0.3 g ml-1 at 0.1 M ionic strength, the colloid osmotic pressure of alpha Le, alpha Ln and alpha H were approximately 2.6 x 10(5) dyn cm-2, 1.6 x 10(5) dyn cm-2 and 1.0 x 10(5) dyn cm-2, respectively. In a similar manner, osmotic pressure rose more steeply with concentration of beta L than for beta H, so that at 0.3 g ml-1 at 0.1 M ionic strength the colloid osmotic pressures of beta L and beta H were 2.6 x 10(5) dyn cm-2 and 1.1 x 10(5) dyn cm-2, respectively. The osmotic pressure of alpha Le dropped as ionic strength was increased from 0.02 to 0.4 M. For beta L and beta H, osmotic pressure dropped as ionic strength was increased from 0.02 to 0.1 M but was nearly the same at 0.1 M and 0.4 M ionic strength. The data for steer alpha Ln and beta H were similar to previous reports for calf cortical alpha L and beta-crystallins, respectively. The osmotic pressure isotherms for alpha Le, beta L and that previously reported for steer cortical extract were nearly identical, whereas the nuclear crystallins (alpha Ln, alpha H or beta H) generated slightly higher pressures than those previously reported for steer nuclear crystallin extracts. In all cases, osmotic pressure rose more steeply with concentration for the cortical crystallins than for the nuclear crystallins.(ABSTRACT TRUNCATED AT 400 WORDS)
Kenworthy, AK; Magid, AD; Oliver, TN; McIntosh, TJ
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