Structural Characterization of a Soluble Fraction from Lens‐Capsule Basement Membrane

A soluble fraction was isolated from anterior lens capsule basement membrane by treatment with a dilute, acidic solution of sodium citrate. This fraction had an amino acid and carbohydrate composition similar to that of the intact basement membrane. Upon electron microscopy, this fraction was found to contain two morphologically distinct components: thin strands measuring 2–3 nm in diameter and globular units measuring 5–20 nm diameter. Treatment with pepsin destroyed the globular component leaving the filaments intact. The amino acid composition of the pepsin‐treated extract was similar to that of basement membrane collagen. Treatment with bacterial collagenase resulted in the destruction of the filamentous components leaving the globular components intact. These data indicate that the filamentous structure was collagenous whereas the globular component was not. Reduction of disulfide bonds resulted in an apparent dissociation of the globular unit from the filamentous component. The data indicate that the protein extracted with sodium citrate represents a structural subunit of the lens capsule basement membrane.