The molecular structure of lubricating glycoprotein-I, the boundary lubricant for articular cartilage.

Lubricating glycoprotein-I (LGP-I) was prepared from bovine synovial fluid by density gradient sedimentation and gel-permeation chromatography. The LGP-I sample obtained was able to lubricate articular cartilage in a manner equivalent to that of whole synovial fluid. Chemical, physical, and electron microscope measurements were carried out to determine the structure of the LGP-I molecules. The molecular weight calculated from sedimentation equilibrium measurements was 2 X 10(5), and the solute distribution obtained indicated that LGP-I was relatively monodisperse. The s(0)20,w value was 4.84, and the intrinsic viscosity was 92 ml/g. The molecular weight and diffusion coefficient calculated from later light-scattering measurements was 2.06 x 10(5) and 1.10 x 10(-7) cm2/s, respectively. The electron microscope measurements showed that the LGP-I molecules had a number average length of 204 nm, a weight average length of 222 nm (with a standard deviation of 54 nm), and a width of 1-2 nm. These data and the kinked appearance of the molecules indicate that LGP-I is a partially extended flexible rod. The hydrodynamic measurements also indicate that LGP-I has the same structure in solution, although the apparently high s(0)20,w value, compared to other rod-like molecules, suggests that due to its flexibility LGP-I can occupy a more compact domain than would be expected based on its extended dimensions. The name of "lubricin" is suggested for this lubricating glycoprotein.