The Chemistry of Antigenic Components Isolated From Glomerular Basement Membrane

Abstract

Fractions of varying molecular size and chemical composition were isolated from glomerular basement membrane (GBM) and characterized inimunochemically. Reduction and alkylation in 8 M urea results in a fraction (GRAU) with an amino acid and carbohydrate composition similar to the whole membrane. Antiserum to this fraction (anti-GRAU) gives two precipitin lines upon immunodiffusion. Reduction and alkylation in the absence of 8 M urea results in a fraction with significantly less hydroxyproline and hydroxylysine which gives two precipitin lines of identity with GRAU when reacted against anti-GRAU. One of these precipitin lines is given by a fraction which can be obtained either after extraction with 8 M urea or as the undigested residue after treatment of GBM with collagenase. This fraction has a molecular weight of over 200,000, has low amounts of hydroxyproline and hydroxylysine and a glucose to galactose ratio of 0.25. The second precipitin line is given by a smallmolecular weight glycoprotein which is retarded on Sephadex G-200 and is obtained after digestion of GBM with collagenase. This fraction lacks hydroxyproline and hydroxylysine and has a glucose to galactose ratio of 0.08. The collagen-like protein represents a third antigenic component; it can be obtained after limited digestion of GBM with pronase or pepsin. It has a glucose to galactose ratio of 0.95. It fails to give a precipitin line with anti-GlUU; however, it is antigenic and gives rise to precipitin antibodies in rabbits. Basement membranes from various tissues contain variable proportions of these antigenic components which are responsible for the cross-reaction seen among these extracellular matrices.