IDENTIFICATION OF LIVER ENDOTHELIAL-CELLS AS THE PRIMARY SITE OF IGM CATABOLISM IN THE RAT

Rat IgMs, both monoclonal protein from ascites fluid and total serum I gM, were purified by sequential gel filtration and metal chelate affin ity chromatography on immobilized zinc-iminodiacetate. Two monoclonal IgMs, IR202 and IR968, chromatographed identically on gel filtration, but required different pHs for elution from the zinc affinity column. IR202 behaved like a euglobulin, being readily precipitated in low-ion ic-strength buffers, while IR968 remained soluble under these conditio ns. IgM was isolated from serum in 30-50% yield by chromatographic pro cedures similar to those used for the monoclonal proteins, and 20-30% of the isolated serum IgM was precipitable as a euglobulin. The half-l ife of both monoclonal and serum euglobulin IgMs was 0.8 days, while t he polyclonal globulin and IR968 had half-lives of 1.8 and 2.8 days, r espectively, in the rat circulation. The tissue and cellular sites of catabolism of the monoclonal IgMs were determined after labeling with the residualizing label, dilactitol-[I-125]tyramine. For both proteins the liver was identified as the major tissue site of catabolism, acco unting for 60-80% of degraded protein in the body. When liver was frac tionated into parenchymal and nonparenchymal cells (NPC), the NPC were found to account for 86 and 69% of protein recovered in liver, for IR 202 and IR968, respectively. Separation of NPC into endothelial (EC) a nd Kupffer cell populations by elutriation centrifugation revealed tha t EC contained the majority, similar to 70% of total NPC radioactivity from either IgM. Based on the ratios of endocytic indices (mu l of pl asma/10(6) cells/day) for each cell type, the EC also had a higher eff iciency for uptake of both IgMs, approximately threefold greater, than for the fluid phase marker, polyvinylpyrrolidone, or for rat serum al bumin. We conclude that hepatic EC are a major site of IgM catabolism, regardless of the heterogeneity in physical and biological properties of various IgM populations.