Rat α 1 ‐microglobulin

Rat α 1 ‐microglobulin was isolated from the urine of rats treated with sodium chromate, and was purified by the use of gel chromatography, affinity chromatography on concanavalin‐A–Sepharose and ion‐exchange chromatography. The protein was heterogeneous in charge, had a tendency to form dimers, and was associated with a brown‐coloured chromophore. The size of the protein (25 kDa) was similar to guinea pig α 1 ‐microglobulin but smaller than the human protein, when measured with sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Immunological cross‐reaction with human and guinea pig α 1 ‐microglobulin was demonstrated. The concentration of α 1 ‐microglobulin in rat serum was 16.4 mg/l (SD = 8.5 mg/l, n = 13) and rat serum α 1 ‐microglobulin was eluted from a gel chromatography column at two different positions corresponding to monomeric α 1 ‐microglobulin and IgA. The latter α 1 ‐microglobulin activity could be absorbed by anti‐IgA serum. Rat α 1 ‐microglobulin and albumin were continuously released into the medium of rat hepatocyte monolayers, and α 1 ‐microglobulin was isolated from the medium by the use of immunoprecipitation with anti‐(α 1 ‐microglobulin). Tritiated leucine, added to the medium, was incorporated into the protein, suggesting a de novo synthesis of α 1 ‐microglobulin by the hepatocytes. The size of hepatic α 1 ‐microglobulin was similar to that of purified urinary rat α 1 ‐microglobulin, when determined with sodium dodecyl sulfate/polyacrylamide gel electrophoresis.