The synthesis of serum albumin and tissue proteins in slices of rat liver and liver tumour

Abstract

The preparation of rat-serum albumin with ammonium sulfate fractionation and zone electrophoresis on columns of "cellulose" is described. The homogeneity of the albumin was investigated by boundary electrophoresis and immuno -electrophoresis. Anti serum, prepared by injection of rat-serum albumin into rabbits, was used to determine the amount of albumin in extracts of tissue slices which had been incubated in a bicarbonate buffer for periods up to 4 hours. Tissue slices were incubated with C14-glycine and the incorporation of radioactivity into the soluble tissue proteins, precipitated with trichloroacetic acid, and into the albumin precipitated by antiserum, was studied. A net synthesis of serum albumin occurred during the incubation of slices of liver from normal rats, and of slices of liver and liver tumor from rats bearing liver tumors induced by feeding on 4-dimethylaminoazobenzene. Radioactive ami no acids were incorporated into both the soluble tissue proteins and the serum albumin of such slices. Immuno-electrophoretic studies indicate that the albumin present in liver and liver-tumor slices is identical with serum albumin. A net synthesis of serum albumin did not occur when rat-kidney slices were incubated, nor did the serum albumin contained in such slices become radioactive when the slices were incubated with C14-glycine. However, the soluble tissue proteins did become radioactive under these conditions. When slices of liver tumors consisting mainly of hepatoma cells were incubated with C14-glycine the serum albumin in the slices became radioactive and there was a net synthesis of albumin. However, when slices of liver tumors consisting mainly of cholangioma cells were incubated under the same conditions, the incorporation of radioactivity into the serum-albumin fraction was very small and there was a net loss of serum albumin during the incubation. Such slices did, however, incorporate radioactivity into the soluble tissue proteins. The incorporation of radioactivity from C14 glucose by slices of rat liver and liver tumor was also studied. Radioactive rat-serum albumin was prepared from the serum of a rat which had received an injection of Cl4-lysine, This albumin was used to determine the combining ratio of serum albumin to antibody in precipitates of these proteins. The ratio varied according to the concentration of albumin in the solution to which the antiserum was added. The effect on the radioactivity of the soluble tissue proteins and serum-albumin fractions of incubating varying amounts of liver and liver-tumor slices with a constant amount of Cl4-glycine was studied. Slices of rat liver and liver tumor were incubated with C14-glycine and the radioactivity of the albumin and soluble tissue protein fractions was determined in the medium and slices independently. The results are interpreted as suggesting that serum albumin is "secreted" both by liver slices and by liver-tumor slices. From a comparison of the results obtained with liver slices and liver-tumor slices it is concluded that the hepatoma cells in liver tumors retain the ability of the liver cells to synthesize serum albumin. Determinations of the absolute amount of serum albumin synthesized by slices indicate that the tumor slices synthesize only 30-50% as much albumin as do a similar weight of liver slices. Studies on the rate of incorporation of radioactive amino acid into the serum albumin in the tissues are in agreement with this observation.