In Vitro Synthesis and Secretion of Albumin by Morris Hepatomas 5123C and 7800¹

Morris hepatomas 5123C and 7800 were incubated in vitro with radioactive l-leucine in Krebs-Ringer-bicarbonate-saline, and the incorporation of radioactivity into albumin precursors, serum albumin, and trichloroacetic acid-precipitable proteins was measured and compared to that which occurs in normal rat liver slices. At the end of a 10-min pulse incubation with radioactive l-leucine, Morris hepatoma 7800 incorporated 0.25% of the total protein radioactivity into an albumin fraction while Morris Hepatoma 5123C incorporated 0.5% and rat liver slices showed a much larger proportion of incorporation (4.8%) into albumin. The pulse-labeled tissues were further incubated with nonradioactive l-leucine for different periods of time, and they all released radioactive proteins into the incubation medium. Only 5% of the total protein radioactivity released into the medium by Morris hepatoma 7800 and 7.8% of that released by Morris hepatoma 5123C were incorporated into albumin while, in contrast, rat liver slices released 46% of its protein radioactivity as albumin. Both hepatomas were capable of releasing 80% of the pulse-labeled albumin into the medium at the end of a 75-min chase period. Analyses of the nascent albumin within the tissues of Morris hepatomas 7800 and 5123C and in rat liver slices showed that, at the end of a 10-min pulse incubation, the intracellular albumin fraction could be identified as a precursor of serum albumin (proalbumin) and that, as the chase incubation proceeded, proalbumin was converted intracellularly to another form of albumin which was chromatographically distinct from serum albumin. The radioactive albumin which was released into the medium, however, closely resembled serum albumin. Like normal liver, Morris hepatomas 5123C and 7800 are able to segregate nascent proalbumin in the microsomal cell fraction, and only a small amount (8%) was found in the soluble cytoplasmic fraction. Also like normal liver, a messenger RNA fraction isolated from Morris hepatoma 7800 can be translated by an in vitro protein-synthesizing system into a larger albumin precursor containing l-leucine in positions 7, 8, 9, and 10. These studies indicate that Morris hepatomas 5123C and 7800, while synthesizing smaller amounts of albumin than does normal liver are, however, capable of normal processing of precursor into serum albumin and appear to secrete serum albumin into the incubation medium.