Labeling with 14C amino acids of albumin-like protein by rat liver ribonucleoprotein particles

The kinetics in vivo of the synthesis of albumin-like protein and albumin in rats

The incorporation of radioactivity into total protein, albumin-like protein and albumin in liver and into total protein and albumin in blood plasma, was studied for a time period of 2 h after intraportal injection of L-[1-14C]leucine. 1. In the liver, radioactivity began to increase in albumin-like protein before 2.5 min and in albumin at about 10 min after injection. In the plasma, radioactive albumin appeared at about 15 min. No albumin-like protein could be detected in the plasma. 2. Maximum radioactivity was reached first in albumin-like protein, then in hepatic albumin and finally in albumin in the blood-stream. The maximum specific radioactivity of albumin-like protein was 15 times higher than that of extravascular hepatic albumin which, in turn, was 6 times higher than that of plasma albumin. 3. The increase of radioactivity in albumin in the blood corresponded almost quantitatively to the decrease of radioactivity in albumin-like protein. 4. It is concluded that the albumin-like protein is the precursor of albumin in vivo. It is converted into albumin 5-6 min before the appearance of newly synthesized albumin in the blood-stream.

Microsomal components in relation to amino acid incorporation by preparations from the developing rat brain

1. After incorporation of [(14)C]valine in vitro, cerebral microsomes were separated into membrane-bound and free ribosomes by sucrose-density-gradient centrifugation. 2. In preparations from both 4-day-old and adult rats, free and bound ribosomes incorporated [(14)C]valine. Free ribosomes could be found as polysomes, which were highly active. 3. Microsomes labelled with [(14)C]valine in vitro were fractionated after deoxycholate treatment into a preliminary sediment, sedimented at 105000g (5min.), and ribonucleoprotein particles, sedimented at 150000g (70min.), to determine the role of membrane-bound ribosomes. In the adult the ribonucleoprotein particles retained most of the radioactivity, whereas in the young the preliminary sediment was as highly labelled as the ribonucleoprotein particles. 4. The labelled preliminary sediment from young preparations was both ribonuclease- and deoxycholate-resistant, and the nature of this material is discussed in terms of a possible structural component of microsomal membranes.