The Biosynthesis of Rat Serum Albumin: VI. INTRACELLULAR TRANSPORT OF ALBUMIN AND RATES OF ALBUMIN AND LIVER PROTEIN SYNTHESIS IN VIVO UNDER VARIOUS PHYSIOLOGICAL CONDITIONS

Abstract

Abstract The incorporation of l-[14C]leucine was measured during a 16-min time interval in rats under various physiological conditions. Rates of albumin and liver protein formation in control rats 5 to 7 weeks of age were 3.13 and 28.8 mg/100 g of body weight per hour, respectively. The half-time of replacement of liver protein was 18 hours, appreciably faster than half-times obtained from measurements of catabolism of labeled liver protein. When synthesis of protein was inhibited by cycloheximide, loss of albumin from hepatic microsomal particles continued at its previously existing rate until the cells were nearly depleted of albumin. Rates of secretion measured in this manner were 0.34 and 0.74 mg per g of liver per hour in adult rats and young nephrotic rats, respectively; corresponding rates of synthesis determined with [14C]leucine were 0.34 and 0.77 mg per g of liver per hour. The agreement between the results obtained with these independent techniques adds some confidence to the use of the [14C]leucine method. The rate of albumin synthesis was unchanged when measured at 10 p.m. compared to 10 a.m. in rats on a 12-hour light-dark cycle. Fasting for 18 hours caused a 40% reduction. In adult rats the rate of albumin synthesis was 34% and that of liver protein 52% of the rates in control rats. No significant increase in albumin synthesis was detected in the presence of nephrosis caused by puromycin aminonucleoside, even when a supplement of protein plus hydrocortisone was given. The dynamics of the transport of albumin and its release from the cytoplasmic particles were similar in all of the conditions studied. The amount of albumin in hepatic microsomes, however, varied directly with the rate of albumin production over a 4-fold range. It is suggested that the stimulus for increased secretion is a small increase in the quantity of albumin in the microsomal particles.