Quantitative importance of biliary excretion to the turnover of hepatic lysosomal enzymes

Abstract

The turnover rate of an individual protein is a function of the rates of synthesis and loss of that protein. For most intracellular proteins, loss occurs through digestion by lysosomal or cytosolic proteases. Although a significant proportion of hepatic lysosomal enzymes is released from the hepatocyte by excretion into bile, the contribution of biliary excretion to the turnover of hepatic lysosomal enzymes has never been measured. Thus, we used in vivo pulse-labeling to determine the half-lives of two hepatic hydrolases, beta-galactosidase (beta-gal) and beta-glucuronidase (beta-glu). Each enzyme was purified by immunoisolation from hepatic lysosomes that were isolated at various times after injection of rats with 3H-labeled leucine. The decay curves for the specific radioactivities of beta-gal and beta-glu were used to calculate the half-lives of the proteins, which were 3.8 and 5.1 days, respectively. To determine the percent of total hepatic contents of each enzyme that was lost per day by biliary excretion, we collected bile from bile fistula rats for 24 hours and then used radioimmunoassays to quantitate the amounts of beta-gal and beta-glu in bile and liver samples of the same rats. We found that approximately 4% of the total hepatic contents of both beta-gal and beta-glu was excreted into bile per day. Finally, we used these data to calculate that 31% and 41% of hepatic losses of beta-gal and beta-glu, respectively, were due to biliary excretion. These results suggest that extracellular release through biliary excretion is a major mechanism contributing to the turnover of lysosomal hydrolases.

Bile Formation and Secretion

“…These individual amino acids are partially reclaimed from the bile by specific amino acid transporters that also line the luminal membrane of the bile duct epithelium (46, 388). Leukotrienes and their metabolites and other inflammatory cytokines, including tumor necrosis factor (TNF)-α (which is also synthesized in bile duct epithelial cells), appear in bile (398, 453, 464). …”

The inhibition ofEscherichia coli lac operon gene expression by antigene oligonucleotides-mathematical modeling

“…According to the N-end rule (Alberts et al, 1994), the repressor, having methionine at the N-terminal (Beyreuther, 1980), should be very stable. The decay rate constant of the repressor is not greater than that of ␤-galactosidase (Lee and Bailey, 1984b), so the half-life of the repressor should be not less than that of ␤-galactosidase, which is 3.8 days (Nakano et al, 1995). With this in mind, it is reasonable to conclude that oligos targeted to the lacI or the lacI promoter cannot significantly inhibit lac operon gene expression.…”

Effect of benz(a)pyrene and constant light exposure on rat liver lysosomes and biliary excretion of lysosomal enzymes

“…Therefore, there is little likelihood that different pathways of LE transport are activated in liver cells. Published data show that 24-h excretion of LE reaches 3-5% of their content in the liver [2,12]. The 2.5-fold increase in the excretion of LE under the influence of BP did not rapid decreased their amount in the liver; the estimated 24-h reduction of enzyme activity was 10%.…”

“…Activity of lysosomes and vesicular transport in bile are characterized by diurnal biological variations [8,12] and depend on changes in light/dark regimen (e.g., constant illumination, CI). This exposure can be considered as a stress factor modulating the reaction of liver cell vesicles to BP [7].…”