Heterogeneity of carboxylesterases in rat liver cells

Hepatic Retinyl Ester Hydrolases and the Mobilization of Retinyl Ester Stores

For mammals, vitamin A (retinol and metabolites) is an essential micronutrient that is required for the maintenance of life. Mammals cannot synthesize vitamin A but have to obtain it from their diet. Resorbed dietary vitamin A is stored in large quantities in the form of retinyl esters (REs) in cytosolic lipid droplets of cells to ensure a constant supply of the body. The largest quantities of REs are stored in the liver, comprising around 80% of the body’s total vitamin A content. These hepatic vitamin A stores are known to be mobilized under times of insufficient dietary vitamin A intake but also under pathological conditions such as chronic alcohol consumption and different forms of liver diseases. The mobilization of REs requires the activity of RE hydrolases. It is astounding that despite their physiological significance little is known about their identities as well as about factors or stimuli which lead to their activation and consequently to the mobilization of hepatic RE stores. In this review, we focus on the recent advances for the understanding of hepatic RE hydrolases and discuss pathological conditions which lead to the mobilization of hepatic RE stores.

The distribution of non-specific carboxylesterases and glutathione S-transferases in different rat liver cells. Effects of vitamin A deficiency

Non-specific carboxylesterases (carboxylesterases) and glutathione S-transferases (GSTs) are two groups of drug metabolizing enzymes responsible for hydrolysis and glutathione conjugation of xenobiotics. This study was conducted to determine the following: (1) the distribution of carboxylesterase and GST activities in different rat liver cells, (2) the effects of vitamin A deficiency (A-) on the absolute activities and on the distribution of carboxylesterases and GSTs in rat liver. Rat livers were fractionated into parenchymal and non-parenchymal cells by means of collagenase perfusion and differential centrifugation. Non-parenchymal cells were further fractionated by means of Percoll density gradient centrifugation into a layer of Kupffer cells and another layer containing stellate and endothelial cells. Carboxylesterase and GST activities were determined in these fractions. show that: (1) both carboxylesterases and GSTs were mainly localized in the parenchymal fraction, (2) there was no significant difference between male and female rats with regard total activity or distribution of carboxylesterases and GSTs in rat liver cells, (3) A- caused a highly significant reduction in carboxylesterase and GST activities in total liver homogenates and parenchymal cells. This reduction was not ameliorated by administration of retinoic acid 18 hr before sacrifice of animals. These results open up a new era of investigations about the potential role of vitamin A in the regulation of detoxification enzymes.