Biochemical Studies on Bile Secretion

Abstract

Bile is a complex aqueous fluid produced by the hepatic parenchymal cells; the flaw rate and composition varies widely from one species to another. The major constituents of bile are water, bile salts and pigments, cholesterol, phospholipids and inorganic electrolytes. A mechanism proposed for the secretion of bile must not only explain the presence in bile of normal constituents, but must also account for the capacity of the biliary system to excrete and in many cases to concentrate organic anions and cations with widely differing physicochemical characteristics. Furthermore, the overall production of bile is not confined to one site: the process begins with some substances that subsequently appear in the bile (e.g. bilirubin; xenobiotic substances) crossing the plasma membrane of the parenchymal cell at the sinusoidal aspect; enzymes present in the endoplasmic reticulum metabolise and conjugate many of these compounds. Other enzymes conjugate primary or secondary bile acids with glycine or taurine, while bilirubin is conjugated, for example, with glucuronide or sulphate. These biotransformed compounds must then pass from the endoplasmic reticulum to the canalicular site. Their route is uncertain, but it has been suggested that it may involve the Golgi apparatus.