Biochemical and histochemical analyses of lecithin:retinol acyltransferase from polar bear (Ursus maritimus) livers

Abstract

Vitamin A is a fat-soluble vitamin required for many physiological functions. It is known that polar bears (Ursus maritimus) store a large amount of vitamin A within hepatic stellate cells (HSCs). However, the molecular mechanisms for high-capacity storage of vitamin A by polar bear HSCs are unknown. We biochemically and histochemically analyzed lecithin:retinol acyltransferase (LRAT), an enzyme responsible for conversion of retinol to its storage form, retinylesters, in the liver. Surprisingly, LRAT activities of the liver microsome fractions from polar bears weighing 100–500 kg were considerably lower than those obtained from rats (Rattus norvegicus). Although there is a possibility that the quality of polar bear liver specimens was reduced during transport, intrinsic LRAT activity of polar bears might actually be lower than those of other mammals. Fluorescent immunohistochemistry revealed that LRAT and cellular retinol-binding protein (CRBP) I were well co-localized within the liver lobules of polar bears. Taking into account that retinol bound to CRBP I is prevented from degradation and becomes a good substrate of LRAT, we hypothesized that CRBP I effectively conveys retinol to the LRAT, thereby circumventing the low enzymatic activity of LRAT in polar bear livers. As the top predator in the Arctic, polar bears have access to large amounts of vitamin A via the food web. We suggest that polar bears acquired an efficient mechanism of esterification of vitamin A within the liver during the course of their evolution.